How do mobile robot charging contact bars work? How are they safe?

Thread Starter

Kayto99

Joined Feb 26, 2023
11
Hi all,

Question for any of you who may know: How does mobile robot charging work safely?
I notice a lot of delivery robots are designed with these large contacts (boxed in red below), but how are these designed to prevent them from being live contacts in case of customers accidentally shorting them together?

1677456075923.png

Summarizing my questions below:
What is preventing these from being live contacts, both on the battery side of the robot and the charging side on the dock?
Is there any examples of parts that are commonly used?

Thanks in advance for helping me understand this.
 

MrSalts

Joined Apr 2, 2020
2,767
They likely have short circuit protection - just like most wall adapters do these days. Shuts down transistors that protect the circuit from over-current or reverse current. A low value resistor is used as a current sensing resistor and diodes used to prevent reverse flows.
 

WBahn

Joined Mar 31, 2012
30,253
Summarizing my questions below:
What is preventing these from being live contacts, both on the battery side of the robot and the charging side on the dock?
There are several ways to go about this. The simplest would simply be for the voltage being used to be so small that coming into direct contact with it would not present a safety hazard. When I worked for NIST we had our main 4000 A busbars mounted bare on the wall and you could walk up to them and put your hands across them when they were at full current and you didn't feel a thing because they could only deliver a maximum of three volts. We had no warning signs up or anything. The only reason we told people not to do it was that it tended to inject noise into the measurements being made.

But I suspect they do something a bit more sophisticated than that, which would be to perform a negotiation of some kind when the robot docks with it. When sitting alone, it is configured as nothing more than a communications port with almost no current drive capability. Think of a phone line lying on the floor with the wires bare. Not much of a threat (though it would probably be much more of a threat given how traditional phone systems work). When the robot docks, all that can happen is that the robot and the docking station start talking to each other and only if that conversation goes as expected is the charging power actually connected to the contacts within the docking station.
 

MrChips

Joined Oct 2, 2009
30,986
The exposed contacts supply low voltages.
Think about the exposed terminals of batteries, iPhone charging cables, etc. They do no harm.
 

MisterBill2

Joined Jan 23, 2018
19,059
Usually in a charging system there is communication that goes on prior to providing power. That allows for devices with different requirements, and also keeps busy fingers out of trouble. So prior to providing power there needs to be a few exchanges, such as a request for identification, followed by responding with that identification, followed by a verifying connections and finally the delivery of power. The exchange is a bit like cashing a check at a bank was 30 years ago.
 

Thread Starter

Kayto99

Joined Feb 26, 2023
11
Usually in a charging system there is communication that goes on prior to providing power. That allows for devices with different requirements, and also keeps busy fingers out of trouble. So prior to providing power there needs to be a few exchanges, such as a request for identification, followed by responding with that identification, followed by a verifying connections and finally the delivery of power. The exchange is a bit like cashing a check at a bank was 30 years ago.
Is there any example of what components you could use for this? I'd assume you'd need a microcontroller, and some sort of relay or load switch.

Could something like this be used?
https://yamar.com/product/sig100/
 

WBahn

Joined Mar 31, 2012
30,253
Is there any example of what components you could use for this? I'd assume you'd need a microcontroller, and some sort of relay or load switch.

Could something like this be used?
https://yamar.com/product/sig100/
That's for sending data over a live power line. That's different than what you are looking for here. Here you want to send power over a live data line.

But you could probably craft something using that approach -- you would still need a relay or solid-state relay or some kind of logic-controlled switch to isolate and connect the power to the port terminals.

You don't have to physically use the same connection points for power as you do for communication -- though that makes the physical interface cleaner and possibly more reliable.

Even if you go with data-over-power, that particular solution might not be economical. If the data communication needs are minimal, then it might be possible to design, or find, an approach that is more cost effective.
 

Thread Starter

Kayto99

Joined Feb 26, 2023
11
That's for sending data over a live power line. That's different than what you are looking for here. Here you want to send power over a live data line.

But you could probably craft something using that approach -- you would still need a relay or solid-state relay or some kind of logic-controlled switch to isolate and connect the power to the port terminals.

You don't have to physically use the same connection points for power as you do for communication -- though that makes the physical interface cleaner and possibly more reliable.

Even if you go with data-over-power, that particular solution might not be economical. If the data communication needs are minimal, then it might be possible to design, or find, an approach that is more cost effective.
Yes I'm thinking some of these with only two noticable contact bars must be using data over the power line although not necessary if they exposed more contacts.
Maybe something more like this but with a higher max charge current: https://www.analog.com/en/products/max20340.html
 

WBahn

Joined Mar 31, 2012
30,253
Yes I'm thinking some of these with only two noticable contact bars must be using data over the power line although not necessary if they exposed more contacts.
Maybe something more like this but with a higher max charge current: https://www.analog.com/en/products/max20340.html
You seem to be moving away from a generic question like you originally asked (how can the be done while keeping it safe?) toward a specific application. There's no way we can comment meaningfully one the pros and cons of "something more like this" unless you provide a lot more information about what it is you are actually trying to accomplish.
 

