# Strange Wheel Base Issues

No, curve B is for the HR16-400 and I am assuming that is the part in the power expander (I don’t have one available to open up). Ignore curve A and all the other curves. The specs for these devices are a bit vague, all we can be certain of is that.

From the datasheet.
4A is the minimum hold current (Ih). ie. 4A will not trip the device at 25 deg C
6.8A will trip the device, but it will take some time.
At 20A (5*Ih) the maximum time will be 1.7 seconds.

So if two wheel 393s and two 269s are plugged into in one circuit breaker, are they unlikely to trip the breaker at 25C during a match?

If you gear your robot correctly or operate it in such a way as to not stall the motors, you will be fine. If you over-gear your mechanism or stall your motors the breaker will pop.

It depends.

If the motors are running at half speed then total current draw will be approximately (1.8A x 2) for the 393’s and (1.3A x 2) for the 269’s. Total is 6.2A, the graph implies you should be able to run for perhaps 60 seconds or more, it’s hard to tell. If you were to drive against a fixed object and had enough traction to stall the motors then total current draw would be over 12A and would quickly trip the breaker, however, bear in mind there are also PTC devices in the motors which will probably trip sooner. My recommendation would be to split left and right drive across two circuits.

Our plan is to divide the four motors through three circuits; one in one circuit, another in the second cortex circuit, and two in the third circuit.

But the free current for the 393s is 0.15A. Would this not be full speed without resistance?

Also our 269s are on the scissor lift, and that has never gone anywhere near stall current because of the enormous torque ratio that the lift has.

Yes, but you have no torque at full speed.

Take a look at this.

Ideally you want to be working somewhere around half speed which is where full power is developed. Search the forums, there are already threads that discuss the ideal motor speed for efficiency far better than I can.

We have rewired the robot as suggested, now with two wheel motors in the cortex (one on each breaker) and two in the power expander. There are a few problems now:
-The stalling issue is as persistent as ever
-Even though only two wheel motors are plugged into the power expander, unplugging the power expander battery stops all four wheel motors, but not the scissor lift or intake motors. We tried the wheels even with the robot off the goround, but the motors still failed to work.

We are extremely confused as to what is wrong now, and more suggestions would be very helpful.

Please describe in more detail the configuration or at least confirm it’s something like this.

cortex port 1 - drive motor left 393
cortex port 2 - power expander - drive motor left 269
cortex port 9 - power expander - drive motor right 269
cortex port 10 - drive motor right 393

Are the motors chained/geared together? If so then when you remove the power expander battery the remaining motor on the cortex has to back drive the motor which had power from the power expander - this will not work well. Even if they are not connected the robot may be too heavy to drive using only two motors rather than four.

Are any of the motors on the same side reversed? If so then they are fighting each other. Is any of the wiring reversed causing motors to work against each other?

If everything is wired correctly then you must have some other mechanical issues. It’s hard to imagine how spreading the motors over more circuits could have made things worse.

The drive motors are all 393s. The 269s that are in the power expander are scissor lift motors. The front two 393 motors are in the power expander, while the rear right drive 393 is in port 3 and the left rear drive is in port 8. Neither rear wheel is in the power expander.

Yes, the wheels are geared together just like on the Protobot’s wheel base but with 6 wheels instead of 4. If they had been fighting each other, then the robot would never move at all.

We know that the robot’s weight isn’t stopping the rear drive motors, because we have tried driving them with the robot lifted off the ground.

Now things are just weird. When we tried bypassing the power expander completely, all four drive motors stopped, but the lift and intake motors continued to work. It seems the drive motors do not work when only plugged into the cortex. This would explain why putting two drive motors on the cortex and two on the power exp. made the stalling worse, as two motors did not run at all and caused resistance to the other two. It would also explain why unplugging the p.e. battery stopped all four motors. We think the drive motors might be told to get their power from the power expander through code, but as I don’t have access to our code now there’s no way to be sure. Is there something in code that dictates where a motor’s power comes from?

We don’t believe we’re any closer now to solving our original problem. When returned to the original wiring is when the robot works best, but the stopping issue remains the same.

Our robot had drivetrain issues just days before our first tournament and here’s the list of things we tried:

1. Decreasing friction.
2. Adding two more motors (the night before the competition).
3. Change gear ratio to more torque (not inside the motors but from 84 : 36 to 60 : 60). It was tremendously slower but there was no more problems with the drivetrain.

Then in between our 1st and 2nd tournament we ordered 2 more HS motors (one of them on the robot broke and we didn’t have any spares so we had to put 2 regular motors on the drive, but it was still better since it was a 6 motor drive instead of a 4 motor drive). Then we changed our speed to 60 : 36 and the motors ran just fine and we stayed with the ratio since it gave us ability to push robot around also.

If the options on the top list are not possible for you, you can always to put clutches instead of the metal connectors in between the axles and the drive motors. I know you don’t need them since the gears are metal inside 393 & 269 but the point is to limit the amount of torque so when the amount of torque that’s needed to move the robot exceeds the physical limit of the clutches, they will start clicking like crazy. And yes, the drivetrain should still be able to move. Our robot last year had a 60 : 36 with 4 in. wheels and we put the clutches in and they click like crazy but we never had any overheating problems because they would just click when we accelerate / change direction too fast and thus decreasing the stress on the motors.

Hope that helps.