# Motor, power, and control for small simple robot

I want to make a small, simple, walking, single motor driven robot for a project in a 400 level ME class, I need to order these parts soon and was looking for some help in the selection.

I want the motor to turn at about 30-60 rpm under load. I’m not sure what the load will be but suspect easily under 1 kg-cm torque. I was thinking this servo motor should work well:

diybin.com/products/TowerPro … Servo.html

The main question is what should I use to power and control it. I really just want the easiest and cheapest solution as this is already going well beyond the prof’s expectations for this project. A simple on off switch for control would suffice.

Then I wanted to use a rubber band to loop the motor drive shaft into the leg drive shaft, are there any simple band-holding wheels that can be attached to the motor drive shaft easily?

I decided to go with this stuff:

motor and gearbox: pololu.com/catalog/product/994
pulley set: pololu.com/catalog/product/109
rocker switch: pololu.com/catalog/product/1406
mounting bracket: pololu.com/catalog/product/1086

Then I can use a 20:11 pulley ratio for 55 rpm no load, and the torque should be over 7 kg-cm.

I can solder a standard 9v battery to the switch and switch to the motor. My only question is how is the internal resistance of the motor work? would I need to add a resistor to the circuit to prevent going above 1.6A? If not, what amperage do you think the motor would run at with standard 9v battery? I guess something around 1A at 9V would be pretty ideal.

Hello.

You should definitely not use a standard 9V battery for driving motors, especially when you’re talking about wanting to deliver an amp. I recommend you use a NiMH battery pack with an appropriate voltage and capacity for your particular application. You might find this blog post on battery capacity helpful.

While these motors are intended for use at 6 V, there is generally some flexibility in their operating voltage, and we say you can use them at voltages from 3 - 9 V. Basically, the higher the voltage you run them at, the shorter their lifespans will be. I do not recommend using a resistor to limit the current as you’re just dumping power that way (and finding a resistor that can handle the kind of power you’re talking about is not necessarily easy or cheap). Instead, you should lower the voltage you are using, or you should consider using a motor driver or motor controller to limit the motor power.

The torque should vary approximately linearly with current, so you can generally treat the torque-vs-current curve as a line between zero torque at the free-run current and the stall torque at the stall current. You should also consider what kind of speed you need and pick the motor that can deliver the torque you want at your desired speed. If you don’t need a lot of speed, get a gear ratio that provides more torque so that your motor will draw less current.

- Ben

Thanks Ben, that helps a lot. I don’t want to spend much on batteries so that’s why I was hoping for using a standard 9v. I don’t think it’ll use more than 1 kg-cm of torque and the motor is rated for 5 kg-cm, plus I’ll use a 11:20 pulley ratio so the torque should be over 7.5 kg-cm at least, I’m guessing that’s at 6v 1.6A then.

So do you think a straight connection to a 9v would be fine with 1 kg-cm? Also, I’ve got one that’s running at 8v right now.

Otherwise what cheap, compact, 6v battery/battery pack is there?

I don’t know what this means.

No. I’m not sure how to be more clear than what I said in my previous post: “you should definitely not use a standard 9V battery for driving motors.” You’re welcome to try it, but I don’t expect it to work well, and I think it’s a really poor choice.

I linked you to some appropriate batteries we carry, and you can search around for cheaper ones elsewhere. I recommend you stay away from alkalines and instead look for NiMH cells or battery packs.

- Ben

I don’t know what this means.[/quote]

The robot shouldn’t require more than 1 kg-cm or torque to drive it. With the specs of the motor+gearbox and the pulley ratio I’ll use to connect the gearbox output to the robot’s drive shaft it should be able to provide over 7.5 kg-cm (safe estimate) of torque depending on how well it scales with the pulley. Since the motor is rated 1.6 amps, and you say the amperage scales linearly with torque I’m guessing it would only use 1.6 amps if it was delivering over 7.5 kg-cm of torque to the robot’s drive shaft. At 1 kg-cm of torque which I estimated it will use it should be using a maximum of .213 apms. Then I’ll only be using a battery that’s delivering 8v currently.

I know a 6v NiMH is ideal and you’d have to recommend that; but what I’m asking is would it not be REASONABLE to use 8v alkaline at .213apms (less than 7.5 time it’s max) since it can work at 9v and 1.6 amps, I’m also not planning on running it that long.

What is an “8V alkaline”? Alkaline cells have nominal voltages in increments of 1.5V, so 8V is not a natural alkaline battery voltage. The sense I am getting is that your 8V alkaline is a partially discharged 9V battery, which would be completely unreasonable. 9V batteries are comprised of alkaline cells slightly smaller than AAAAs (that’s not a typo, I really mean quadruple-As). They simply are not capable of delivering high currents, and if you try to draw large currents, such as by connecting a motor, the battery voltage will drop significantly, potentially causing all sorts of problems and weird behavior.

As an aside, how could you possibly interpret my statement that “you should definitely not use a standard 9V battery for driving motors” as an indication that I might consider using a 9V battery “reasonable”? I’m not sure why you keep pushing for my approval here, especially after I very specifically said in my last post: “You’re welcome to try it, but I don’t expect it to work well, and I think it’s a really poor choice.” If I now say it’s reasonable, I’m either contradicting my previous statements, or my definition of “reasonable” is so weird as to not really be meaningful.

When you say something like “since it can work at 9V and 1.6 amps”, it sounds like you think the issue here is with the motor. My objection to your power supply (the 9V battery) has nothing to do with whether the motor can tolerate it (that is a separate issue).

- Ben

Okay, now I understand why it won’t work. I thought your objection was to the motor’s tolerance.

So besides a 5 cell, 1/3 AAA NiMH. Do you know of anything smaller that would work well? And would you expect the voltage to drop much if it did happen to use 1A, if so I’m thinking maybe a 7.2V cell would be better.

I don’t recommend 1/3-AAA cells for those motors, and going to a higher voltage will just make the problem worse.

- Ben