Hospital robot suffering noise issues - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2020, 07:44:45 am
content: HelloWe are bidding to start work on a hospital robot…. “Hospitabot”. It’s a wheeled robot which goes round the hospital and can pick up parcels and then carry them and then deliver them to other parts of the hospital.It also has radio receivers on it to give it specific commands when needed.We don’t yet know the full spec, but we know enough to have grave concerns. Please confirm or refute our concerns?.......................I am told the power is that given by its 24V, 20Ah battery.All of its electric motors are Brushed Motors. We are also told that there is a noise problem with its comms system.Can you confirm that Brushed DC motors are a poor choice here, and a swap to BLDCs is in order? …. because all the electronics and comms and motors are packed tightly together, and as is surely known to all, brush noise is very aggressive.There are offtheshelf SMPS’s on the robot, eg the JTH1524S15 for example….(and others of the same family)JTH1524S15 24VIN 15VOUT, 15W DCDC modulehttps://www.xppower.com/portals/0/pdfs/SF_JTH.pdfThey have told us that the power supplies are causing noise problems with the comms systems.Would you agree that these type of DCDC modules are optimised for Thermal performance and Efficiency…not for EMC? Do you agree that they probably switch the FETs on superfast, resulting in super high dv/dt and resultant noise issues?Also, maybe the underside of these DCDC modules is not metal shielded? , and so maybe the underside can output noise which can couple to nearby electronics and cables?In fact, would you agree that when electronics is packed tightly together like in this “Hospitabot”, it is often better to NOT use offtheshelf SMPS modules?, ……but instead, to use custom designed SMPS’s, with relatively high value FET gate resistors so as to damp the V(ds) switching transitions (lessen the dv/dt). This will reduce noise issues.They are suspicious that the Hospitabot’s cable looms have not been routed correctly, and not been made correctly, and are picking up noise.Also, the above JTH1524S15 power module has a recommended input filter as in the attached.If Vin = 24V gets hot plugged into this module, then, its input will ring up to 48V, and that will damage the module. Would you agree with this? (PDF schem and LTspice sim attached of input filter).Also, regarding the comms system noise, I am presuming that where possible, converting to differential signalling methods will reduce noise? Also, use of common mode chokes in signal lines.Also, we are thinking that using common mode chokes at the output of the battery will be good.Also, common mode chokes at the input to all DCDC modules?The one noise combatting situation that comes up repeatedly in similar things, is the “grounding” of the circuit ground to the chassis. ….Where should this be done, and how many times.? Eg a grounding wire connects chassis to circuit ground, but where best to connect it? Also, can it be beneficial to connect chassis to circuit ground with more than one wire in more than one place? Of course, in the hospitabot, there is no earth ground, so we assume that from a common mode filtration point of view, the chassis is effectively to be treated as “earth”. Also, would you agree that all metal chassis parts must be electrically connected together? (for purpose of noise combatting).Also, the Y capacitors, that connect chassis to circuit ground. Would you agree that the place where they actually connect can impact on noise? Also, over-use of Y capacitors from chassis to circuit ground can actually make common mode noise worse?

author: jpanhalt
author_rank: Super Contributor
timestamp: July 18, 2020, 10:03:50 am
content: Those BOTS have been operational since the mid-1990's in American hospitals.You are "bidding" on this project. You have been given certain specifications. Are you sure the entity asking for your bid wants your opinions? That is not to imply that some of your concerns are not legitimate.

author: fmkit
author_rank: Contributor
timestamp: July 18, 2020, 10:36:34 am
content: just avoid long I2C linesPS: COVID BOT:

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2020, 10:29:48 pm
content: Thanks, we are wondering if an earthing strap may solve the noise issues....(as in like the attached)

author: m98
author_rank: Frequent Contributor
timestamp: July 20, 2020, 08:52:14 pm
content: If the robot runs with brushed DC-motors, they probably also heavily contribute to the noise. Especially when the brushes are worn-down.Multiple grounding points on the chassis won't do anything, you usually want one star grounding point, as you're using your chassis as an unpredictable antenna rather than a shield if you have current flowing through.And how should an earthing strap solve the issue of noisy power supplies inside the system?

author: fcb
author_rank: Super Contributor
timestamp: July 23, 2020, 11:01:25 am
content: What does "bidding" mean?Do the robot's currently work? Are they causing EMC issues with other equipment? Are you trying to add more stuff to them - using them as a base?Brushed DC motors in good condition can be surpressed fairly easily, and possible have been already???If the 'comms' system was good enough before - why isn't it good anymore? On a robot you shouldn't need differential comms as the distances are short and noise/grounding predictable, could be a big bag-of-worms Treez..

author: ocset
author_rank: Super Contributor
timestamp: July 29, 2020, 06:42:01 am
content: Thanks, thats a good point, will look into itWe have one particular "hospitabot" (robot) which is fairly stationary most of the time...it just takes things off a table and stacks them in the right place on a row of shelves nearby. Can we use a stepper motor driver for this robot? Say about 400W? I realise we wont get as smooth torque as a BLDC motor, but who cares, this thing is just shunting back and forwards over small distances. A stepper motor would be quieter than a Brushed DC i believe? And steppers are easy to control. Sometimes this robot needs to travel long distances, but that should be fine with a stepper i would have thought?.....non smooth torque isnt the end of the world here.

author: Berni
author_rank: Super Contributor
timestamp: July 29, 2020, 11:26:58 am
content: There is no magical motor type X is better than motor type YYou can quiet down a DC motor a lot by correctly grounding it, providing filtering right on the terminals etc, or making it noisy as heck by giving it long loop areas. Similar applies to stepper or BLDC, they can be both electrically and audibly whisper quiet with a good motor driver, or they can whine and scream while running and spewing out tons of EMI in the process.If there is an interference problem, grab a spectrum analyzer and shove an E or H probe up in every spot to see where it is coming from. Plug thing in and out to see what makes a difference. Then once you have located the problematic devices you can tackle them one by one with targeted fixes. Important to measure before and after the fix to see how much of a difference did it make.

