Electronics > Mechanical & Automation Engineering

Hospital robot suffering noise issues

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ocset:
Hello

We 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? [/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.

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.

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]

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?

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.

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? [/i] (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?[/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?  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?

jpanhalt:
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.

fmkit:
just avoid long I2C lines

PS: COVID BOT:

ocset:
Thanks, we are wondering if an earthing strap may solve the noise issues....(as in like the attached)

m98:
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?

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