Robot loses connection: Any clue why?

Technical Control System
1

Hey all
We are working on an offseason robot, and we are pretty close to finishing. The only problem is that when we try to drive, the robot will lose connection. It is fine when we “drive” with the wheels off the ground, but when we set them on the ground, no connection. We are using victors and 4 motors (in two cimple boxes). The connection is only lost for a few seconds, and then the connection is regained. I was thinking it was the current draw that is doing something, but what?
Thanks,
Michael

2

Is the cRio completely electrically isolated from the robot chassis ?

Verify it with a meter.

3

Michael,
If the battery voltage should fall to 5.5 volts or less, the connection on the Crio analog module #1 will sense this and disable all outputs. In your particular case, are you sure that all motors are connected in the same direction? It sounds like one of the motors is running in the wrong direction such that when placed on the floor, you are generating enough load to cause the battery voltage to fall. When the robot is off the floor, I bet one side is running much slower than the other side. It is also possible that you have a loose electrical connection somewhere in the battery path. (bad Anderson connector, loose battery terminal, loose hardware on the PD, loose hardware on the main breaker) A loose connection will drop significant voltage when you are drawing a lot of current. This symptom also occurs with a discharged battery so it confuses a lot of teams as to exactly which part of the electrical system in which the failure is occurring. I am assuming you are using the 24 volt output on the PD for the Crio as well.
There is always the possibility under these conditions, that part of the drive is binding with the weight of the robot on the floor.

4

Make sure your 12->5 converter for the radio is connected to the regulated 12v supply.

5

Hi

This may seem obvious, but I’ve seen it with rookie teams preparing for competitions on Thursdays.

There are several ways to build a drive system that WILL NOT run when placed in contact with the ground. In most cases when you try to run it, the motors stall and cause a moderatly charged battery voltage to fall to levels where the radio/c-rio resets. This can not be seen on the console… because the system has reset.

Some things to check for when the robot is in the air:

  1. Are all the wheels travelling in the correct direction? (a 4 wheel drive robot with one motor hooked up in reverse will stall instantly.)

  2. Is the same gear ratio used on all motors? (Incorrect choice of sprockets will cause the wheels to spin at different speeds. Motor will stall immediatly.

  3. Are you trying to skid steer with too much traction, or too High gear ratio? A narrow base robot with traction wheels needs a LOT of torque to turn on carpet. If you can drive OK straight, but fail on turns, this is the likely cause.

6

I was wondering if anyone has any data (not theory) on this.

For example how much more torque* does a 30" wheelbase by 24" trackwidth require than a 24" wheelbase by 30" trackwidth, both vehicles having the same wheels, motors, and drivetrain?

  • or current, since it’s easier to measure, or voltage, since the motors are stalled

7

In the drives Phil describes, we have measured near stall currents on the motors used. This type of drive essentially creates excessive friction in the bearings used on the wheels.