Building a testing machine. What kind of global rules - requirements? - Page 1
Forum section: electronics / general

author: Houseman
author_rank: Regular Contributor
timestamp: December 18, 2021, 04:54:53 pm
content: Hi forum, don't know if it's the right place, but.In the need of building a testing machine that tests electrically (12V low voltage) some manufactured products, do You know what kind of "general requirements" apart from the specs imposed by the customer and apart from the general safety test imposed by law do I have to care about when building It? Could You please point me to some books / references that helps to make light on such a new subject?Any help appreciated.Best regards

author: AndyC_772
author_rank: Super Contributor
timestamp: December 18, 2021, 05:34:17 pm
content: What is the context here?If you've been asked by a manufacturer to build a piece of test equipment, it's up to them to specify what testing they want doing, and what the pass / fail criteria should be.You can, of course, help them out by using your own expertise and experience to help define what that test specification should be. Ask relevant questions about the function of the product, the intended end use, published specs, costs, quantities, standards compliance, and so on. If you don't have the necessary experience to do this, then you need to be sure that the customer does, and that they can be sufficiently clear about what you're required to do.If you're concerned about ensuring the product meets legally required safety standards, you should first ask whoever designed it if they have an FMEA (failure modes & effects analysis) document. This should list all the potentially hazardous failure modes which the designer has identified, and between you and the customer, you should work out whiich (if any) of these may apply to a unit which you're required to test.It could well be that, because it's running from 12V, there's very little to consider from a safety point of view, since it cannot generate a hazardous voltage or apply one to any external connector. It is, however, usually a good idea to measure the supply current being drawn, since this can be a good indicator that something isn't working as it should; high current means something is getting warm, and lower current means something isn't powered up or enabled.The bottom line here is 'ask the customer'. If they can provide full details of what they need, then that's great - get the spec in writing, and follow it. If they're not able to provide enough information, and you yourself don't have the experience to work with them to fill in the details, then decline the job and walk away.

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 19, 2021, 02:08:56 pm
content: We always try to trace all measurements back to the connected test equipment (a DMM in most cases) and not having anything inside the testbox which needs traceabiity, which cannot be traced back to the DMM. (Shunts for high currents are often a problem). And we try to build in self-test options as far as possible. For example, include enough relays to be able to test all your relais contacts and so on.

author: mendip_discovery
author_rank: Super Contributor
timestamp: December 19, 2021, 06:55:09 pm
content: Build in the ability to verify it is within specification, if used a lot then plan on it being a basic test done each day/shift. For example, have a unit that will pass and a unit that fails. Record the tests/results. The reason is if the test rig fails then you will need to retest all stuff tested since the test rig was last verified.Find the customers specifications and ensure the equipment used is better than the item under test. Let us say you have a spec of +/- 0.01 V at 12 V using a meter with a specification of +/- 1% + 0.05 V isn't going to be good enough.Keep calibration/adjustment records for the units you will be using. Actually, check that the certificate you have is within specification, and record it. Log the readings and use that to calculate its long term stability, which helps you decide if 1, 2 or 5 years is enough if you need to bring it down to 6 months.I recommend getting an ISO 17025 calibration lab to do calibrations on the test gear to the 17025 requirements. If there are specific points you would like tested then ask the lab to include them, for example, if you do a lot at 12 V it isn't a great issue to ask for 11, 12, and 13 V to be tested. You can assume linearity but depending on the auditor it might help you out of a hole.If you send the meter off for calibration then expect it to be gone 7 days or so, plan to have redundancy. Also handy if it fails and you have another to go in there.Also, have a good read of ISO 9001, you need to be ensuring that you are visually looking to reduce risks.

author: daqq
author_rank: Super Contributor
timestamp: December 19, 2021, 09:08:11 pm
content: General advice for general question based on what annoyed me on other devices and some of the mistakes I've made:Make it easy to use to minimize the level of training and qualification needed to use the device so that production and testing does not need to employ PhDs.Make it easy and straightforward to calibrate and/or verify, without any disassembly and ideally without trimpots and similar. Include precise instructions for all calibration/verification procedures in manual.Make it robust, it might need to survive users who may not be the brightest crayons in the box and treat ESD and fragility warnings as a challenge.Take a moment to consider the user interface side of things. Make sure the main output (good or bad) is clearly and distinctly indicated. No one wants to scroll through a 5 page generated HTML page to find out the device is bad. Other indicators (beeping, different colored LEDs etc.) are also good. Input is also important - be sure to have a straightforward command interface, ideally just pressing one button will start the test.The output should be useful - good/bad is all well and good, but "Bad because of excessive leakage current on input 7" is better.

