In many applications of electromechanical actuators, knowledge of the linear position is essential. As the gear ratio between the rotational position of the shaft and the axial position of the piston can be considered as constant, it is possible to obtain the linear position by assessing the angular position of the motor shaft.
Which options for motor feedback do we offer?
There are different options to obtain the rotational position of the shaft. Besides the physical principles to measure the rotating angle these differ in their ability to provide absolute or only relative values of the rotation. For the IMI Norgren ELION range, two types of feedback systems are available. One relative feedback system, called ‘Resolver’ and an absolute multi-turn feedback system with an EnDat-interface.
How does relative feedback work?
Relative feedback systems measure the rotational position during one rotation only. After one revolution of the shaft, the step counter is set to zero again. The measuring is based on the phase deviation of the induced current in two measuring inductances. To gain an absolute position, the software of the drive counts the number of total revolutions and adds this to the zero position. Both end positions must be taught during a homing sequence. This can be realized for example by a low torque movement from one end position of the actuator to the other. The end positions are determined from the motor current. The rotational positions of the end positions are then saved in the drive. In case of a power down, the absolute position is lost, and a new homing sequence is mandatory.
The system offers a rotational resolution of 12 bit, i.e. one rotation of the shaft is divided into 4096 measuring points leading to an angle of 0.088° per step. Neglecting deviations in spindle pitch, this equals a linear resolution of 0.097 mm/step.
How does absolute feedback work?
The absolute feedback system is based on an inductive measurement principle. A high frequency signal is modulated regarding its amplitude and phase. The rotational position is calculated from the signals of different inductances on the circumference. In contrast to the resolver, the absolute feedback measures the total number of rotational steps, thereby measuring an absolute angular position. This position is stored in a non-volatile memory. In case of a power down, the position stays saved, and the positional feedback does not need to run a homing sequence when power is back up again.
The rotational resolution of the EnDat-Encoder is 19 bit, i.e. one rotation is divided into 524,288 steps, equalling 0.00069° per step. Assuming a constant pitch of the ball screw, this leads to a maximum resolution of 7.6*10-5 mm per step at the maximum pitch of 40 mm. Due to deviations in the pitch, the accuracy of the measurement of the linear position is lower.