Hi guys

How do you deal with the PID gain problem in batteries power device?

Say I got a 12V battery, it power a 12 DC motor, the feedback loop is good. But if my battery run down to 11.5V, my PID gain is still the same, so my motor will not performance as good as at 12V.

How this problem is usually solved in practice?

• Monitor the battery voltage and dynamically adjust the PID?
• Monitor the speed of the motor and motor driver take speed as input?
• Someting else?

In my case, I am refering to a balancing robot I am working on, I find that when battery voltage getting low (eg 50% discharged), my robot doesn't performance as good as when the battery level is 100%.

Am I making sense here?

 

...I find that when battery voltage getting low (eg 50% discharged), my robot doesn't performance as good as when the battery level is 100%.

I'm wondering if you are discharging your battery too far. That will dramatically shorten its life.

 

I'm wondering if you are discharging your battery too far. That will dramatically shorten its life.

Yes, I agree I should not discharge the battery too far. That's relative easy to deal with. (at least that's what I think)

But my problem is, as the battery discharge, say from 100% to 80% (I think it's acceptable for a typical SLA battery), internal resistance increase, output current drop, voltage drop, those can reduce/upset the gain (or turning parameters) of the PID.

What can I do to solve these? or should I ask how would Segway solve these? Or do they just use an expensive lithium battery pack or something?

 

Forget to mention, I am using DC brush motor here, will it be different if I use a brushless DC motor?

 

Your gain will be the same, however your error and subsequent output increase. One thing that does change is the dynamic response of the system, which would benefit from adaptive tuning. I'd suggest increasing your supply voltage for starters.
You might also scope your motor currents to see if further tuning could improve any ringing or oscillations that suck up current.

Maybe you could show us a vid of your project in action.

 

Here are a couple of videos of my robot in action:

http://youtu.be/4qYa0xzUhpE
http://youtu.be/pD3hjkkQnEE


It's OK, but it still not performance as good as I would like it to be. It seems to work better on carpet than on a solid flat floor.

On a solid flat floor, it will just going faster and faster and fall.

any tips?