Hi all, I’m working on a balancing bot using a Raspberry Pi and Pololu IMU and need some help.
I’m not sure if i have chosen the right motors and power supply as I seem to be having issues.
Every so often, my motors seem to stutter and not seam as powerful.
First thing I want to check is the power supply I’m using;
2 x 9.7:1 Metal Gearmotor 25Dx48L mm with 48 CPR Encoder moters , Stall current at 6v = 2.2 A
Raspberry PI = 700 mA, (3.5 W)
MinIMU-9 v2 connected to Pi.
Dual H-Bridge Motor Driver for DC or Steppers - 600mA - L293D
To power this, I have a Anker® Astro3 10000mAh Multi-voltage (Dual USB Output: 5V, 2A; DC Output: 9V / 12V, 2A) <http://www.amazon.com/gp/product/B005NGKR54/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1>
From this power supply I am powering the Raspberry Pi @ 5V) nd the motors at 9V.
Second is the motors;
As you see above, my power supply outputs 9v… would this be an issue driving these motors?
Could weight also be an issue? These motors 11 oz-in Stall Torque@ 6V… my bot weighs about 1.5ponds.
It looks like that power supply might be a little under powered for your application. Although we do not have it characterized or have a lot of experience in balancing robots (my balancing robot project is currently on its side), the motor might draw more that 2.2 A, possibly double, when it switches the direction of rotation (as we would expect it to do many times while balancing). Since you have two motors, I suggest using a power supply that can deliver more current.
I think you might be able to run these motors at 9 V; however, you will trading off the added performance with life expectancy of the motor. If you are not looking for the additional performance, I suggest limiting the duty cycle to 66% (66% of 9 V is 6 V).
thank you for the reply Jeremy.
If I was to get one of the pololu battery packs to power just the motors, would this do?
#2244 Rechargeable NiMH Battery Pack: 6.0 V, 350 mAh, 3+2 2/3-AAA Cells, JR Connector
The batteries you linked to probably will not be able to power your system. I recommend using AA batteries or bigger. Something like our 6V AA battery packs might work; they should be back in stock in a few weeks.
Some additional comments:
A 600 mA motor controller will croak when the motor wants to draw the 2.2A stall current – or even the double of that, if it’s quickly reversing direction.
At 9V, the motors will draw 2.25x what they draw at 6V (square of 9/6), so up to 5A per motor, and double that for direction changes. That’s not bad in itself; it gives more power, but repeated operation at that voltage/power will quickly wear them out.
PWM reduces the power/amp/wear, assuming you’re using a good PWM controller. You need a PWM controller that can control 10A per channel to correctly control your motors under the situation you suggest.
If the worst case current is 10A per motor (at 9V) then the worst case overall current is 20A. Your power supply is 2A, so it will clearly croak when actually trying to do quick changes. “Croaking” may mean turning off because of overcurrent protection, or reduce voltage for constant current limiting, or overheating and dieing if not properly protected, or some combination of the three.