Motion Control System

Drive is a common term used in the context of Motion Controlling, for instance, AB PowerFlex 700, Danfoss VLT AQUA Drive FC 202, etc. These AC or DC Motor drives are used for controlling the speed or velocity of the Motor, however, Motion products such as Kinetix or SEW Servo drives control the position of a motor. Applications such as filling machines that require repeated, precise movements are good candidates for motion control not only at low speeds but also at very high speeds, for example, VCI 250 Master-Tech Wolf Machine whose speed is 150 bags/min.

At its simplest level, a motion control system typically consists of :

• A motion controller
• A servo drive that is connected to the motion controller
• A servo motor that is connected to the drive
• A load that is connected or coupled to the Servo Motor

A motion control system is usually comprised of two Closed-loop Control Systems. First, the motion controller issues a velocity or torque command to the servo drive, and the servo drive provides the position from the motor’s encoder as feedback.

The servo drive commands the motor by providing current or torque while the motor’s encoder responds with another closed-loop feedback signal to report its actual position. Typically, the motor is connected to some sort of an actuator, which is the part that moves the load, the motor will turn the actuator to make the load move.

A drive, motor, and load altogether comprise an axis. Axes move either in linear or rotary directions.?In a Linear axis, the movement of the load is along a straight line while a rotary axis rotates about a center point. Motion controllers can be configured to reset their position units to zero at a predefined position. This predefined position is called the unwind or rollover position.?

The rate of acceleration and deceleration are important factors to consider when selecting a motor and drive for an application.?If the motor or drive chosen is too small, the system may not be able to accelerate the load fast enough to make the desired production rate. On the other hand, deceleration requires kinetic energy to be pulled out of a moving system, the load may coast out of control and damage equipment. Servo motors produce torque, the amount of turning force needed to overcome the inertia and friction to accelerate the load.

A motion Profile defines how the load is expected to move. A highly dynamic motion profile requires rapid changes in velocity, and has high inertia, it will need high motor torque, and therefore, a larger more expensive motor. The amount of torque required to move an axis is a function of how fast the load needs to be accelerated. Torque can be minimized by minimizing the inertia of the system. The total inertia of a rotary mechanical system consists of the inertia of the motor as well as the inertia of the load. Load inertia can be reduced to either reducing the mass of the load or reducing the size or radius of the load.

Motion Feedback Devices

Motion feedback devices are used to report actual position data back to the motion controller. There are two primary methods for obtaining position feedback, via encoder and auxiliary encoder. Linear motion feedback devices are typically embedded directly into linear motors and actuators. Common rotary motion feedback devices are resolvers and encoders. Encoders being the most commonly used feedback devices.

An encoder is a device that converts mechanical motion into electrical signals to determine machine position and/or velocity. Motor encoders provide motion feedback signals that include motor commutation, position, velocity, and acceleration feedback. Basic rotary encoder construction consists of a source of light, a focusing lens, a light detector, an etched rotating disc that determines resolution, a signal converter, and an output driver. As the shaft rotates, LED Light passes through the code disk and is detected by the photodetector. This signal is then converted to a corresponding pulsed output. Incremental Encoders and absolute encoders are the two types of encoders. Incremental encoders generate a train of pulses. Absolute Encoders retain position even after power has been removed and then reapplied. Absolute Encoder can be based on Optical, Magnetic, or Capacitive technology.

The output of an encoder can be transmitted in either parallel or serial form. Communication protocols for serial output encoders are SSI, EnDAt, Hiperface (High-Performance interface), etc.

Servo Drives can be integrated with the control systems via SERCOS, EtherNet/IP, Modbus, DeviceNet, etc.