Mongoose-like robot

[upand_at_them]

This is sort of a question about Blueroom's Mongoose robot. I'm using the same gearbox and bridge, but it keeps going into thermal shutdown...even after just a few seconds running. I wondered if the Mongoose runs into this as well?

I've got the Tamiya gearbox, Tamiya tank treads, and 754410 H-bridge. I was hoping to use this as a test platform for some ideas. I was surprised about the thermal shutdown, since it's not going fast at all.

Mike

 

[blueroomelectronics]

The tank treads are much harder to run than tires. Use the highest gear ratio available to keep the h-bridge from overheating. It'll be slow with those treads.

You could also thermal epoxy a heat sink to the 754410.

 

[upand_at_them]

Thanks, I'll try the gear ratio. I looked before I left for work and I'm not using the highest ratio.

I also found these, which draw much less current: Pololu - Brushed DC Motor: 130-Size, 6V, 11.5kRPM, 800mA Stall

 

[blueroomelectronics]

With a high gear ratio the tank will be slow but shouldn't overheat the h-bridge. The RM-130 motor is near identical. Also add either one or three 0.1uF caps across the motor terminals.

 

[nickelflippr]

I ran into the same problem, and had to run 114.7:1 ratio on the Tamiya double gear box with that setup. Mine's kind of heavy with 6 AA nimh batteries, and so that's about as high a speed as I'm going to get without stalling. Also running one 754410 H-bridge per motor, kind of a safety factor, and ready for higher amp motors like a Tamiya Hyper-Dash 2?

 

[bryan1]

For an easy maybe fix just piggyback another 754410 ontop of the existing one and solder each pin to the lower chip. This will double the current to 2 amps and hopefully be enough to get you going.

Cheers Bryan

 

[upand_at_them]

Good idea, thanks. Right now I'm building a new base and waiting for those low-current replacement motors to arrive, since my voltage regulator was burning up too.

 

[blueroomelectronics]

I wouldn't use a voltage regulator on the motors, plus if it's burning up then you're design is just wasting energy. Can you post a schematic?

 

[upand_at_them]

No schematic, but it's simple: a TO-220 7805 (1.5A) feeds the power rail for the motors, PIC16F886, and the H bridge (754410). I needed to regulate the motors, because I'm using a 7.2V LiIon pack and feeding them that much voltage significantly reduces their life.

I do like your idea of using 4xAA NiMh. But my AA's are currently tied up with another robot.

 

[blueroomelectronics]

Don't regulate the motor power, there is a 1.4V drop anyway with the SN754410 and your 3V motors will be fine with a couple of extra volts. Using PWM will effectively lower the voltage to those motors. The 886 may have a low voltage monitor built in (the 18F2525 does) and you can monitor the battery voltage with a simple resistor divider (and control the PWM max duty accordingly)
Did you add the 0.1uF caps across the motor?
Pololu - 9. Dealing with Motor Noise

 

[upand_at_them]

Okay, I'll see. More than 3V really does shorten the life, though:
https://www.pololu.com/picture/0J501.895.gif

Yep, got caps on the motors.

I won't be making any changes soon. Today I'm working on my line-following bot for the competition in a few weeks and I'm using the mini-mill to make a new servo mount and arm and get the steering code written.

 

[upand_at_them]

New motors arrived today. The H-bridge doesn't even get warm now. The V-reg only barely so. I even went back to my original gear ratio. I'll get rid of the V-reg once I get some more AA cells. I hope to use it for testing proximity and line sensors soon.

 

[upand_at_them]

Strange behavior...I have to use a high duty cycle (>80%) to get the gearbox to move at all. Even with no load (off the floor) and even with the tank treads removed. I also noticed that if I stall one of the motors it causes the other to flip direction. Is this normal?

 

[upand_at_them]

Replying to myself here...The new Pololu motors like a higher voltage. They're quite weak at 3V. Feeding the H-bridge from the 7.2V battery is much better and I get a wide duty cycle range now.