Providing accurate speed and rotation of a motor of a drilling assembly allows accurate monitoring of a drilling operation, for example, direction of drilling, rate of penetration, vibration, and stability of the drilling assembly. A bi-stable magnetic device is used to provide better control of torque and rate of penetration and prevention of slip-stick issues. One such device is a downhole motor that includes a stator and a rotor with a magnetic blade. The magnetic blade includes at least one of a magnetic edge, a magnetic surface, and a magnetic core, or is entirely or partially magnetic. A sensor is positioned in the stator to detect the passing of the magnetic edge of the blade of the rotor. Detection of one or more magnetic blades may provide rotational speed or other status information of the drilling assembly.