Re:Test~Structural, Vibration and Fatigue, Consulting, Training and Resources
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Servo-Hydraulic Test Controller Review
Please note that this review is now archived, and will not be updated. The information was current in the fall of 2004. For latest product information, please see our new Buyers Guide.
This is an unbiased review of servo controllers offered by a group of vendors in the market of automotive structural testing. I have been able to review all these controllers “in-the-flesh” either at the manufacturer’s facility, at installed locations, or at my facility. My intent is to leave personal opinions out as much as possible. All these products have both strengths and weaknesses, and it is up to you to read the facts and make a decision based on your own situation. Every lab has unique requirements, and while one controller might be a perfect fit in one situation, it might not be in another.
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Price and reliability are important, but they are left out of this review. It is difficult for me, without using these controllers day in and day out, to assess how many bugs there are. However, since this is an important issue, there is a link at the left for you to add your own comments. Your comments will be added to the end of the report so that folks can get a better overall picture. I also invite the manufacturers of these controllers to add their own comments.
Secondly, I didn’t want to get into the middle of the often-controversial pricing issue. If you are interested in purchasing a controller in this category, I suggest you get a quote from all these folks, and assess their local service and support. Also, just because a feature is mentioned in this review, it doesn't mean it is a part of the standard package. Carefully review the quotation to ensure you are getting everything you need. Take a look at the options and compare apples with apples.
The controllers reviewed are all PC and/or instrument based, and have applications designed for automotive structural fatigue testing. They all provide the ability to develop complex test procedures.
This review covers controllers supplied by the following companies (in alphabetical order):
· FCS-COM (formerly COM, Inc.)
While there are other players in this business, they do not have turn-key PC or instrument based controllers that provide extensive capabilities for automotive fatigue testing. If you are aware of a controller that should be reviewed, please let me know. This review is intended to be a floating document, and will be updated, as new products are made available. Other players in this market who, in my opinion, do not have controllers in this category are: DTE www.dynamic-testing.com, Zwick www.zwick.com, and Team www.teamcorporation.com. Please note that this review does not cover products that fit in the general category of random vibration controllers, or programmable controllers that allow you to build your own, such as the Moog M3000, or the Delta Tau P/MAC UMAC. To be included, controllers must have the capability of constructing block-profile tests or playback time histories.
This company is probably best known to you as COM Inc. COM, was a division of Enprotech, but it was purchased in the fall of 2003 by FCS Control Systems, a Netherlands company (FCS). COM, based in Ann Arbor Michigan, was incorporated in 1972, and spent its first 20 years working on custom automation for test labs and production lines. Their market is split between automotive and aerospace. In 1992 they began developing a test site controller in partnership with GM. The SIMCON 2100 controller was launched in 1995. Initially a Test Site Controller, it did not include servo-control. It was partnered with analog controllers provided by other vendors. In 2002 they released embedded digital servo-control capabilities.
FCS started as Fokker Control Systems. They originally developed test systems to test aircraft, and have since broadened their base to include all applications of servo-hydraulics for structural testing. They have developed a controller that meets the needs of all their markets. This has resulted in a product that brings many of the unique aerospace testing techniques into the automotive world. FCS also purchased the British company Kelsey Instruments, now called FCS-Kelsey.
Since FCS purchased COM, they have integrated their products. Joe Morrill, a leading light in the testing community, manages FCS COM. He has many of years experience at GM Proving Grounds, and presents white papers on testing techniques around the country.
This is a division of Instron, and was formed from the structural test division they had in England, the Schenck HydroPulse group in Darmstadt, and the Schenck Pegasus group in Detroit. Very little remains of the later group (Schenck Pegasus), and the operations of this division have all now been consolidated in the former Schenck facility in Darmstadt (as of January 2003). From the standpoint of their controllers, they have done a good job of focusing on one platform: the 8x00, which will be reviewed here. This must have involved some difficult decisions because the Schenck Pegasus group had some powerful controller products at one time. However, I think the decision was a good one. It is better to focus on a single unified platform rather than spread resources over several competing products. This is the controller platform used across the Instron Corporation. Instron has representation worldwide, and is well established in the material testing market. Schenck and Instron independently fought hard to become major players in the structural testing market, and as a single company, they continue to aggressively pursue a strong position. Unfortunately, through the turmoil of acquisitions and mergers, they are only just finding their feet as a single company with one voice, and are now positioned become a formidable competitor.
Their website can be found at www.instron.com/ist
I don’t think anyone would argue with the statement that MTS is the largest player in this market. Formed in 1968 from a spin off of Research Incorporated, they have become a juggernaut that has taken a strong position. They have many controller platforms that are used in this market, some share the same hardware platform, and some share the same software platform. This has lead to a lot of confusion around which product fits where. I will attempt to clear this up. While I am going to focus on the FlexTest® GT and SE controllers, I will mention the FlexTest IIs, FlexTest IIm, TestStar® II, and TestStar IIs to help you navigate through the maze. MTS is well represented worldwide, and while they have acquired several companies, their core vehicle testing business is largely unaffected. MTS is based in Eden Prairie Minnesota, and has representation worldwide.
Their website can be found at www.mts.com
This UK based company has been in the business of building servohydraulic test equipment for many years. They have established a strong niche in the area of testing Formula One cars, although they have products in all areas where servohydraulic test equipment is used. Their current controller is the newly released Pulsar® Digital Control System (Summer 2003). It is a rewrite of the DSC 2000 system, and currently uses the same hardware. Servotest is planning to release new hardware in the summer of 2004. The Pulsar system is installed at several locations around Europe.
