Steering Motor Signal Question

[Tengato]

ROBOT Forum:
I'm trying to control a steering motor that turns front wheels the way basic RC cars do. It's a small 6V motor connected to a high ratio gearbox and a simple mechanism that converts the rotation into the left/right steering action. Also, there is a spring that centers the steering position when the motor is relaxed. The thing that's throwing me off is that the range of motion is very mechanically limited - the motor can't turn more than 5 revolutions before the steering mechanism blocks it. Imagine a scenario where I need to keep the wheels turned to the right for a long time; if I simply apply a constant voltage to achieve this, the motor will be stalled nearly the whole time. This will probably wreck the motor and the H-bridge and drain power from the rest of the robot.

So... what kind of signal should I send to the motor? The only thing I can think of is a pulse to maybe allow the spring to keep the motor from stalling.

Thanks for any suggestions!

[hgordon]

It sounds like you're trying to run your motor as a servo without the benefit of feedback. You need some kind of return signal from the steering mechanism which can be used as the basis of an "error" signal (the difference between actual and desired position) for generating a control signal to the motor.

[HarryBotter]

I'd think the simplest way to set up akermann steering on your bot would be with an ordinary RC servo using PWM. You wouldn't need a spring and could implement proportional control over the whole steering range.

If you want to modify your setup it would help if you told us how you're controlling the motor now. With a bit more information I'm sure there are others here who have resolved similar challenges that would be willing to help.

[Tengato]

I just want to replace the circutry in a toy RC car with my microprocessor. I only need three levels of control over the motor: left, right and off, so I'm trying not to over-engineer this. If I had an oscilloscope I could just see what kind of signal is coming out of the toy's original circutry... I'm pretty sure it's a pulse but I don't know the frequency or the duty cycle %.

[HarryBotter]

I'm not sure how pulsing the motor would help if you don't have a way of telling the microprocessor where the steering arm is. You'd need feedback of some sort whether from a pot, a limit switch, a measure of load on the motor or some other way of detecting steering arm travel.

[hgordon]

A typical hobby servo takes a pulse that ranges from 1 millisecond to 2 milliseconds, where a pulse of 1.5 milliseconds should center the servo, while longer or shorter pulses will move the servo to one side or the other. Generally, you want to update the pulse about 20 times per seconds. Some servos work over a wider range (0.5 - 2.5 ms), but still center at 1.5ms. You should be able to write some simple software to generate this pulse, and some microcontrollers already have servo interface routines built in.

[carterson2]

Hi,

I want to steer my lawn mower. Its a snapper so I can extend the steering axle...

Q:What kind of motor should I get????


I have no clue.

Any leads appreciated.

jpruett
http://www.gpscruise.com

[Kevin Berry]

A lot of this depends on how you are going to power it. The simplest approach might be to raid the power window motor and rack/pinion from a junkyard, and use a cheap 12V Sealed Lead Acid. Those motors are used extensively in medium to big bots as a power source.

Another choice, again you'd have to gear it down, is a hacked Harbor Freight drill motor with it's battery pack. Those run at 600 rpm so you'd need another reduction stage.