Rig Collision Detection System

[dmac123]

Dear Eng-Tips Community,

We require a collision detection system for our robot with the plan to electrically isolate the rig from the adjacent structures and putting it at 24 VDC with a 100ohm resistor. Then when the rig contacts any of the adjacent structures the voltage will change as the overall resistance of the circuit changes with the additional path to earth. What we did not account for is that all the motors (7 in total) are earthed to the servo drives. The only thing I can think to do to mitigate this is to put resistors on the PE lines for all the motors to increase their resistance - see posted circuit diagram. Then if the rig contacts one of the earthed, adjacent structures the resistance of the overall circuit will change causing a step change in the voltmeter reading which I can then read with our DAQ. Let me know if you think this will work. I'm a bit weary about increasing the resistance of the PE lines from the motors which are used for motor's earth leakage current.

Thanks for your assistance.

 

[waross]

Is your robot trailing an extension cord, or trailing a grounding conductor.
If you PE ground point is local to the robot, you won't have a circuit to grounded structures.
You may need a more sophisticated detection system.
Beware of unintended consequences if you add resistance into ground paths.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dmac123]

Hi waross,

There is custom loom cable which boundless all the motor power cables and encoder cables, which trails the robot and where each motor's power and encoder cable are connected to its respective servo drive housed in an electrical cabinet.
If I check continuity on a meter between the metal surface of the robot and cabinet earth, there is continuity.
Agreed on adding resistance to the ground paths - I think we might need something more advanced that what I proposed above.

 

[waross]

40 years ago I spent time in a General Motors assembly plant.
The autonomous robots, carrying automobile components around the plant had proximity detectors that would halt a robotic transporter if a person or object came within a couple of feet of the intended path.
The technology is out there and has been for decades.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dmac123]

Agreed on the proximity sensors/switches. Looks like that will be the most pragmatic approach.
Thanks for your responses.

 

[cranky108]

A number of years ago there was some projects made using to distance detection system in polaroid cameras. I believe it used ultra sonics for that. It was used for some early robot experiments.
Then there was a book written about some early robot experiments that used a ribbon switch for a contact detector.
But why all the 5K resistors in parallel? Are you trying to create a smaller resistor, or for heat dissipation?
Not that I need to know, but more for self interest. In my line of work, the thermal rating can be as important as the resistor value.

 

[waross]

A real cheap and easy solution may be to surround your robot with a flexible rubber hose, similar to what was once used at full service gas stations (anyone else old enough to remember those,outside of a few selected jurisdictions) to announce the arrival of a customer.
Surround the robot extremities with a soft rubber hose connected to a simple pressure switch.
Almost as simple as a hammer. Sometimes old tech still works.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!