Power Expander Problems - Help!

Hello. Our robot is a six-bar lift, which four motors are driving the arm. We tried to use power expander to provide power for the arm, but we have encountered an issue which the power expander would stop working if we keep pressing the UP button when the arm reaches its maximum height. This phenomena also occurs when the arm reaches its minimum height, and we keep pressing the DOWN button, but everything else is fine. However, if we connect four arm motors directly to cortex, this phenomenon doesn’t occur. In addition, if we try to use power expander to provide power for high speed wheel motors, there is always a blinking green light on the power expander, which causes the wheel motors stop running. BTW, we did not program the power expander. Thank you.

Just a bit of a further explanation with the chassis…

The 4 wheels on our chassis are where the only high speed motors occur, thus we thought first to put the power expander to those 4 motors. After completing our bot and testing it out just days before the competition we couldn’t figure out what was going wrong because the drive kept dying after about 20 seconds of test driving. We couldn’t quite fix the problem so we tried it with normal speed motors and it seemed to work OK.

The problem isn’t the motor controllers or motors themselves because it works fine when we run it through the actual cortex (the 4 motor lift and chassis). The only ideas we had left are to try a different power expander and to use a half charged battery to see if maybe there was too much current being sent by these super-charged batteries and maybe triggering the PTC. We will try both of these tomorrow after school. If anyone has any ideas as to the source of the problem please post it right away as our last competition before states is this saturday.

Thank you to anyone who read all of this.

It sounds like you’re tripping the PTC in the power expander, as you suggested. VEX batteries charged by VEX chargers wouldn’t cause the tripping though; the tripping happens when the motors draw too much current from the power expander.

If your robot works fine with all motors connected to the Cortex, you might as well compete that way; you’ll just have to replace batteries more often.

Another setup to consider would be splitting your subsystems across both the Cortex and power expander (for example, front drive motors on the power expander and rear drive motors on the Cortex).

We will try your suggestion with splitting the chassis between the power expander and cortex as you mentioned. Also, we would solely run the cortex but we are running 12 motors on our bot at the moment.

Do you think the normal speed motors on our 6 bar are drawing too much current (when we wire the power expander to it) and tripping the PTCs as well? What happens is that when we start the lift on the ground and push the up button it rises up well until it reaches a certain point and then at that point the lift just starts twitching as if the motors are fighting one another, spinning in opposite directions. We can’t add anymore rubber bands or else they actually lift the 6 bar out of the 18" limit.

I don’t really have enough information to make an accurate judgement.

I can provide, however, a quick and simplified explanation of what you’re experiencing:
When the arm reaches its extremes and you continue to try and run the arm in the direction of the extreme, the motors try and draw more and more current as they [to no avail] try and overcome the mechanical stops. Though the increased current may not be enough to trip the PTCs in the motors, the combined increase in current from all the motors plugged into the power expander is enough to trip the power expander PTC. If the load is distributed across the Cortex circuits and the power expander circuit, less current is being drawn per circuit and, therefore, there is less chance of tripping a circuit.

I assume by “certain point” you do not mean “hard against a physical stop.”

Either the motors are stalling individually (which means one or more motor PTC devices have hit their max current over time), or more likely, the cortex half is stalling. That is, hitting the current that causes the PTC to “trip.” (It really just becomes a very large value resistor. Until it cools back down a bit.)