Robot Steering

Introduction

Quite a few Robots have steering problems initially. These notes are to help you set up the mechanics and electronics.

In order to steer in a straight line, any machine which is driven by two motors driving separate wheels, must have wheels of the same diameter and these must rotate at the same speed when presented with the same driving voltage. The controllers have to assume this to be the case. A standard controller can’t compensate for mechanical problems, though this is possible: read on.

Testing: mechanical or electronic?

If your robot does not steer straight, the first thing you need to decide is whether the problem is mechanical or electrical. To do this, drive both motors from the same controller.

Disconnect one motor from its controller and re-connect it in parallel with the second motor so that both are driven from the same controller. Make sure that both wheels turn in the same direction, reverse the motor to make this happen.

You won’t harm the unused controller by leaving its motor output disconnected, but make certain that the motor wires are safely tied where they cannot touch each other or anything else.

Once you have both motors working from the same controller (in parallel) whatever you do at the controls will feed the same voltage to both motors and if the robot does not steer straight, the problem is mechanical. If the robot does steer in a straight line, then the problem is electronic and you need to check the setting up instructions for the DCI and controllers and make sure you have done this correctly.

Mechanical problems

Does the robot steer along the same path in forward and in reverse, as shown left?
If so, then the possibilities are:

• one wheel (the outer) is larger than the other
• one motor (the outer) is faster than the other
• One motor (the inner) has high friction: bad brushes, bearings or similar.
• One motor (the outer) has weak magnets: you may have put too much current through and de-magnetised it! That will make it go fast.
• Your gear ratios are different.

Does the robot’s steering bend one way in forward and the other way in reverse? This is shown in the second diagram.
If this is the trouble, then your motors have offset brushes, see our FAQ sheet.

Electronic cures for these problems

Fitting tacho feedback around the motor will allow the controllers to electronically compensate for these mechanical problems. See our FAQ sheet for more information.

Electronic problems

Generally there are few mismatch problems in the electronics providing these have been set up correctly. So check that you have everything set as instructed.

Tacho feedback

Our circuits archives over at 4qdtec.co.uk have a circuit for a tacho feedback system which 4QD can supply as an additional plug-in board to suit Pro-120 or NCC series controllers. Fitting tacho feedback can compensate for mechanical problems, but its main advantage is that it enables your machine to deliver full power at slow speeds as the system measures the actual rotation speed of the motors and keeps this at the required speed by adjusting the voltage fed to the motors. So you can have full motor current and still be travelling at slow speeds!