What is Peristaltic Pump Control?

The motion control challenge involved with peristaltic pumps derives from two specific characteristics of how the pump functions. The first is that these pumps typically rotate slowly, 60 RPM all the way down to a complete standstill depending on the application. The second is that the load experienced by the motor varies dramatically as the squeezed tube engages, or disengages from the roller.

The graph to the right shows this. For most of the rotation cycle the rollers traverse the tube without major changes in reflected torque, or pressure in the transferred liquid. However at the points where the roller disengages from contact with the tube and then re-engages with the opposite portion of the tube there is a large pulse of both torque and fluid pressure.

There are several motion techniques to address these challenges. The first is use of a position control loop rather than a velocity control loop to handle the slow rotation speed. Especially when using digital position encoders, velocity estimation becomes increasingly inaccurate as velocity lowers so most controllers use a position control technique such as a position PID loop. Nevertheless if a high resolution encoder is used velocity loops may provide acceptable performance even at low speeds, and may have some advantages for delivering a more consistent flow of fluid.

Another common technique is the use of direct drive motors to rotate the pump. Brushless DC motors with high resolution encoders driven by a position servo loop can achieve very high accuracy and can react quickly to the dramatic changes in load. By comparison gear motors inevitably have compliance of the engaging gears which means they will not be able to follow their commanded position profile as accurately.