Motor/Servo for Tank Tread Robot

[mr_kobayashi]

Hi

I need help in selecting a motor for a robot with tank treads. The main constraint is I can't use more than 4 AA's and the robot weighs about 7kg so I'm thinking of using two hi-tech 311 servos for the two front wheels since they look to give quite decent torque and RPM. What do you experts think about this? Is a DC motor superior? I'm considering those as well, but the ones I found that have an operating voltage of 6V or less have high RPM but low torque, even considering with gearing the torque for two servos seem superior (not to mention less hassle!).

Cheers

[Ro-Bot-X]

If you want a motor, you can get a 6-12V geared DC motor. There are different gearing rates with different RPM and torque. Motors can be used with less voltage than nominal and a little higher sometimes. You will also need a motor driver and will use 2 or 3 controll pins from the microcontroller. Cheap but powerfull geared motors you can get from electric screwdrivers (7.2V will work great at 6V).

Servos have the driver and the gearing inside and use only one controll pin from the microcontroller. Also they move as you send them pulses, so you can measure the distance traveled for one pulse and then you will be able to send x pulses to travel a certain distance, or to turn a certain number of degrees. Of course, this works if there are no obstacles in the way, but you will use sensors to determine them...

[Admin]

This tutorial is designed to help you calculate an answer:
http://www.societyofrobots.com/mechanics_dynamics.shtml

For treads, there is a lot of high friction, so you probably want your motors 120%+ stronger (multiply requirements by 1.2+) than what you calculate.

Search this forum for treads, many would argue it isnt worth the effort.

[hgordon]

It is not practical to expect that you will be able to drive a 7kg robot with treads off 4 AA batteries.  Depending on your target torque and speed, I'm guessing that you will need at least 4 amps of drive current at 6 VDC, and stall currents will probably exceed 10 amps.

[Admin]

I agree with hgordon.

[mr_kobayashi]

OK thanks for the tips, I think I'll just have to reduce the weight of the robot.

This is a bit off topic, I noticed on the tutorial here on modifying servos for 360 degree rotation, it doesn't involve replacing the pot with a resistor ladder which somebody told me is the way to do it. What is the difference between doing that and just gluing the pot in place? Is that method just better for the $50 robot?

[hazzer123]

Its just a bit easier to glue. The glued variable resistor acts like a fixed reisistor. No soldering involved.

[mr_kobayashi]

OK but both methods should work the same right? Essentially there are no differences if you do either method correctly?

[hazzer123]

Yes you are correct