Their website can be found at www.servotest.com
Tiab is a small UK company. It was founded in 2002 by Conway Young, a motorsport R&D expert. He felt that the control products that were available for servohydraulic testing were cumbersome and expensive. Using government funding, he has developed what he believes is a simple, versatile, inexpensive solution.
The Tiab website can be found at www.tiab.co.uk
The products I will focus on are below. This review will be updated as information on the new products becomes available.
· FCS—SmarTEST ONE®
· FCS COM (formerly COM, Inc.)—SIMCON 2100
· IST—Labtronic® 8800/8400
· MTS—FlexTest GT/SE
· Servotest—Pulsar Digital Control System
The main focus of this review is the controller hardware infrastructure, multi-station capabilities, servo-controller and tuning, events and limits, optimization algorithms, block programming, and time history playback. While I will mention a little about the real-time iterative simulation packages that are available as add-ons for each platform, I am not providing a review of them here. That might well come at a future date. I am also not going to talk about pricing, or support. These vary on geographic location. Make sure you get quotations from all these manufacturers, and research their local support, parts availability etc. before making your final decision. Also, make sure your quotations all include the options you need. Just because a capability is mentioned in this review does not mean it is included in the standard package.
As far as reliability is concerned, please add your own comments. Since I do not use all these controllers daily, I have no idea about how many bugs and workarounds there are. I know some of them, but not all, so it is not fair for me to comment.
By taking a step back, and looking at the architecture of all these controllers you will notice that they all implement a “client-server” model. What I mean by this is that the real time operations of the controllers occur in a processor in the controller itself. The PC is not involved with the time-critical functions. The Windows® Graphical User Interface (GUI) takes commands issued by the user, and packages them into instructions that the test processor understands, the test processor then modifies its behavior according to the new set of instructions. Information piped-up to the PC from the test processor is interpreted, manipulated, and displayed to the operator in a user-friendly manner. The Instrument based controllers offered by FCS-COM, MTS, and IST are all stand-alone versions of their PC applications. A more simple, dedicated user interface has been developed, which runs on a small screen on the front of the device itself, obviating the need for a PC in most applications. If, however, the user wants the power, flexibility and screen real-estate offered by the PC, they can connect one to any of these three controllers, and operate it remotely.
The new Pulsar software currently runs on the DSC 2000 hardware. It is based on Digital Signal Processor (DSP) architecture. All the real-time controller code is executed on the DSP. The dedicated PC acts as a front-end, and communicates operator interaction to the DSP. The single DSP resides on a board that is on the PC bus. This PCI card uses the TMS320C44-60 64 MHz chip. All software is downloaded to the DSP on boot-up. This means that software updates are possible without having to upgrade firmware.
The DSP board communicates with up to four conditioner boxes via a XBUS. Each box has 17 slots, which can be populated with the following (configuration limits depend on a complex combination):
· 2 channel conditioner
o Both DC and AC conditioning
o 16bit/20 kHz
· 1 channel valve driver
o Multiple two stage or
o Single three-stage (in standard configuration)
o Single channel high-level input
· 4 channel high level input
o 500 Hz 4 pole low pass anti-aliasing
o +/- 10 volts
· 6 channel high level output
o +/- 10 volts
· 16 channel digital I/O
o All channels are fully bi-directional
One box typically handles a maximum of 6 servo-control channels. With four boxes, a single PC can handle up to 24 channels, which can be split among up to 16 test rigs. A/D and D/A conversion is 16 bit at a maximum aggregate loop closure rate of 8192 Hz (divide by the number of channels to get maximum rate per channel). Loop closure typically occurs at 1024 or 2048 Hz, although it is possible to run as fast as 4096 Hz and as slow as 102.4 Hz. Maximum rates are difficult to define because of the infinite number of configuration options.
Pump and manifold control is performed using the Digital I/O module. E-Stops are hardwired and do not go through the software. Up to 8 test stations can be supported.
Servotest is in the process of developing a servohydraulic controller based on a new hardware architecture. The newer Pulsar hardware solution, due to be released by the Summer of 2004, is based upon an external DSP (Texas Instruments TMS320C6711-200) which links to any PC using FireWire (IEEE1394a). The synchronized Pulsar I/O is comprised of up to 24 nodes distributed using Optostar fiber optic technology. Each node, which may be configured for actuators or transducers handles up to 6 channels of servo-control, which may be configured for AC/DC transducers, 2/3 stage valve drivers, high level analog I/O etc. Alternatively, the node can be configured for up to 12 transducers. Another node is used for pump control. The new system will also feature intelligent transducers, with built in identification/calibration. Click here for a diagram.
This is the controller that was previously offered by COM Inc. It is still available. The system is based on the VME architecture, although they have developed, and delivered a new PCI-based system. These boxes are connected to the Ethernet, and any PC on the network can communicate and send commands to them. Boxes can contain multiple chassis, and come in many sizes. They can also be subdivided to multiple rigs (as many as there are channels). Each box includes two processors, which run all the real-time code, and a networked disk. The network is therefore not used for any time-critical activity. Each group of 6 servo channels is controlled by a DSP board. A single chassis can handle a maximum of 12 servo channels, or 32 analog control channels (or any combination). Up to 15 additional chassis can be added to expand the system with up to 16 Channels per expansion chassis. A DSP board is comprised of the following:
· 2 channel conditioner
o A/C or D/C based on daughter board options
o Maximum of 3 per DSP
· 1 channel valve driver
o 3 stage valves require one conditioner from above
o Maximum of 6 per DSP
· 32 high level analog or digital I/O
The standard raw sample rate is 2048 samples per second (10240 samples per second for the PCI based system), and anti-aliasing filters are typically set at 80 Hz (switch selectable). Loop closure is between 2048 Hz. and 8K Hz. depending upon the number of channels. Data is decimated to the required sample rate (no digital filtering/down sampling).
