Products > Robotics and Automation

Hospital robot suffering noise issues

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donotdespisethesnake:

--- Quote from: treez on July 18, 2020, 10:29:48 pm ---Thanks, we are wondering if an earthing strap may solve the noise issues....(as in like the attached)

--- End quote ---

 :palm: If 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.

rstofer:
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.

penfold:

--- Quote from: treez on July 18, 2020, 07:44:45 am ---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? [/I]

--- End quote ---

Not 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 controllers


--- Quote from: treez on July 18, 2020, 07:44:45 am ---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? [/i]

--- End quote ---

No, 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 sources


--- Quote from: treez on July 18, 2020, 07:44:45 am ---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? [/i] (PDF schem and LTspice sim attached of input filter).

--- End quote ---

That'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 am ---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?[/i]

--- End quote ---

Differential 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 problem

Can you share any details about the radio receiver? What frequency it operates on, how often do they command/respond etc

wizard69:

--- Quote from: treez on July 18, 2020, 07:44:45 am ---Hello

We are bidding to start work on a hospital robot…. “Hospitabot”.

--- End quote ---
Which means nothing to us and you fail to address this later in the thread.   What do you mean by "bidding"?

--- Quote --- 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?.......................

--- End quote ---
Why are you "bidding" on it then?

--- Quote ---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? [/i] …. because all the electronics and comms and motors are packed tightly together, and as is surely known to all, brush noise is very aggressive.

--- End quote ---
Known to all????????   The motors impact on the system depends upon implementation, bad voodoo can happen with any motor install.

--- Quote ---
There are offtheshelf SMPS’s on the robot, eg the JTH1524S15 for example….(and others of the same family)

JTH1524S15 24VIN 15VOUT, 15W DCDC module
https://www.xppower.com/portals/0/pdfs/SF_JTH.pdf

They have told us that the power supplies are causing noise problems with the comms systems.

--- End quote ---
Whom 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".

--- Quote ---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? [/i]

--- End quote ---
Nope!

--- Quote ---Also, maybe the underside of these DCDC modules is not metal shielded? [/i] , and so maybe the underside can output noise which can couple to nearby electronics and cables?

--- End quote ---
I really don't know what you are getting at here!    You are apparently making assumptions about a device you haven't even looked at.

--- Quote ---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? [/i], ……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.

--- End quote ---
Nope!!!    I'd go even further and say a custom power supply for this type of hardware is asinine.

--- Quote ---They are suspicious that the Hospitabot’s cable looms have not been routed correctly, and not been made correctly, and are picking up noise.

--- End quote ---
An who is this again making these announcements?

--- Quote ---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? [/i] (PDF schem and LTspice sim attached of input filter).

--- End quote ---
Nope!

--- Quote ---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.

--- End quote ---
Differential 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.

--- Quote ---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?[/i]
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.?[/i]  Eg a grounding wire connects chassis to circuit ground, but where best to connect it? 

--- End quote ---
Star grounding!

--- Quote ---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?

--- End quote ---

Of 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.   

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