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The motion control challenge involved with wheel-driven mobile devices differs in one very important respect from virtually all other motion control problems: wheels are not solidly attached to the surfaces they contact and rely on friction for creating motion. This means controlling the motor's angle position, velocity, or torque output doesn't allow the controller to perfectly predict the location and trajectory of the mobile device.
The primary reason for this is wheel slip, which means the surface of the wheel loses contact with the surface it is travelling on. A secondary reason is that the surface itself may be bumpy and uneven. The net result is similar - even if the wheels precisely rotate a distance that should result in (for example) one meter of robot movement, that actual distance travelled may be less or sometimes more. And even if the robot wheels are commanded to turn the robot to a precision direction it may end up pointing in a different direction.
The most common motion technique to combat this is to drive the wheels with a velocity loop controlled by an outer loop position controller. A velocity loop is critical for keeping wheels synchronized and moving at the desired speed. Compared to a position loop used for the same task, a velocity loop won't suffer from the problem of multiple wheels 'fighting each other' as a result of small surface skips and bumps. This is because velocity loops don't 'remember' and try to restore position.
The outer loop controller measures the robot's orientation and location and generates, for each wheel, a commanded angular velocity (again depending on the wheel configuration) to change the robot's direction or speed so that the system motion matches as closely as possible the planned robot travel path. GPS is a common method for determining the robot's position, but other devices such as magnetic guide tracks, cameras or lidar may be used. Gyroscopes are sometimes used to supplement these sources if highly accurate orientation information is needed.
The diagram below shows a generalized outer loop controller which uses the qa (actual robot position to generate the linear and angular velocities the robot has to follow to reach the trajectory). This robot motion information is transformed to velocity commands for each driven wheel and output to a velocity loop. The velocity loop, one for each driven wheel, inputs via an encoder the wheel position from which it derives the velocity and then calculates a velocity error which is passed through a PI (proportional, integral) filter and output as a torque command to the amplifier.