author: Houseman
author_rank: Regular Contributor
timestamp: December 28, 2021, 09:15:33 am
content: Quote from: AndyC_772 on December 18, 2021, 05:34:17 pmThanks AndyC_772, your reply did really made me think a lot more clearly about the specs I have to ask.The first answer from them will be: We want a machine that cost less. So maybe it's also a trade-off between automation task that lowering the budget has to be done by the operator (i.e. connect an external oscilloscope by hand to 20 outputs instead of implementing a MUX and having an automatic sampling procedure).Thanks again.

author: Houseman
author_rank: Regular Contributor
timestamp: December 28, 2021, 09:18:27 am
content: Quote from: e61_phil on December 19, 2021, 02:08:56 pmHi e61_phil, so also hardware parts need traceability? What do You intend with that? I thought traceability was needed only in software pars of the product.Regards

author: Houseman
author_rank: Regular Contributor
timestamp: December 28, 2021, 09:25:33 am
content: Quote from: mendip_discovery on December 19, 2021, 06:55:09 pmThank You so. Good point of ISO 17025 calibration lab,I didn't think about that being so inexperienced.I totally agree with You and am also afraid this will affect the total cost of the device that needs to be as low as possible.. It's a hard choice, I have to go through to make them understand how important it is the test is needed to prove the specs are not slippering away in the year of the testing.Thank You again

author: Houseman
author_rank: Regular Contributor
timestamp: December 28, 2021, 09:33:38 am
content: Quote from: daqq on December 19, 2021, 09:08:11 pmGreat practical advices, daqq, Thanks a lot!!One question to ask. Since the test to be performed often will be one year long, how can I guarantee the testing device will maintain the given specs in one/two years without stopping?Calibration of course... but what to calibrate if I use for example fixed linear regulators for the power supply (it's just an example) or a micro controller with an oscillator with enough ppm precision.Do I need to think about adding complexity to the basic parts in the schematic?Best regards

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2021, 11:56:51 am
content: 1 or 2 years is not that long. It is normal for DMMs to have a 1 or 2 years calibration cycle and most are still well within specs even after quite some time.Of cause it depends on the conditions. 1 year in space or outside in the dessert sun is different from 1 year in a nice and cosy office.If the instrument was OK for the 2 year will only be known afterwards. A good design can only guarantee a low chance for failure. It need the calibration step after the measurements to be sure the instrument has not drifted too much.There are parts available that are reasonable stable, though they may be not the cheapest ones.How much effort is needed depends on the degree of accuracy needed. A linear regulator just for the supply is usually OK for a long time within the usualy 5 or 10% tolerances for the supply. With more crictical demands there are ways with multiple reference and redundant part, possibly even a complete 2nd instrument / set of sensors and parallel recording. If the 2 halfs give different result one can still later decide which had part survived better.Usually the idea is to keep it simple and add complexity only if needed or it gives a big advantage.

author: mendip_discovery
author_rank: Super Contributor
timestamp: December 28, 2021, 01:51:08 pm
content: Quote from: Kleinstein on December 28, 2021, 11:56:51 amIt is commonly accepted that the meters should stay within specifications for years. The issue from a 9001 process is how you manage that risk. If you use that meter for 2 years and pass products for 2 years and at the end of it it turns out to be at 75% of the spec there is room to say that potentially you have passed products that are out of specification. You now have no idea at what point that unit started to give incorrect readings. Then you have to look at product recalls and if you are competing with a US firm you know they will crow about it for months so that is costing you plenty.When picking a meter there are various "rules" that people used to use such as a 4:1 ratio so the meter has to be 4 times better than the item you are measuring but that is considered old skool. What you need to do is calculate the test gears measurement capability and then use that to work out a decision rule. Seehttps://www.isobudgets.com/statements-of-conformity-and-decision-rules/. So your spec of +/-10 V and use a meter with spec/unc of 2 V you might have to get cautious if it gets to 8 or 9 V away from nominal as you can't be sure its in your specs or not. But getting a meter with 0.5 V means you don't have much of a worry until 9.5 V.

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 28, 2021, 06:06:57 pm
content: Quote from: Houseman on December 28, 2021, 09:18:27 amHi,with traceability I meant tracebility of measurements against standards like in a normal calibration.In our testers we have a lot of build in stuff, but we always try to trace the values to an external instrument. For example: We need to simulate several Pt100s to test the readbacks. For this task there are severeal simulation resistors build in, but we don't rely on the stability of the resistors. We measure (automatically) the value of these resistors with the connected DMM. Therefore, the tester itself never needs to be calibrated, but the external DMM. That also reduces downtime, because you can simply swap the DMM, send one for calibration and go on with a second one. For HV dividers, frequency counting and so on it is done in the same way. You only need to guarantee, that your tester hardware will not influence any reading. This is normally done by checking all the relay contacts against the DMM as well.Best regardsPhilipp