Building a Test System
A test engineer has to make sure that the products they are assigned to test are operating correctly now and in the future. Before any products can actually be tested, a test system platform must be designed and built that can implement the engineers vision for an effective test. This entry will focus on building a test system from scratch. This entry will consider the high level issues as well as some of the low level design details.
Building a test system is not the first task when a new product needs to be tested. Before you can start designing and building the system you must know what products are being tested and what measurements need to be made to test those products. In other word, first come up with a test strategy and then build your tester to implement that strategy. A test strategy is things like, what are the failure modes to check for and how much time and money should you spend on test.
Before you can buy or build anything you also have to make some decisions about what type of strategy you have for your test system itself. That is, you need to decide on the architecture of your system.
Here are some of the factors that go into deciding on tester architecture.
What area of application is this test system going to be used? If you have a test strategy in place you will know this but it can affect the type of test system hardware you use to implement your system.
1. R&D testing
If you are designing a test system for use while products are being developed, you need to make parametric measurements and have a flexible, informal setup that is fast to use. This may not be much more than an oscilloscope, but a more formal tester with some general software utilities may be called for.
2. Design validation and verification
This is a little more formal than the R&D test system in that while it should be flexible for multiple different measurements, it may have to be able to formally record the results of the testing to create a record that the design worked as conceived.
3. Manufacturing test
A manufacturing test system has to be fast and, hopefully, inexpensive. It has to be much more robust and automated. This is where most of the consideration of this entry will be.
What is the scope of products for this system to test? Again, this will be most likely answered by your test strategy but you will need to know if it will be expected to test only as single product, multiple products or multiple product lines.
What is the expected lifespan of this test system? Is it expected to be a general purpose test system that can test products which have not been designed yet? My though on this is it is better to try to make your systems simple and cheap, only testing a single or narrow line of products. I have personally seen several attempts to build universal, future proof systems/ITAs/fixtures only to have these projects grow out-of-control in time, budget and problems.
Trying to build the ultimate end-all or universal system seems to be a trap that engineers often fall into. This may be a case of engineers trying to invent interesting but unnecessary projects for themselves.
Some other factors to consider include expertise and hardware available, along with the development time and budget. If you have a lot of test developers with a lot of Labview experience, you may want to take advantage of that. Is there any existing hardware that your company already owns? Perhaps you have a number of older PXI systems with some hardware your people know well that is no longer going to be needed. Assuming this will not be an obsolescence problem that may be a time and cost savings to your project. Obviously, you have to build a test system that is appropriate to the budget you have available. Often, development time can trade off with cost. For example, you may be able to develop tests from more general-purpose instruments versus buying instruments (at a higher cost) designed for your exact test. Like an instrument that implements some special serial data protocol.
The general architecture of a test system is usually based around a main bus. There are a number of bus options to choose from. A rule of thumb is that you should try to select a bus that has instruments available to perform 80% (if not all) of the measurements you need to make. Also, how well do the busses work together, like sending triggers and clocks across the bus types. An embedded controller for a PXI chassis will have GPIB and USB ports, but you should be sure the instruments can be integrated well. All of the test bus options trade off strengths and weaknesses including the following.
The bandwidth is a measure of the rate data can be sent across a bus in MB/s. So, as the bandwidth goes up the bus is able to transmit more data in a given period of time. This is an important consideration if you are trying to gather some kind or real-time high-speed data and will have to log it as fast as it’s acquired.
Latency is the delay in the data transmission. This is different from the bandwidth where the bandwidth is more of the raw ability to transmit data; the latency is a more practical measure of how long it will take to transmit. Ethernet for example has pretty good bandwidth but poor latency.
3. Message based versus register based communication
This is an instrument distinction you will often see that traces back to the bus the instrument is based on. Message based may be a little slower due to having to interpret the commands. Register based will directly write to the instruments registers, for fast binary data transfer. It used to be that register based instruments were much more difficult to program than message based instruments, but that is not really visible to the programmer anymore as the programming is all done with high-level drivers that mask the complexity. However, the drivers may slow down the performance gain of the register based.
4. Range of data transmission
All the different bus types specify a maximum cable length for the instruments. LAN would be the best choice for long distances.
5. Instrument setup and software configuration
It may be relevant that some instruments are plug and play while others require a system reboot.
6. Ruggedness of the system and connector
How rugged is the connector for a particular bus? For example, USB is not a very rugged connector and can be easily un-plugged.
There are several busses to choose from and I’ve probably missed several. Here are some popular ones
1. PXI/VXI/PCI (Card Cage)
PCI card based instruments can be placed directly into a regular PC to put together a very simple test system. A PXI system and its newer variations are based on the PCI bus but go into a chassis to make a more rugged and dedicated test setup. The PXI chassis can then run off of an embedded or external computer and can work along side other standalone instruments.
