Step motors are popular in part because they are low cost and easy to use. But experienced users know that if the system demands more torque than the motor has available, the motor stalls. Compounding the situation is the fact that stepper systems don’t require feedback, so other devices in the system are unlikely to be aware of the stall. However, by adding an inexpensive encoder, the drive can monitor the motor’s actual position directly, closing the feedback loop and avoiding many of the limitations traditionally associated with stepper systems.
By adding an encoder to the system to track rotor position, the drive can know precisely how far and how fast the motor has moved. Thus, the first use of encoder feedback is position and velocity verification. A PC or PLC can monitor position and velocity over RS-232, RS-485, CANopen, Ethernet or EtherNet/IP.
A step motor is constructed with a permanent magnet rotor that turns on bearings, and a stator, a series of electromagnets surrounding the rotor. When the drive sends electric current through the stator, the resulting electromagnetic field forces the rotor to react. If the stator current is properly controlled, a step motor can perform impressive feats, accelerating at precise rates to exact speeds, and stopping on a dime with great accuracy.
Because step motors produce less torque at high speeds, they can stall if overloaded. The second use of encoder feedback is the immediate detection and reporting of stalls. The driver can be configured to signal other equipment by the state change of a digital output or by providing this information over the communication interface.
The encoder also allows for real time monitoring of the torque producing lead/lag angle, the angular difference between the magnetic fields of the stator and rotor. Applied Motion drives can use this information to precisely control the motor’s torque and prevent unexpected motor stalls, the third and ultimate use of encoder feedback. Stall prevention is enabled by checking the “Stall Prevention” box in our ST Configurator™ set up software.
Encoders can be purchased with Applied Motion motors ranging from NEMA size 11 to 34. The encoder comes pre-assembled to the rear shaft of the step motor and connects via cable to the encoder feedback port available on many of our drives, including our DC powered ST5 and ST10 and the AC powered STAC5 and STAC6.