Hospital robot ("Hospitabot") has noise issues

Hello

We are due 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 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?

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)

1. You need an oscilloscope. It can be cheap, preferably classical big one, something like this model. First you need to measure noise levels, you can measure noise at power input of each module. Usually I use oscilloscope's "Auto" button, if it did not choose scale correctly, I use default mode, scale to some millivolts / milliseconds, press CH1 button and select "AC", so large signal voltage will be centered. Most importantly, with an oscilloscope you will able to see if your modifications resulted in noise reduction.

2. I think any motors are ok, and your problems may be solved using LC filters. Motors are probably have metallic case, and main focus must be on power wiring rather than additional shielding. Most of electronic modules may be put into metallic box, even communication modules if external antennas may be attached. Also you can put electronics in plastic enclosure and use aluminum foil adhesive tape for shielding.
Check this article:

3. You can try any SMPS modules, but need an oscilloscope to find real source of the problem. Module from your link have 300kHz switching frequency and potentially may induce some "audio band" noise to mix with communications upconverter, but may be solved with large LC output filter. It is very unlikely that there is some very special strong noise coming out from those modules. Underside of module does not matters unless it strongly radiated by RF communications module and works as modulated scatter. You can ask them why DC isolated module is used.

4. Input filter makes sense. From datasheet there is no minimum load, but must ensure that you far away from maximum capacitive load.

Recommended actions:

- use oscilloscope to measure noise at battery output and at each module input, write it down, take a pictures of noise waveform
- do some research on hospital equipment EMI requirements, is it ok to use chosen rf modules, etc.
- write down list of modules, input voltages and power consumption
- optimize power schematics, maybe some modules can share single voltage converter
- if there are time constraints, do some hot fix with C / LC filtering
- maybe need to rethink and simplify the whole design: battery, mini computer, camera, motors, motor controller, wifi module, backup low speed communications module. All this components put together in an optimal way.

Thanks,
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?

It is not necessary to connect everything to the chassis ground. I do not think you need Y-capacitors or common mode chokes. Just check links below about placing capacitors on the motors.

I would tear down the robot, put all components on a big table, connect them as intended. Each component is visible and easy to access.
Make motors run constantly or turned on by mechanical switch, they may be powered directly if robot remote control is complicated.
Start to investigate source of noise using an oscilloscope. I do not think you need to add common mode chokes. Maybe put only _one_ emi suppression filter for most sensitive module (for example, some these BNX filters).

All components and modules are on the table, connected, powered on, motors are running. Going from battery to first DC voltage converter module. Output from this module goes to several power consumers. Put temporary 100uH/100uF LC filter at each consumer input. Connect oscilloscope to voltage converter module output, try to remove LC filters and observe noise performance. Probably two LC filters will be enough, one before control board, and one before communications module. And put capacitors on each motor.
check these about capacitor placement on the motor:
1) 2)

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

It will not help in reducing noise from brush motors.
I think it may be useful on concrete floors with antistatic flooring (something like this or **broken link removed**) to prevent static discharge through person taking/placing stuff on the robot. Here is an old paper on dangers of static discharge in operating room
Need to do more research on current requirements, depends on where this robot will be used. This must be put in instruction manual and warning on a robot (something like can't be used in places with ... hazards), because static discharge or motor spark may be a problem.
But many equipment have such motors: vacuum cleaners, floor cleaners, air conditioners, etc. Must keep in mind that hospital cleaning personnel may have some training and instructions where they can't use vacuum cleaner, but doctors and nurses may be unaware of that, so robot operators must be provided special instructions if required.

About current noise problem. Please check if there are capacitors on the motors:
1. capacitor on each motor
2. LC filter for communication module power line
and preferably measure with an oscilloscope.

Georgy.Moshkin said:

But many equipment have such motors: vacuum cleaners, floor cleaners, air conditioners, etc.

Click to expand...

Apart from noise problems, brushed DC or universal motor isn't suitable for long life, don't expect more than a few 1000 h. Hence it's not used for continuously operated drives, e.g. air of conditioners. I guess that motor life time would be a sufficient reason not to use brushed DC for the robot application. BLDC is state of the art anyway.

I was wrong about using any motor is ok, brushed motor lifetime is too short. Totally agree about "none DC brushed motors in air conditioners".

Supposing there are multiple SMPS’s in the robot. Will it help to have all the SMPS’s on any PCB synchronized with each other? When I worked in an Automotive place they used to sync all the SMPS’s on a PCB, and it was said to be to reduce noise problems.

Sync'ing is kind of not great, because it means that the switching instants are all lined up, and kind of add up with each other...however, if they are not sync'd, then every now and then you get the different SMPS's switching say about 100ns apart from each other, which causes a very harsh, high frequency noise "explosion". Would you agree?

How many voltages in your system?
Why do you use JTH1524S15 (1500V DC Isolation) in a battery powered robot? I think simple step-down makes more sense here.

1. I think BLDC motors may work in some range of voltages and may be powered directly from battery with some MOSFETs controller board. So motor part does not require any separate power supply.
2. If you use some industrial PC, it is probably can operate directly from battery too (12 to 24V) . This needs some research, and you may try contacting manufacturer to clear things out.
3. It seems that such robot may need maximum two SMPS to provide two high current voltage lines, and few LDOs to some low consuming parts.
For example it may have high current 12V and 5V.

We 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.