VXI is kind of old news at this point but they are pretty much the same kind of system as PXI. I have worked with both but I don’t think VXI was ever that popular. PXI, however, is an extremely popular choice and has heavy backing from National Instruments, so it’s fairly easy to get these systems going with Labview.
2. GPIB (Rack and Stack with standalone instruments)
GPIB is usually used to connect stand-alone instruments. These types of instruments are different from card cage types in that they have a user interface on the front of the instrument for manual control or they can be programmatically controlled via the GPIB and PC. Because of the interface options, instruments like this may be a better choice if you want the option to do more manual, bench-top testing. They are also often mounted in test racks along with a computer to create an automated system.
This is another option for test systems where you are not actually building the test system you just buy it and configure it to do what you need. Integrated circuit test systems often come like this. There are a lot of specific purpose testers, like manufacturing defect analyzers, flying probes testers, shorts and opens testers and in-circuit testers. Of course, you can perform all of these types of tests on systems you build yourself too. I have worked with test systems form Eagle Test Systems (now part of Teradyne) that are specifically for testing analog, mixed signal components and boards. While you still select the instruments you want they all come from the same manufacturer. They also develop their own programming environment. The advantage is you start with a nicely integrated test system, which is easier for some tasks (like automation with a handler) the disadvantage is it is more expensive.
For remote monitoring applications Ethernet based instruments are available. A newer standard based on Ethernet is LXI. LXI adds triggering and synchronization to these instruments.
There is also instrumentation that can connect to a PC via USB that connect via plug and play. However, they are short range and do not a rugged connector.
Test System Structure
All test systems have a general structure with the following components
2. System Control and Software
7. Mass Interconnect/ITA
Each area here merits some discussion and consideration. The remainder of this entry will cover the each of these areas.
The instruments are the heart of a test system, and selecting your instruments is the most critical part of building the system. Selection of instruments is based off of knowing what measurements you are going to make. One approach may be as follows.
Steps for selecting instruments.
1. List the input and output parameters you need to measure on your DUT.
This could involve listing all the pins or test pads or whatever the access method is and then determining what needs to be done with them to get the measurements you need.
So, do you need to measure voltage at a pin and report that? Do you measure the input and the output at two pins and use the values to calculate the gain?
2. List the accuracy and resolution needed for each measurement.
Actually, you only need to look at the worst-case accuracies. For example, if you know that one of the measurements is really low current, you don’t really need to worry about all the other higher current measurements. As for resolution, this might show up if you need to accurately digitize some high-speed signal.
In an application where you want to digitize some high-speed pulses for a certain length of time, a digitizer typically will trade off memory depth (number of samples) with acquisition speed. If you want to digitize really fast for a long time, that may take a more capable instrument.
As a rule of thumb, the instrument accuracy should be at least 4x the desired accuracy of the measurement.
3. Check for redundant testing and instruments.
Now you should be getting a better picture the instruments you will need in order to make all the measurements you need, but you still have not purchased anything yet. This is a good time to step back and see if you are duplicating any testing or duplicating any measurement capability. So, don’t just buy a DMM to do a measurement because that is what you would normally use to measure voltage. Maybe a DAQ that you are going to need for some other reason will have sufficient accuracy to measure that voltage, or maybe a power supply has the capability to measure voltage itself.
4. Will there be any need to do things in parallel? How many simultaneous DC signals will you need? This can affect the number of DAC channels you may need. How many waveforms will you need to apply simultaneously? If this is greater that maybe four, go with a card cage, otherwise a standalone rack instrument might be all you need.
5. Will you need any specialized instruments to implement specific test standards like a boundary scan controller?
6. Other instrument specs to consider – Noise, power, offset compensation, dynamic range, isolation.
Some common instruments:
1. DMM – highest accuracy voltage and current measurements, slower speed
2. Arb, Digitizer, DAQ – Analog and digital I/O, able to generate patterns
3. Counter – sometimes part of a DAQ, counting and timing pulses, measure frequency.
4. SMU – Forcing and measuring analog voltages and currents. Goes beyond a DAQ for specialized applications like really low currents.
System Control and Software
In reality the first decision you may make in your head is what programming language you want this system to be. National Instruments Labview is the most popular choice at the moment. Some test engineers don’t like Labview but programming languages is one of those things that people really like to argue about. I feel Labview is the easiest to get a lot done fast, but it can also be easy to get yourself in trouble if you don’t know a few basic things about Labview. I plan to write another entry all about Labview in the near future. There are other choices like NI’s LabWindows/CVI and Microsoft Visual Studio .NET languages. NI clearly cares about and wants you to use Labview and these others are less common.