The SmarTEST ONE controller is a member of a family of SmartTEST controllers. The SmarTEST hardware architecture allows extension up to 256 simultaneous control channels in one test
The architecture is based on a controller board, called the SCU, which takes-up two slots in a chassis. The SCU standard configuration is as follows:
· Two DC (loadcell) conditioned inputs
· One AC (LDVT) conditioned input
· One high level analog input
· Two high level analog outputs
· One servovalve output (current or voltage)
An optional daughter board provides LVDT conditioning for a 3-stage valve driver, position encoder and acceleration transducer (ICP). The SCU connection panel comes in many different configurations. FCS provides connector panels to match any custom requirement, including direct connection with existing cabling (MTS, Schenck, others). A fault management system switches in backup A/Ds in the event of a failure. Interlocks are daisy-chained between boards.
The multi-station SmarTEST ONE chassis includes the following:
· Intel Processor
· 20GB hard drive
· Ethernet or Fiber Optic networking
· Ten slots available to support up to 4 servo channels and a additional I/O cards
· Control of up to 12 channels
An expansion chassis can be added that adds a further 16 slots (8 channels). For multiple hydraulic stations a hydraulic switch unit can be added for up to 4 manifolds. This will interface with DC, AC or Proportional solenoids. Since the controller board takes up two slots, the SmartTEST ONE typically has four control channels, and two spare slots for other boards that include:
· Digital I/O boards (24v opto-isolated)
o 8 in 8 out
o 16 out
o 16 in
· Analog I/O boards (+/-10v 16bit differential)
o 8 in 8 out
The maximum channel count for a single test with the SmartTEST ONE architecture is 12. This is the maximum number of channels that can be driven from the single small screen on the front of the chassis. If more channels are required, FCS uses the larger SmartTEST architecture, which allows multiple boxes to be slaved together through the fiber optic or Ethernet networks.
A standard PS2 keyboard can be plugged into the front of the device, to simplify operation. A common configuration for the SmarTEST ONE is to mount it into a rack with a keyboard and monitor. The controller can also be operated from a networked PC, where a single window provides the same user interface at the front panel
In the standard configuration, the adjustable loop closure rate is typically 2500Hz for multiple channels, and 10kHz for a single channel.
The fundamental building block of the architecture used in the Instron controller is the Integrated Axis Controller (IAC). This one board contains two DSPs (one for control, one for signal conditioning), and all the resources required for a single channel of control. It has four slots for combinations of the following:
· 1 channel Sensor Conditioning/Control Module (SCM)
o Patented over-sampling system (see Sampling and Filtering below)
o A/C D/C Encoder, or high level input
o Valve driver
o Maximum of 4 per IAC
o RS232 for Remote Man-Machine-Interface (MMI)
· 1 channel Sensor Conditioning Module (SCM)
o Patented over-sampling system (see Sampling and Filtering below)
o A/C D/C Encoder, or high level input
o Maximum of 4 per IAC
o RS232 for Remote Man-Machine-Interface (MMI)
The IAC also includes the following:
· 4 digital inputs
· 4 digital outputs
· 1 high level input
· 4 high level outputs
· Transducer linearization
IAC boards may be configured for control, or data acquisition only (using the different SCMs above). These IAC boards are installed into different chassis configurations depending on the application. The 8400 is an instrument-based single channel controller with one IAC, the 8800 comes in the form of a tower, or rack mount. In its tower configuration, it takes up to six boards. Multiple towers can be slaved together, and slaved with 8400s, or up to 30 IAC boards can be installed in a rack. The chassis also contains four digital inputs, four digital outputs, four analog outputs and one analog input (+/-10v). Channels can be assigned to up to eight Test Groups (independent test rigs). A tower can handle a maximum of four Test Groups. While it is theoretically possible to have more than one axis of control per IAC, it is important to note that the entire board runs at one sample rate.
Communication with the host PC is via the IEEE488 parallel interface (also known as a GPIB). It’s the same communication used for the standard printer port on a PC. A single PC is therefore dedicated to the control system.
An optional remote MMI can be used for each axis of control through an RS232 interface to the IAC. This provides remote access to common servo-loop functions at the test site. Also, a jog handset is available for the 8400.
Loop closure is at 5 kHz.
While this review focuses on FlexTest GT and SE, I think it fair to first talk about all the PC-based controllers in the MTS lineup. MTS controllers are split into two hardware platforms, and two software platforms. Unfortunately, the division between these platforms is not along the same lines. One hardware platform I will call FlexTest II, the other I will call Model 493. The software split is a little easier to explain, since there is only one controller that stands alone: FlexTest IIs. So, in a nutshell, here is the breakdown:
· The FlexTest IIs controller is built for specific applications. For example, it controls MAST tables, crash sleds etc. It is not intended to be a user configurable system. It allows MTS engineers to design front-end interfaces that are application specific.
· The FlexTest IIm, is a platform for large channel-count systems, its capabilities grow as processing horsepower increases. It is currently sold with a maximum of 16 control channels and four test stations, although MTS can configure it with 24+ channels for applications such as 6 degree of freedom road simulators.
