# How many encoders should I have?

I'm planning a robot vehicle and plan to get one or more motors that have encoders built in.

How many encoders do I need I really need in order to gather solidly accurate information about speed and distance? I'll get four if it makes a difference but I'm having trouble seeing much benefit beyond one. Would averaging across two or four encoders help to mitigate the impact of slipping wheels?

There's about a \$20 price difference with and without the encoder, so it would be nice to minimize the number of motors with encoders.

EDIT: This is for a four fixed wheeled robot.

• four fixed wheeled robot ..... do you mean that you are using skid steering? – jsotola Feb 14 '19 at 21:38
• That is correct. – Rob Reuss Feb 15 '19 at 17:37

Typically you will only need 2 encoders to estimate the velocity and the angle for a differential vehicle. If you are using more strategies than only the odometry for the localisation, you can estimate the slipping with your range sensors or your IMUs.

Using more encoders is not an strategy that I have seen many times, except for critical applications were the safety is a great requirement.

In any case, the answer will highly depend on the kinematics of your vehicle. I guess you might be thinking in a 4 fixed wheel vehicle. If that is the case, you will definitely need the 4 encoders, since slipping will be continuous when turning and not an exception. But again, if you are using a differential kinematics (which is way easier to model), I'd go only with 2 encoders.

• Thanks, that's what I was looking for. Edited the question to indicate that yes, this is four a four fixed wheel robot, so per your suggestion I'll get 4 encoders. Should be fun trying to make sense of the data! – Rob Reuss Feb 14 '19 at 16:33