If you are building a test system that will be used in a manufacturing environment you will need to have a user interface and a test executive. Well, you will need these things to have a serious test operation, I’ve seen places just getting started that didn’t have much of either. The test executive is what controls the test flow, handles sending your test results to a database and generates test reports. NI has a product that does all this for Labview called Teststand. NI also has templates for manufacturing test user interfaces that work with Teststand. Engineers love to come up with reasons why they should write their own test executive and user interface, but I would start with what NI provides and only go off on your own when you reach a serious road-block.
Switching can somewhat be seen as just another instrument but it really calls for a strategy all of its own. There are the following choices when it comes to switching.
1. No switching
2. Switching in the test system only
3. In the DUT interface hardware (ITA/Fixture) only
4. In both the test system and the interface hardware
1. No switching
This is pretty rare, you almost always want the flexibility of some sort of switching but the switches add a little bit of resistance in the measurement path. For very sensitive measurements it may be required to connect the instruments directly to the DUT. This is also faster because you don’t need the software to operate the switches and wait for them to settle. The downsides are that it’s not flexible and not expandable. It can also be expensive because each instrument I/O point is completely dedicated to a signal DUT test point.
2. Switching in the test system only
It’s easy to add switching to a test system by putting the switching in the test system itself. There are lots of PXI matrix cards and other types of switching modules available. This makes it easier to develop your tests because all the switching is controlled with software drivers that the manufacturer provides. If you leave open space in your card cage this is also an easy way to add more switching in the future.
A downside can be that it can create some long signal paths. If you take the approach that all of the instrument outputs go out to a mass interconnect, to the DUT hardware, then back through the interconnect, through the matrix and back to the mass interconnect to finally arrive at the DUT, that can be a lot of wiring. This path is shown with the arrows in Figure 1.
Some of the problem can be avoided by using parallel sense lines wired all the way to the DUT, but wiring length should be considered. Another problem can be that you are leaving long wires attached to your DUT that can act as an antenna for noise when that line is switched out.
Figure 1. A long signal path through a mass interconnect and a matrix card.
3. In the DUT interface hardware (ITA/Fixture) only
If all the switching goes in the DUT hardware, this should prevent a lot of the signal length problems described above because with the switches close to the DUT you don’t have to loop back to the system. This also solves the problem of have long wires on you DUT when switched out.
The downside here is you need to design some sort of hardware to control your switches with software. There are relay driver components that can go on a PCB to provide the logical control of the relays, but you still have to figure out how to implement those. Another problem here is if all of your switching is going into a test fixture, in order to have any sort of reliability in your system, you’ll need to design a PCB.
You have to have the tools and capability to design a PCB, designing a PCB is expensive and it’s not easy to change if there are mistakes or easy to expand in the future.
4. In both the test system and the interface hardware
This is the most flexible option but also the most expensive. It’s flexible because, as was discussed, sensitive measurements are switched in the fixture and when accuracy is less of a concern the tester matrix can be used.
There are a few different switching topologies available.
1. Simple Relays or FETs
As with all things, try to keep the switching as simple as possible.
Power and load relays are often separate from the instrument signal routing relays.
Designing and assembling the test rack is mostly just a list of things to remember to help ensure success. Even if you are just putting together a card-cage with a fixture, monitor, keyboard and mouse, these are things to consider.
There isn’t much to the physical rack itself, you can buy them from many vendors and the instruments are a standard size to fit them. There are lots of rack mount PCs available also.
Consider the following.
- Use the cables recommended by the instrument manufacturers
- Secure all cables with strain relief
- Make sure all cables are long enough
- Use cable sleeves to keep things organized
- Label the cables with what they are used for
- Leave some room to expand within the rack
- How much floor space with the tester take and how much do you have
- Leave space so everything fits, including the instruments and other hardware but also cables, power sources and PC.
- Put wheels on the system
- Make sure it fits through the door and it can be transported. Also, make sure it will fit in the space where it will be used. I have seen testers that were too tall to fit in the room where they were supposed to be used.
- Interlocks – all the parts of the system should have interlocks where the tester will not operate unless a switch is closed that (ideally) is connected to a safety shield
- EMO – Emergency shut-off switch somewhere easy to find.
- Heavy stuff low – put heavy power supplies and UPS low in the rack so it will not tip over easily
- Balance fixture shelf – if there will be a heavy fixture shelf where the DUT is connected, be sure this is not making the rack unstable.
- High voltage points enclosed and disabled with interlocks
5. Environment of use
- Ruggedized for its environment
- What temperature and humidity is the room where the test system will be?