· The TestStar IIs is a single channel PC based system primarily aimed at the material testing market
· The TestStar IIm is essentially the same as the FlexTest GT, but for the material testing market
· MTS offers two products, the FlexTest II CTM and FlexTest GT Supervisor that provide control for existing analog systems, the later being upgradeable to full Digital Control. This is a path for labs that are currently fully analog, and want to gradually transition to digital control
· This review will focus on the FlexTest GT and SE controllers, based on the TestStar II (model 493) hardware platform. The FlexTest SE is a single channel version of the FlexTest GT. It is instrument-based (no PC required) and can be combined with others for multi-channel tests
The TestStar II architecture is based on a VME chassis. In the case of the FlexTest GT, the chassis is mounted into a box that contains: the processor, interlock board, and 32 slots available for the following:
· Digital Universal Conditioners (DUC)
o A/C or D/C
o 24 bit 100 kHz sampling
· Valve driver
o 2 stage standard (3 stage optional)
o Maximum of 8
· 16 channel digital I/O
o 16 channels of input and 16 channels of output
o Breakout box with simple connection interface
o One only (as an option)
· 6 channel high level input
o 16 bit resolution
o Maximum of 3 (as an option)
o 8 station GT only has room for one
· 6 channel high level output
o One only (as an option)
· Remote Station Control Interface board (see below)
o Up to four stations (as an option)
In the case of the FlexTest SE, the box that contains: the processor, interlock board, user interface, and the following:
· Up to three DUCs (2 standard)
o A/C or D/C
o 24 bit 100 kHz sampling
· Up to two valve drivers
o 2 stage standard (3 stage optional)
· 1 channel high level input (more optional)
o 16 bit resolution
· 3 channel high level output
· 4in 4out digital I/O
o One I/O pair for hydraulic interlock
o One I/O pair for run/stop
o Two I/O pairs for user definition
The boxes are connected to a PC via a dedicated Ethernet (SE's do not require the PC). If the PC also resides on a corporate network, it needs a second Network Interface Card (NIC). The maximum channel count for a single GT box is 8, assignable to up to eight test stations (test rigs). It is possible to have multiple PCs connected to a single GT box, allowing a “PC per Station.” The SE is a single station device. Multiple SEs can be connected together for a higher channel tests, and SE boxes can be combined with GT boxes in single or multiple stations.
A Remote Station Control (RSC) device allows an operator to operate common control commands through a MMI close to the test system. One RSC is used for each station, and connects to the GT chassis via a RS 422 connection. You cannot use the RSC with 6 or 8 station boxes, or with the SE.
Loop closure rate is up to 6 kHz.
The Tiab controller is a dedicated single station box that communicates with a dedicated PC though a USB. The box contains a DSP board and currently provides 4 channels of control and 8 analog inputs. All signal conditioning is performed using off the shelf signal conditioners. The valve drivers are either current or voltage. 48 assignable discrete opto-isolated I/O channels are also provided.
The box includes a safety interlock that gracefully shuts the system down when the USB is unplugged.
Loop closure rate is at 5 kHz.
The DCS 2000 system includes a PC, and boxes that contain the X-Bus. These boxes (up to four) are 150mm H x440mm W x480mm D. The interface for the actuator is via a snake that has robust military connectors at both ends. The snake terminates in a breakout box for the individual connections to the actuator. The benefit of this approach is that there is only one cable to run for each actuator, and (presumably) more control over noise etc. A similar approach is used for the hydraulic control, where a remote box provides for pump and solenoid control, along with an emergency stop mushroom. The system comes packaged with an Uninterruptible Power Supply (UPS). 19-inch rack mountable options are available.
The SIMCON 2100 is built into a 19-inch rack, and integrated into a workstation, with the hydraulic control interface and connections. The connectors are MTS compatible, so that customers can effortlessly switch between controllers. There is also a smaller desktop version.
The SmarTEST ONE is built into a dedicated box with carry handle, which may stand alone, or rack mounted 177mm H x450mm W x280mm D x9.2Kg. It includes its own color screen and dedicated user interface on the front panel. The VGA (640x480) screen is clear and bright, but small. However, as a standard feature, a VGA connection is available to connect a bigger screen. Field cable connections may be MTS, Schenck, IST compatible, or user defined.
The Labtronic 8800 comes in a tower configuration or rack mount. The tower dimensions are 650mm H x280mm W x570mm D. Each IAC has two SCM connectors on the front, via a pull-down hatch, and two at the rear. Hydraulic control is incorporated into the top of the tower through an angled panel, and the user has the ability to label each of the hydraulic buttons with a removable mask. Hydraulics can also be controlled from the PC. Connectors are a combination of Limo BNC, and “D” connections.
The 8400 is built into a dedicated box with carry handle, which may stand alone, or rack mounted. It includes its own monochrome screen and dedicated user interface on the front panel. Connections are the same as the 8800.
The FlexTest GT looks futuristic. The box looks a little like a PC on steroids: 457mm H x127mm W x508mm D. Connections are all D type. Except for high-level I/O, which are BNC (as far as I could remember, this is true for all controllers). A rack mountable chassis is also available.
The SE is built into a very light dedicated box with carry handle, which may stand alone, or rack mounted 130mm H x430mm W x430mm D x8.6Kg. It includes its own color screen and dedicated user interface on the front panel. Connections are the standard MTS D type.
The Tiab offers a range of enclosures. From a purpose designed bench top box that has optional ears for 19inch rack mounting to systems in off-the-shelf packaging. Tuchel connectors are offered as standard. Canon, BNC, Lemo or other customer requirements can readily be met.