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 11:30:45 am
content: Oh my, I hope that only a small number of patients will be killed and this does not turn into massive massacre considering who makes this robot.

author: statorworks
author_rank: Newbie
timestamp: August 04, 2020, 12:51:33 am
content: Like others have said, the chassis grounding can be a double edged sword, in case these connections get corroded etc.In my experience it is better if you daisy chain your power with ground wire of enough thickness so all energies can return to the battery's negative pole with the least effort. Think of it as the drain pipes in your house being wide enough so your toilet doesn't back up.If you can, you may add additional ground lines for parts that must return significant energy (i.e. toilet has dedicated pipe to the street)As far as noise induced over the air, I've seen it happen by the wiring of stepper motors being run at 24V, as the driving spikes are quite aggressive. Regardless of using brushed or steppers, I think it's better each driver itself sits near the motor, as opposed to carrying the driving pulses along a significant length of the machine harness.I would also make sure the power supply as well as each board has enough bulk capacitance to assist with surges.Just my $0.02

author: donotdespisethesnake
author_rank: Super Contributor
timestamp: August 09, 2020, 10:45:30 am
content: Quote from: treez on July 18, 2020, 10:29:48 pmIf that's your idea of a solution, I don't think the right people are bidding on this project.I wonder if the customer knows you are getting your "expertise" on robots by trawling web forums.Really though, I suggest you limit your bids to stuff you understand, maybe SMPS.

author: rstofer
author_rank: Super Contributor
timestamp: February 19, 2021, 02:31:59 am
content: The golf cart business has been using brushed DC motors forever. All of the issues have been resolved as far as speed control is concerned.https://www.4qd.co.uk/product-category/controllers/The electronics package to drive the controller can be isolated with high speed opto-couplers.

author: penfold
author_rank: Frequent Contributor
timestamp: March 07, 2021, 03:36:58 am
content: Quote from: treez on July 18, 2020, 07:44:45 amNot unless you want to increase the cost and add additional sources of noise, brushed are bad for voltage noise, but that is more easily handled than the noise currents coupled via BLDC controllersQuote from: treez on July 18, 2020, 07:44:45 amNo, XP are generally pretty good EMC-wise when the appropriate input filtering is done, they should be over a ground plane to shield the underside though.A custom PSU may help, less on the low dv/dt or di/dt but it gives you ultimate control in designing with the specific problems in mind, even just physical separation between components when needed and frequency selection and synchronisation to limit upset caused to the RF bit.. the intermediate frequencies could be close to some of your noise sourcesQuote from: treez on July 18, 2020, 07:44:45 amThat's unlikely when you add in the characteristics of the front of the PSU and a few parasitics,Quote from: treez on July 18, 2020, 07:44:45 amDifferential signalling is always good, its just nicer, common mode chokes rarely hurt, but if that's the source of your noise then there will be a design fault elsewhere which needs to be taken care of rather than masked. Isolated communications are better,Since the battery doesn't naturally have a common mode return path (except through capacitance to surroundings) and you would need a monstrous choke to combat this or Ys cap at the battery terminals. Impopper overuse of Y-caps can be problematic, but again, its likely something else which is causing the problemCan you share any details about the radio receiver? What frequency it operates on, how often do they command/respond etc

author: wizard69
author_rank: Super Contributor
timestamp: March 24, 2021, 07:09:11 pm
content: Quote from: treez on July 18, 2020, 07:44:45 amWhich means nothing to us and you fail to address this later in the thread. What do you mean by "bidding"?QuoteWhy are you "bidding" on it then?QuoteKnown to all?? The motors impact on the system depends upon implementation, bad voodoo can happen with any motor install.QuoteWhom is telling you this? People often blame the machine for their inability to understand it.Now that being said power supplies fail and often they don't go completely. Replacing power supplies is actually something that happens frequently in automation. Sometimes a failed supply is obvious and sometimes you have to catch it when it is having a "moment".QuoteNope!QuoteI really don't know what you are getting at here! You are apparently making assumptions about a device you haven't even looked at.QuoteNope!!! I'd go even further and say a custom power supply for this type of hardware is asinine.QuoteAn who is this again making these announcements?QuoteNope!QuoteDifferential signalling can solve a lot of problems but that doesn't imply that your current system has problems. You would need to actually verify what the problem is.QuoteStar grounding!QuoteOf course location matters. It doesn't matter if you are restaurant or a cap if you are not in the right place nothing will stop by.I really don't know what the point is in all of these questions. It kinda sounds like you are contracting to try to repair something but are making all sorts of assumptions without the knowledge to make reasonable plans. When working automation the real art is in finding the root cause of a failure. Sometimes that is not easy.Consider this I work automation in a large factory with plenty of robots and other automation. The plant is maintained by multiple shifts of technicians and engineers. If I walk into a "passdown" the last thing I want is a detailed report on what the last tech thought was wrong. Rather I'm better off taking a look first myself; otherwise you can get drawn down the wrong path based on what somebody else thought. I might read an E-Mail or log latter but i want a fresh opinion based on observation.In a nut shell if this robot is having problems you need to determine root cause or causes. Guessing at this point serves no purpose. Further it is not unreasonable to believe that the machine has been poorly maintained.