Servo Controller Seizure

scootklein

My servo controller just recently started acting up. It works fine for a while, and then all attached servos seem to enter a "seizure" mode. I've checked the serial communication with the board and everything is maintained. The same behavior happens when the usb cable is attached and i'm running the parallax program for servo control. Video link is posted below.

http://people.engr.ncsu.edu/sbklein/servo.mov

Any help is appreciated.

Mike Green

What's your power source? Usually problems like this are due to power shortages of some sort.

scootklein

11.75 volts LiPo battery regulated down to 5V for the microcontroller and for servo power.

It's just strange to me that everything works fine until ~20 seconds after turning it on and using it.

Post Edited (scootklein) : 2/3/2009 7:05:12 PM GMT

Mike Green

Your regulator may be overheating and shutting down. You've got a voltage drop of about 7V. Servos can draw a lot of current when moving, commonly 250mA or so when lightly loaded, up to 1A when heavily loaded or stalled. Most regulators shut down when the current drawn is over 1A or 1.5A and they all shut down when they overheat. With a 1A load, your regulator needs to dissipate 7W. That's a lot unless it has a good heatsink.

scootklein

You're right, seems to be the thermal shutdown.

Is it really 7W dissipation though? I thought these were switching regulators that didn't waste the excess voltage, but rather used capacitor/inductor networks to establish the 5V and high-frequency on-off of the DC supply to maintain it?

Mike Green

I don't know what kind of regulators you're using. If you do have a switching regulator, then you're not likely to have thermal shutdown going on, but you could still have current limiting. Most regulators are linear regulators although switching regulators are becoming more commonly used. You can buy switching regulator modules that are "drop in" replacements for common linear regulators, but they're more expensive and slightly larger.

SRLM

I had this same sort of problem (1A regulators not up to the job.), so I went ahead and made a board design using four 3A regulators that allows you give a total of 12 amps to servos. I posted it on GG here. You can get a board, or you can just use the general concept. I got the regulators at Mouser. Note that they have a different pinout than the other regulators.