The bandwidth and accuracy of a control system depends on the type of sampling and anti-aliasing used. Anti-aliasing is necessary whenever the acquired data is used for frequency domain analysis of any kind (all real-time simulation packages fall into this category). There are various techniques employed by the manufacturers.
The first method is the traditional method of analog filtering. High quality analog filters are used at the front end before the data is sampled. These filters must have excellent gain and phase relationships across channels in the pass-band to ensure the data is not time shifted unequally. The roll-off of the filters must be sharp enough to maximize the ratio between sample rate and bandwidth.
The SIMCON 2100 controller uses this technique. Filters are adjustable through hardware jumpers. They are 5-pole Bessel filters. Note that these are optional; be sure to specify them if you plan to do anything in the frequency domain. The filters are typically set at 80 Hz, which covers the bandwidth most folks are interested in. The data is sampled at 2048 samples per second, and decimated to the rate specified by the operator. Note that the decimation must not drop below twice the frequency at which the filter roll off drops below the noise floor.
The Tiab system can sample data to disk at 21Khz. The analog filtering is set to minimize phase delay. The analog filters can be readily adjusted. Filters are 2-pole Butterworth, with a nominal cutoff frequency (-3db) set at 2.5kHz.
The Pulsar system uses dynamic Butterworth low pass filters with software selectable poles.
Digital Filtering and Down-Sampling
Another technique is to sample at a very high rate, with less expensive “guard” filters. These filters have a gentle roll off, and hence do not have the strict gain/phase requirements of those in the Analog Filtering section. If the sample rate is high enough, these inexpensive filters will be flat in the bandwidth of interest, and the aliasing that occurs does not get reflected into the bandwidth of interest. The data is then digitally filtered and down-sampled, with ideal “brick-wall” filters. This is the technique, employed by most, maximizes the ratio between sample rate and bandwidth, making the most of your storage resources.
Other Proprietary Techniques
The folks at Instron have developed their own patented technique that maximizes transducer accuracy. By sampling at very high rates (40 kHz), they are able to quantize the noise of the transducer to the point where its noise floor exceeds the noise of the quantization process itself. This allows them to extract the most resolution that is physically possible from any given transducer. At the same time, they control the excitation of the transducer to get the most gain they can, while getting down to the thermionic noise level. The user can fix, or set limits on the excitation voltage. Through their patented algorithm, they are able to acquire anti-aliased data at a bandwidth of 1 kHz with 19 bits of resolution. This technique obviates the need for multiple transducer ranges.
MTS accomplishes this by building DUCs that sample at 100 kHz (49.15 kHz to be exact) at 24 bits. This also allows them to work without multiple transducer ranges, but this does not apply to the analog inputs, which are still 16bit. These might be required for simulation (if you are using external conditioners).
The Labtronic 8800/8400, SmarTEST ONE and FlexTest GT/SE products support the “lump in the wire” calibration system, with their own proprietary systems. This is a set of electronics in the wire that identify the transducer to the system, and includes the shunt calibration resistor. With the “lump in the wire” feature, the controller automatically detects a transducer and its calibration parameters as soon as it is plugged-in. Very simple from the operator standpoint, and it minimizes opportunity for error. The MTS and SmarTEST ONE systems work with or without these transducers. The 8x00 requires that transducer cables be modified with the appropriate electronics to be recognized by the controller (they call it being “Instronized”). The Pulsar, Tiab, SmarTEST ONE, Labtronic, and MTS controllers include dynamic linearization of non-linear transducers. The SIMCON 2100 uses a straight-line calibration for command generation windows, but the controller operation is all performed in volts (uncalibrated).
This section covers the Graphical User Interface (GUI) presented to the user. All the systems covered in this review run on a Windows XP Platform, and make use of the standard windows interface, and three of them have their own instrument screens, which can be used without a PC.
One of the more challenging GUI tasks is to handle the operation of multiple test rigs asynchronously. Complete asynchronous operation means that the test rigs all run at their own rate, and can be started and stopped independently. Handling the user interface for multiple stations presents a challenge. Every station requires its own instance of a complex set of windows, and keeping track of which windows relate to which station is very important. Make note of how each manufacturer implements their multi-station capabilities. Note that the FlexTest SE, 8400, and the Tiab controller are all single station devices. If you want more stations, you buy more boxes. The SmarTEST ONE is the only multi-station instrument-based controller with up to four stations (defined by grouping the channels on the function generation/test programming side). The MTS FlexTest GT and IST 8800 are the only 8 station devices (SE and 8400 controllers can also be combined into 8 stations using a PC).
By using the client-server model discussed in the architecture, all the manufacturers provide the ability to write applications that layer on top of the controller. In some cases users can write their own applications using Virtual Instruments (VIs), or Visual Basic®. But in most cases, the manufacturers write the specific application software themselves, and sell them as options. All controllers have a system configuration application, a controller operation application (tuning, set point, spans etc.), and a general purpose, or generic application, that is used for flexible testing of a variety of components, and is probably used 80-90% of the time. This generic application provides the ability to perform block programming, time history playback, etc. This review will focus on the generic applications.
A launch pad is used to start the application.
Advanced users can configure the system using an explorer-style tree view. This allows the user to define resources connected to the DSP, perform calibration functions and define units, identify all analog and digital signals used by the other objects.
Once the station is set-up, the user launches the main window, which provides access to all the functions loaded with the package. This main window, instanced for each station, contains windows for signal generators, actuator control (PID ramp/park position, FGs, spans etc.), park and ramp, meters, scopes and so on. Windows are launched by right-clicking the "desktop" and selecting from a menu, or by pulling down the Edit menu at the top of the screen. By focusing a window (clicking on it) a second window provides setup parameters. This window can remain on the screen so that the user can quickly click around the windows to see their parameters.