Thread Starter

Kayto99

Joined Feb 26, 2023
11
Apologies for the lack of clarity. I'll try to be more specific with an example:

Let's say there is a mobile robot with a charging dock attached to the wall, both have 2 of the spring loaded contact bars as shown in the picture. The contact bars are providing data communication both on charger and robot that once connected to each other will send a signal to supply power and begin charging the robot's battery. When the robot needs to undock a signal is sent to stop supplying power, the robot undocks and only data communication is supplied on the contact bars on both charger and robot as it drives around keeping it safe to touch.

My question is: what kind of IC could supply only data over the power line unless that same IC is detected on the other end of the power line, in which case it would send a signal to switch on power?

Unless I am completely off base here and the solution is not a master/slave component configuration. This is the part of the circuit that I still don't quite understand so I'm trying to find a data sheet to dive into or some other resources to fully grasp how these work.
 

MisterBill2

Joined Jan 23, 2018
19,059
Consider how common and less expensive wireless digital communication is now. No reason to use a special wired link just to talk with a charge station. And then consider that guarded shock-proof connectors have been around an even longer time. So one switch closure to verify the mechanical coupling prior to switching on the charging power is all that is needed.
And I would certainly use a mechanical relay to switch the power because when they are OFF there is much less chance of leakage, compared to any solid state power control device.
 

drjohsmith

Joined Dec 13, 2021
852
Apologies for the lack of clarity. I'll try to be more specific with an example:

Let's say there is a mobile robot with a charging dock attached to the wall, both have 2 of the spring loaded contact bars as shown in the picture. The contact bars are providing data communication both on charger and robot that once connected to each other will send a signal to supply power and begin charging the robot's battery. When the robot needs to undock a signal is sent to stop supplying power, the robot undocks and only data communication is supplied on the contact bars on both charger and robot as it drives around keeping it safe to touch.

My question is: what kind of IC could supply only data over the power line unless that same IC is detected on the other end of the power line, in which case it would send a signal to switch on power?

Unless I am completely off base here and the solution is not a master/slave component configuration. This is the part of the circuit that I still don't quite understand so I'm trying to find a data sheet to dive into or some other resources to fully grasp how these work.
Whats your experience and expected outcome on this discussion,

To answer the first question, is it safe, I can say definately yes.
These things are covered by an abs amazing amount of regulation and paper work,

Your follow on question of how to make it safe,
Very very many options come to immediate mind.

As MrBill says, a short range radio link , say Bluetooth, is easy ,available in just about any micro now.
and a proximity switch, be it mechanical , optical or what ever seems probable,

I'd imagine the charge point has to have a beacon of some type, for the robot to find it accurately, My robot vacuum cleaner has such which could also act as proximity,

As for communication over the power,
Is this AC or DC?
Either way easy to inject an AC waveform on top, either capacitively or transformer coupled.
https://www.physicsforums.com/threa...-ac-small-signal-on-a-dc-power-signal.311708/

https://www.gadgetreview.com/how-to-robot-vacuums-find-their-charger
 

MisterBill2

Joined Jan 23, 2018
19,059
My question now: Is this for a purchased robot? Or is it for a hobbyist-built robot?? Commercial robots legally sold in the US have that issue already solved and implemented. A hobbyist-built robot will need to avoid thoughtlessly creating a shock hazard trough using a poorly thought out cobbled together collection of pieces.
The very simplest scheme would be to have the robotic charging connector on the end of a probe that would enter an opening on the charging power module, so that the charging power connection points would be protected against casual contact, and that a sensor switch would be operated at the point where the connections were made.
This would adequately protect against accidental contacts. There is no adequate protection against determined stupidity, so I offer none.
 

MisterBill2

Joined Jan 23, 2018
19,059
Hi all,

Question for any of you who may know: How does mobile robot charging work safely?
I notice a lot of delivery robots are designed with these large contacts (boxed in red below), but how are these designed to prevent them from being live contacts in case of customers accidentally shorting them together?

View attachment 288471

Summarizing my questions below:
What is preventing these from being live contacts, both on the battery side of the robot and the charging side on the dock?
Is there any examples of parts that are commonly used?

Thanks in advance for helping me understand this.
On top of everything else, it is probable that the voltage is low enough that there is no shock hazard. And even if it were mains power, a simple proximity verification scheme where prior to switching on, the robot processor hand shakes with the charger is processor very simple to do. just a relay in the robot and a relay in the charger. Not a big deal hardware wise.
 
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