RTC Magazine

The world is in motion. A robot transporting delicate silicon wafers during semiconductor processing, a large telescope automatically tracking star position, an automatic sewing arm that precisely stitches quilts, steering control of an autonomous vehicle that tracks position targets from a GPS; the applications are endless.

While the applications are varied, what they have in common is that they require precise and automatic motion control. At the heart of each application is a motion control system, which includes an intelligent motion controller. It is the job of the controller to make sure the mechanical devices get to the right place at the right time. How can a motion controller handle such varying loads and varying types of motion?

There are five elements that make up a typical motion control system: motion controller, amplifier, motor, feedback sensor and a host computer or HMI. Figure 1 shows the block diagram of a simplified motion control system. Each of the elements in the system performs a different function. The Host computer or HMI sends high-level commands such as motion specifications to the controller. The amplifier receives a command signal from the controller and translates it into the appropriate power to drive the motor. The motor converts the electrical energy from the amplifier into torque, which is transferred to the mechanical system. The feedback sensor provides position information, which is sent to the controller. The controller is the brain of the system and processes information received from the host computer and feedback sensor.

There are two main functions of the motion controller. First, it provides a reference position, a function commonly referred to as the motion profiler. Secondly, it compensates for the position error, which is the difference between the reference position generated by the profiler and the actual position received from the feedback sensor

Closing the Loop

Looking at the complete motion control system, we can see that the size of the motor must be selected to accommodate the size of the load. Similarly, the power drive for the motor must be sized to meet the torque and speed required to move the load. The motion controller must be able to perform the two main functions described above and send a proper signal to the motor drive regardless of the size of the load.

When operating in what is called an “open-loop” mode, the motion controller only provides a reference position and does not compensate for error or a difference between the reference and actual position. This mode is most commonly used when controlling stepper motors; where under normal circumstances the actual position of the motor can be assumed equal to the reference position. In servo motion control systems, the actual position feedback sensor is used by the controller to determine the position error. This is “closing-the-loop.” In order for the system to be stable, the motion controller must provide compensation for the position error. The most typical compensation is proportional integral derivative (PID).

The PID filter, as seen in Figure 2, creates three command signals based upon an error in the system. The Proportional (KP) signal is a direct multiple of the error, the Derivative (KD) is a multiple of the rate of change of the error, and the Integrator (KI) provides a command based upon the error integrated over time. The proportional term provides system stiffness, the derivative term provides damping, and the integral provides position accuracy. Proper tuning of a motion control system means that the KP, KD and KI terms are adjusted to achieve the best performance. Many motion controllers offer tuning software, which automatically selects the optimum PID parameters. It is important for a motion controller to have a robust PID tuning algorithm with adequate range and resolution of KP, KI and KD parameters to accommodate a wide range of amplifiers, motors and loads.

Motion Profiling

The other important task of a motion controller is motion profiling. Motion profiling is where the controller generates the reference position. Here, the controller receives motion parameters such as distance, speed, acceleration rate and deceleration rate from a host computer or HMI. From these specifications, the controller computes a continuous trajectory of reference positions.