Pump/manifold controls program run/stop and enable/disable limits buttons are in a fixed window in the top left of the screen, and limits are permanently displayed in a reserved area at the bottom of the window.
The main window uses user-defined wallpaper as the backdrop. This wallpaper can be a schematic of the system, and the user can distribute the windows (meters etc.) around so that they correspond to the actuators shown in the background. This provides a quick visual view of the system.
A feature of this paradigm is that the user can define multiple views of a single system, by defining different windows. One station takes one single instance on the Windows Taskbar, so all the windows associated with a station can be minimized using the standard Windows function. If a limit trips, the window will pop-up automatically.
Pulsar is written in .net (Microsoft) and uses an XML database. Both are current standard environments which will be supported well into the future.
First, the user adds a box, called a QNX Host, to the user interface (QNX is the name of the operating system used in the hardware). This assigns a drive letter to the box, allowing the user Windows Explorer access to its disk, and presents the user with a list of resources that are available in the device. The user checks on the resources that are to be assigned to each station within the box.
The SIMCON 2100 was developed as a Test Site Controller first, then expanded to perform digital control. Its implementation is therefore unique. FCS-COM allows the user to differentiate between logical stations and physical stations. In other words, tests are designed and built first, then assigned to specific stations. All calibration is performed on the logical side of the fence, rather than the physical side. So all functions that occur on the physical side are performed in volts, so that when a test is loaded, it makes the connection between the volts at the physical layer, with the engineering unit assignment at the logical layer. One benefit of this implementation is that tests can easily be redirected to different stations, and stations can contain a mix of digital and analog control channels. The disadvantage is that whenever the user is operating the servocontroller, they are not working in engineering units.
All screens that are presented for a given station have a thick color-coded bar at the top, with a large number prominently displayed to give the station number the screen is referencing. This is a very clear indication of the station the user is working on.
Once the station has been defined, the user launches the Stations Main Screen, which provides access to all the applications that are available, from Servo Control to Test Monitoring. A window is instanced for each test station.
The user operates the system using the 150mm wide color screen on the box and the soft keys on the front, or via an external VGA monitor and/or keyboard. The screen is divided into four main sections. A set of indicators across the top provide a quick review of the test status, two panes tiled horizontally for various displays, and context-specific menu items across the bottom. The horizontal panes each have a set of optional windows that can be displayed, one graphic and one numeric in each half, and each can be split down the middle to allow a display of up to four windows simultaneously. The top pane displays a scope alone, or a scope and numeric table, the latter of which provides a real-time numeric display of system variables (aka DVM). The bottom half manages application specific displays.
The green number in the top left corner of the screen shows the current channel number being manipulated/viewed. The letter 'A' means all channels. The other indicators across the top of the screen are colored green, yellow, or red for alert severity. An additional removable message pane appears automatically when a yellow or red alert is presented
The soft buttons on the front of the device are intuitive, and are used to vary displays and parameters. A big rotary knob selects and adjusts parameters, and is pushed to enter. To the right is an alpha numeric keypad and play/pause/stop/rewind buttons. Soft keys beneath the display are used to select the various menu items. The number of menu levels is limited to three to keep the user from getting “lost” in the menu structure.
The SmarTEST ONE controller handles stations differently than the other controllers. A station is an entity within the command generation layer rather than at the channel definition level. When a user defines a test, they select the channels to be included in the test. Up to four independent tests may be setup and run.
When the user needs to run more complex block programs, or real time simulation, an external PC is hooked to the box via Ethernet. Tests are created on the PC and downloaded to the hard drive on the box. PC utilities also allow file management and upgrading.
The soft buttons on the front of the device are arranged into 4 areas. The display always shows which channel is in control, the status of channel limits, the status of events and the action of the function keys. The display also includes a screensaver, which dims the screen after ten minutes of inactivity. As soon as a button is pressed, the screen saver is cancelled.
Numeric keys are used to enter values for parameters at the various stages of setting up a test. The rotary knob is used for entering parameter values or to generate alphabetic symbols for labeling channels or user state names.
The four start, hold, stop and reset keys have an illuminated display to their right when they are activated. Below the keys is a writable area to enable you to add a specific label for the controller.
The four illuminating hydraulic keys, control the hydraulic pressure to the system and illuminate when activated. The small red stop key, below the Emergency Stop, can be programmed to stop the system in three different ways. In addition, if the system is being operated from a PC, it restores the control to the front panel.
All the operations of the 8400 can also be performed using software on a PC. This is identical to the software supplied with the 8800, and so I will not deal with it separately. For details on operating the 8400 from a PC, read the 8800 information.
The servo-controller functions of the 8800 are performed via an application called RS Console. Available resources are attached to test groups or stations using the Configuration Manager. This window provides an Explorer-style tree view of the hardware, and allows the user to assign the hardware to groups. This application provides a toolbar that sits on the side of the screen and gives an iconic view of the various stations. Via a code of icons and colors, the toolbar provides a quick overview of the test status. While these icons seem somewhat cryptic, I understand that once operators get used to them, they are easy to understand. The toolbar provides the platform to launch the various windows.
Tests themselves are defined in RS BasLab. This is a database driven application that allows the user to define projects, users, specimens, event actions, signals, schedules, etc. From there applications are launched that define the requisite profiles, for example, RS Block is the block programming application.
The Project Manager provides an Explorer-style tree view of all the projects on the system, with easy access to the various files and test definitions. Password protection is used to deny certain users access to critical setup areas of the software. These areas are user-definable.