- Shock and vibration
- Pollution level – will the tester be working in a dirty environment?
- Add some lights if necessary, particularly in the back side of the rack for looking inside
- Work Space – What type of work surface will this tester need in a manufacturing environment?
- Position for operators – will the operator stand or sit? Is the tester comfortable to use. How tall are the people using the tester?
- Input devices – where do you put monitors, keyboards and mice?
- Add fans for air flow, maybe do some initial testing to see how hot the tester gets inside while running to see how many and how effective the fans are.
- Instruments have their own individual fans, think about where these are blowing and if it is consistent with the overall cooling
- Instruments often have specifications for ventilation that should be followed.
8. Signal Integrity and Grounding
- Prevent long signal paths by designing DUT, switching and instruments as close together as possible. Add remote sensing lines for four-wire Kelvin measurements.
- Terminate all grounds the same place, at the power distribution unit.
- Setup a preventative maintenance cycle for cleaning fan filters and performing instrument calibrations
- Replace worn parts
- Provide some documentation or training for how to maintain the system
There are two types of power to consider in a test system, system power and DUT power supplies. The systems power unit will provide AC power to the computer and all the instruments with some centralized controls. There might also be DC power supplies in the system specifically for powering fixtures and DUT hardware, it depends how you want to design it. The DUT power supplies are used to power your DUT itself (like 5V, +-15V rails, etc.), these are really very similar to the other instruments.
Here are some power considerations
1. System Power
- Does the system need power for different regions? 120VAC and 240VAC
- Have an EMO button for emergency power off
- Have a UPS in the system so if the power goes out it can be shut down properly
- Consider the load that your system is. The total draw with all the instruments should be about 70% of the total available.
- Determine a sequential power up method. If all the instruments are connected to one switch that inrush current could overload fuses or breakers.
2. DUT Power – power supplies have lots of specifications that may or may not be relevant to what you are doing. Here are some of them, I won’t try to explain it all here.
- Settling time
- Output noise
- Fast programming of levels
- Remote sensing
- Built in voltage and current measurement – this is another alternative to adding expensive instruments
- Physical size of the unit – power supplies can get big and heavy
- Triggering options
- Programmable output impedance
- Output ranges, multiple outputs
- Over load protections (V and I compliance)
- Lead lengths
I have already mentioned fixtures several times and I plan to write about designing fixtures as a separate article, but I’ll try to write some basic consideration here.
The idea of a fixture is that you have to design some piece of hardware that will interface with the exact pin-out or test-points or whatever the method of your DUT. At one end are all the signal lines from the test system and the other end is the DUT interface. It makes the test system more reusable and also provides a place to put any necessary signal conditioning circuitry or switching.
A fixture might also contain hardware to facilitate testing the DUT, like loads. Loads could be fixed value and location or programmable via switching.
Some thought must also go into designing fixtures for operators to use in manufacturing. Are they rugged and ergonomic so that operators can easily place the DUT in them over and over without error or anything breaking? Are they easy to duplicate and deploy?
ITA and Mass Interconnect
The mass interconnect is a connector block where all of the instruments are wired out and made accessible. The mass interconnect consists of a receiver hardware piece attached to the test system that is populated with receiver modules that wire to the instruments.
The ITA (interchangeable test adaptor) is the other side of the connector that interfaces the test system to the fixture. The ITA and fixture can sort of be a blurry line, which is which. The ITA may be just an interconnect, or it may contain switching and signal conditioning if it is desired to keep the fixtures very simple.
The big advantage of the mass interconnect is it makes your test system reusable and general so different products can be quickly swapped and tested on the same tester.
Figure 2 shows the block diagram of test system to DUT.
Figure 2. Test System to DUT connections
Building a test system can be a pretty big job with a lot of factors to consider. Having a good test strategy and knowing what needs to be accomplished is key to building a successful test system.
The main points are
- Strategy – what kind of testing is being performed? R&D, Deisgn V&V or Manufacturing. What technology will the tester use? PXI-Card Cage or Rack based with standalone instruments.
- Instruments – What instruments will you need to make the measurements you need to make?
- System Control and Software – What programming language and support software will be used and is needed? What will be developed or purchased?
- Switching – Where is the best place to put switching to meet the needs of your testing?
- Rack – There are a number of items to remember to make a useful and useable rack.
- Power – Two kinds of power, system and DUT.
- Fixture – The fixture interfaces the test system with the DUT.
- ITA and Mass Interconnect – The mass interconnect makes the test system generic so that multiple fixtures and DUTs can be tested. The ITA is used with the mass interconnect and works with the fixture.
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