Preventing motor damage

[cowana]

I've got a quick query regarding motor control.

To control the angle of a camera, I am using a geared motor, potentiometer for feedback, and a PIC running a PID control loop.

That all works fine - the system responds to an rc input. The motor is a **broken link removed** - with a 360:1 gearbox, it offers 7.4kgcm stall torque at 6v - mighty!

That's at 23rpm - OK for my camera control. However, as my PID loop pulses the motor mostly, it is actually much slower and weaker. My solution was to run the motor driver and motor at 12v, which increases the speed and torque to better levels.

As the motor is only being pulsed, the average voltage/current is low, so the motor stays cool and happy.

However - the problem:
- After a few weeks of successful operation, I pressed 'download' with out unplugging the motor driver - the outputs went to unknown states, and the motor went full speed in one direction. It moved the camera to the mechanical limit, and with the power of 12v, destroyed the motor's gearbox.

I replaced the motor (£15), and ensured the motor was unplugged for downloads.

- After a few more weeks of successful operation, while working on another part of the system, I turned off a BIG inductive load, which screwed up the power supply and caused a couple of PICs on the system to reset/crash. The motor (again) went in the wrong direction, jammed, and broke the gearbox.

I replaced the motor (another £15), added more capacitors, and have been OK for the past few months.

However, I'm constantly worried that I'm in a bad situation - if something does go wrong with the driver, I could easily loose another motor. The motor is rated to be stalled, but only at 6v, not 12v.

So:
- I realise running at 12v is a bad idea, but I don't want to redesign the system just in case of this unlikley event.
- I feel adding a dropping resistor is a quick (and acceptable) solution, as the motor is OK being stalled at 6v. The motor is driven by an H bridge, so adding an op-amp based solution or something at the motor is a big task.

Measurements:

	     6v         12v
No Load      34mA       73mA
Some load    100mA      250mA
Stalled      670mA      1.2A

Solution:
Adding a 10 ohm resistor in line with the motor would reduce the voltage to about 10v under normal operation (that's fine with me), while reducing the voltage my much more under stall condition.

What do you think of that idea? While a bit power-wasting, it seems to be a quick and workable solution to me. Does 10 ohms sound about right?

Any thoughts appriciated!

Thanks,

Andrew

 

[KeepItSimpleStupid]

What I did for a model RR project was I used an LM317T, an SCR, op amp and crowbared the ADJ terminal on the LM31T. I also had to add a slight delay. Worked fine. Recycle power to reset. Lots of jogging would probably be a problem with this approach.

Other options are a Limit switches or slip clutch. **broken link removed** has a nice adjustible onethat I have used.

One may, however be a design issue. If you did not use NOT ENABLE and DIRECTION signals, then while the ports are defined as inputs the motor moves.

 

[leftfield95]

Hi Andrew,

A self reseting fuse of suitable rating placed in the motor drive cable, should save you coughing up another £15.

Regards,
Leftfield95.

 

[BobW]

I would go with the resistor idea; it's nice and simple. It may waste a bit of power, but the motor isn't in operation all the time anyway. As for the correct value, you may have to experiment a bit.

The fuse idea may be okay, but I've had quite a bit of experience with motor protection, and proper fuse selection can be very tricky. When the motor starts up on normal load, it will very briefly draw the same amount of current as when the rotor is locked. So, the fuse has to be able to tell the difference between normal starting inrush current and locked rotor current, which are the same value, differing only in duration. And a slow blow fuse may not operate fast enough to prevent the gearbox from getting shredded.

 

[crutschow]

The spec sheet says the motor 6V stall current is 692mA. This would require a resistor of 8.7 ohms to drop 6V, thus 10 ohms should be more than adequate to prevent damage if that value works ok for normal operation.

 

[ronv]

Might work, but it may be the shock load caused by the sudden stop that actually kills your gears. My bet is if you put it against the stops and hit it with 1.2 amps it would just get hot, not break the gears.

 

[BrownOut]

Two words: Limit Switches.

 

[misterT]

That's at 23rpm - OK for my camera control. However, as my PID loop pulses the motor mostly, it is actually much slower and weaker. My solution was to run the motor driver and motor at 12v, which increases the speed and torque to better levels.

Tune your PID controller so that it works correctly with 6V.