Instron also provides VIs that allow you to build your own custom applications
The SE has a vast array of controls that have specific functions rather than a few buttons that change context. The main menu items are hard coded into one set of buttons that provide immediate access to the functions of the controller. Once in a menu system, soft menus appear on the right side of the screen that are navigated using the adjacent keys. The display shows the test log, meters, and the current status. If the optional scope is purchased, it sits in front of the test log. Store and recall buttons allow the user to save views as bookmarks, and return to them at any time. All the operations of the SE can also be performed using software on a PC. This is identical to the software supplied with FlexTest GT, and so I will not deal with it separately. For example, the Multi Purpose TestWare® (MPT) described below can also be used to setup block profiles for the SE box. For details on operating the SE from a PC, read the FlexTest GT information.
The FlexTest software is divided into three layers. Station configuration is performed by the Station Builder application. This drag and drop interface has available resources listed on the right, and station resources listed on the left. The user chooses which resources should be applied to which stations.
Once a station is defined, the user then launches the Station Manager. This is instanced for each station. All servo control functions are performed at this level. Once the servo loop is setup, the user can launch specific programming applications from this window. Multi Purpose TestWare® (MPT) is the application used to define block programs and time history replay. Functions for all the applications that are launched from the Station Manager are available from within the same window. Password protection is used to deny certain users access to critical setup areas of the software. The main window includes control for the function generator, hydraulics, and interlocks.
The Desktop Organizer puts a taskbar at the bottom of the screen for each test station. By clicking on one of a pair of arrows, all windows relating to a station can be expanded up onto the screen, or collapsed out of sight. This provides the user with all the available screen real estate for a single station at a time. the Indicator “LEDs” on the taskbar provide the user with a quick look at the state of the test.
MTS provides VIs and a Visual Basic API for you to build your own custom applications.
The Tiab software is written using National Instruments® LabVIEW®. All the VIs are therefore available to write custom applications. However, I will review the stardard controller software that is available "off the shelf" from Tiab.
Each Tiab system runs its own intranet site and can allow protected access to all its operation and data files from anywhere in the world with only standard web explorer software. Webcam and digital camera features link in with Report Publishing and automated Test Sheet Generation.
Once the software is launched, the user selects the relevant configuration file, which may be edited using a simple editor, and is then presented with a Main Control Panel This is the launch-pad for all the functions of the controller. The buttons invoke the following (from left to right, top to bottom):
· Signal Generators
· PID Controls
· Multi-Axis Model (for simulating a real system)
· Trigger Timer
· User (custom) VI
In the middle of the Main Control Panel is a set of buttons and indicators that, in this example, are used to switch hydraulics and limits. The user has the ability to define these buttons and indicators to switch and read any controller two-state variable. This is defined in the System Configuration File .
Servo-Control and Tuning
All the controllers in this review except the SmarTEST ONE use PID control, and include delta-P stabilization (usually an option). I will briefly describe the control system FCS uses in the SmarTEST ONE section.
The more tools in the controller toolbox you have available, the better chance you have of controlling that awkward axis, that is very non linear and/or has a bad resonance. While a lot of controllers have auto-tuning, I have not found an algorithm that works well. I usually tune loops manually. So ease of use is important here. Especially considering that during the tuning process, you need to be able to easily switch between channels, while looking at command/feedback signals on a scope, switch function generation, and have easy access to the sliders for each servo controller setting.
Access to the PID window is discussed in the User Interface section above. The properties of the actuator window includes a tab which allows the user to adjust the PID parameters. The servo controller has the standard Proportional, Integral, Differential, and Feed-Forward (PIDF) parameters with bump-less mode switching. The main window provides access to the function generator; the user launches a scope that has access to any internal signal (feedback, error, command, etc). A square-wave is selected and the process begins on the first channel. Once complete, if mode switching is required, the control mode of the channel is changed on the main screen, Setup>Tuning is selected again, and the scope channels are changed. Once the second mode is tuned, the user moves on to the next channel and the process is repeated. An unlimited number of meters for any system signal are available in a separate window with instantaneous, average, RMS, max/min readings.
Sevotest recommends that the user perform tuning using an "EZflow" process. EZflow will be discussed later, but suffice it to say that this allows the user to predefine processes, one of which is an auto-tuning function which they assure me works well. I was unable to test this under load control with a soft specimen, the condition which usually challenges auto-tuning algorithms. All the controllers reviewed do include auto-tuning, but I have deliberately avoided it because it has not proven to be a useful tool. However, the Servotest algorithm does seem different from the more ubiquitous approach of using swept sinusoids. Servotest uses a random signal, and tunes the system to achieve a user-defined desired frequency response curve that can be saved to disk. This could be useful where you need to reproduce the tuning on two different test systems, say a drive file was developed on one, and you need to replay it on another. It is also easy, using this technique, to perform test quality monitoring (detecting changes in servoloop response over time).
Access to the PID window is via a toolbar shortcut in the Servo Control Application that is launched via the toolbar on the main station window. Feedback is selected via a setup window (no mode transfer). The servo controller has the standard Proportional, Integral, Differential, and Feed-Forward (PIDF) parameters. Access to the function generator is in this same window , along with a graphical display that shows the command. To see the feedback, the scope is used. The scope has access to system responses. Current values and following error are digitally displayed on the Main Servo screen. Servo controller tuning parameters can be saved to a configuration file, and may be recalled later when a given test is loaded.
FCS uses a "Patented Force Loop Model Follower Controller." Don't let this mislead you, it can be used for either force, displacement, or acceleration control. They also have a more traditional PID displacement control loop in the controller. The "Force Loop Model Follower Controller" is not a PID(F) controller, it internally computes force, displacement, velocity and acceleration into the same domain. Force and displacement commands can be rapidly switched on the fly, or even applied together, providing an implicit seamless control mode transfer.
In each domain, they have the ability to perform certain actions to improve control stability. When in the Force Domain, a Breakout force can be defined (that limits the maximum force that can be applied in displacement control). When reached, the controller switches to force control. This is useful to protect the specimen, or personnel.
While in the velocity domain, the user has the ability to add damping. This is useful for controlling sharp resonances. It is similar to the "D" term in a standard PID loop.
Also, in the velocity domain, they provide the ability to control cross coupling. If you have one actuator that causes error in another due to fixture or specimen kinematics, you can feed its velocity directly in, providing the control channel the ability to react more quickly to the external disturbance.
When performing acceleration control, it is very easy to apply a displacement offset.
This control loop also has other features that are not available in the other products: feed forward gains are independently applied to force, acceleration, velocity and position; limits can be applied for maximum velocity, position and servovalve command, that clip the valve command; and the servovalve command can have an asymmetric positive and negative gain for non-linear systems.
To perform the tuning, the Setup item is selected from the main menu, followed by Controller (after selecting the relevant channel). The screen is automatically populated with a scope preset with the command and response, function generator, bar graphs and tuning parameters. The user selects the function generator in the top right of the screen, and enters the parameters for a square wave, and then varies values in the spreadsheet in the bottom right. The loop's performance can be monitored via the scope in the top left, and bar meters in the bottom right that show command, feedback and error (gain and phase). To switch channels, simply exit, select the next channel, and re-enter.
Of course, with this control scheme, there are a lot more parameters to tweak, which can be a blessing and a curse. Auto-tuning is available, but, as with all others, it does not do a good job.
To perform the tuning, the user sets the waveform to a square wave in the respective channel, and with the hydraulic pressure on high, presses the start button. From the home screen, the user presses the Setup and Loop soft keys. Soft keys are then available to adjust the parameters, and a scope, preset with command and response is on the screen. To switch channels, the user returns to the function generator screen, stops the program, and begins again.
When tuning the 8400 from the PC, the procedure is identical to the 8800 (below).
Access to the tuning tool is via a tab on the controller properties window, accessed from the tools menu on the taskbar. A single window provides access to all the items needed to tune a channel: tuning parameters, scope (preset to view command and response) and a function generator. Instron uses a PIDFL loop. The “L” stands for phase lag, which helps to control through difficult resonances. A graphical editor is used to configure the control loop, showing exactly how parameters will be applied, and allows the user to enter and graphically edit the parameters of a filter in the forward path. A system summary table shows all channels, their gains and parameters. This allows the user to compare gain settings etc. to ensure they are consistent between similar channels. Meters are available in a separate window with instantaneous, max/min, mean/amplitude and peak/valley readings. These meters can be sized from full screen all the way down to summary views on the taskbar.
To perform tuning on the device itself, the user selects the tuning button. This populates the screen with the scope, preloaded with command and feedback, and the softkeys provide access to the PIDF parameters for adjustment. Switching channels is easy. When tuning the FlexTest SE from the PC, the procedure is identical to the FlexTest GT (below).
Access to the tuning parameters is via the Station Setup window accessible from the main Station Manager window. Through an Explorer-style tree, the user can select the channel and control mode. An icon selects the tuning parameters. The servo controller has Proportional, Integral, Differential, Feed-Forward (PIDF) and Forward Loop Filter (bandwidth) parameters with bump-less mode switching. The function generator is on the main window, and the relevant channel must be selected there, along with the control mode. A scope is launched from the main window, and its parameters are selected to view command and feedbacks from the relevant channel. To use the mode switch, the user then switches the control parameters screen, switches the function generator control mode, and scope channels. Once the second mode is tuned, all the parameters are changed for the next channel. Up to 9 meters are available in a separate window with instantaneous, max/min, mean/amplitude and peak/valley readings. The meters can be sized up or down by dragging the corner of the window.
The PID settings panel is invoked from the main window. The user then opens a scope and a function generator. The scope is set up along with the function generator, oil is applied to the actuator, the function generator is run and the PID settings are adjusted to give the requisite response. To switch to another channel, the function generator, scope and PID settings window must be switched. Meters (called aliases) are readily available from the main control panel.
Tiab offers "Remote Tuning". This allows mixed-mode control. For example, the controller can follow a sinusoidal program of load when in position control. A rate adjustment allows for soft or stiff test specimens. The algorithm used ensures the adaptive tuning is halted if a valve limit is reached or there is any interruption of the sinusoidal drive signal. The same technique can be used to ensure an actuator in position control does achieve the full amplitudes requested.
One feature that sets this controller apart in this category, is the system modeler. It is possible to build a model of the system to be controlled, and use it to tune the system first. This is useful if the specimen is highly degradable or expensive, and cannot be used for tuning purposes.
struggle on your own...
PULSAR® is a registered trademark of Servotest, SmarTEST ONE® is a registered trademark of FCS-COM, FlexTest®, TestStar®, Remote Parameter Control®, RPC® and TestWare® are registered trademarks of MTS Systems Corporation, MATLAB® and SIMULINK® are registered trademarks of Mathworks Inc., National Instruments® and LabVIEW® are registered trademarks of National Instruments Corporation, Windows® and Visual Basic® are registered trademarks of Microsoft Corp., SOMAT® is a registered trademark of AEA Technology plc.