Technical Note:
Motion Electronics Design

Introduction

Motion control systems consist of three main components: motion controller, motor driver or amplifier, and a motion device. The primary purpose of a motion controller is to control the dynamics of the motion device. The motor driver converts the command signals from the motion controller into power signals required to drive the motor. The motion device is any mechanical device that provides motion and is actuated by a motor. Such motion devices typically contain feedback devices to provide information such as position and velocity to the motion controller (see Figure 1). Each section is further explained below.

Motion Controllers

In a motion system, the motion controller is used to control motion devices such as stages and actuators so that they move or stop in a desired manner. If the motion system is equipped with position, velocity, or torque sensors, the signals from these sensors are fed to the controller. In servo systems, the controller compares the actual signal to a desired value and takes corrective actions.

Common motion systems use three types of control methods: position control, velocity control, and torque control. Position control is used in applications where precise positioning and position tracking are of utmost importance. In these applications, the primary feedback device is an encoder. Velocity control is used in applications such as spindle, conveyor belts, and such where velocity regulation is of primary importance. In these applications, the primary feedback device is a tachometer. Torque control is used in applications such as robotics where the torque applied by end-effectors must be controlled accurately in order to grasp or release objects. In these applications, the primary feedback device is a torque/force sensor such as a strain gauge.

A majority of Newport’s motion systems use the position control method. They use both encoder and tachometer feedback to attain high levels of positioning as well as velocity regulation. The purpose of the motion controller in these systems is to command a motor so that the actual position of the moving mechanism tracks the desired position specified by a preplanned trajectory.

While the main objective of a motion controller is to control a motion device, many advanced motion controllers provide various additional functions such as:

  • Trajectory generation for moving devices from one point to another or for coordinating the motion of multiple devices
  • Interface to let users configure and command the motion system to perform various tasks
  • Monitoring end-of-travel limits, amplifier faults, feedback errors, etc. for safety of the system
  • Digital input/output lines to synchronize external events to motion or vice versa
  • Memory for storing and running on-board motion programs

Furthermore, the output of the motion controller can be configured depending upon the type of motor used to move a motion device.

To control stepping motors, most motion controllers send two digital signals to the motor driver (amplifier). The driver must interpret these signals and provide appropriate commutation to rotate the stepping motor. These signals can be used in one of two ways:

Step and Direction – one signal is pulsed to command the driver to step the motor and the other indicates the desired direction of motion.

Plus (CW) and Minus Pulses (CCW) – one signal is pulsed to command the driver to step the motor in the positive direction, and the opposite signal is pulsed to command the driver to step the motor in the negative direction.

A stepper motor control system does not require position feedback. The motion controller can simply provide the correct number of pulses to rotate the stepper motor the appropriate amount for a desired move. However, for improved positioning accuracy, a more sophisticated stepper motor system can also incorporate a feedback system (e.g., shaft-mounted rotary - or glass-scale linear -encoder) that can be used to directly monitor the position and provide the motion controller with actual displacement information. The motion controller can then provide a quasi-servo closed-loop positioning system that adds or subtracts output pulses to the driver to correct for positioning errors.

For DC Servo motors, conventional motion controllers feed an analog voltage to the motor driver that varies from –10Vdc to +10Vdc. This command signal is often referred to as the DAC control signal. The motion controller adjusts the DAC output in order to make the actual position of the motion device accurately follow a desired position. (See Closed-Loop Control section) To control servo motors in positioning applications, the motion device (i.e. stage) must provide some type of position feedback.

A third mode used for brushless DC servo motors requires the motion controller to send two DAC control signals. These two sinusoidal signals are shifted 90° (or 120°) out of phase and used to directly commutate the motor. This method can also be used to commutate stepper motors, eliminating the need for complicated driver electronics.

Motor Drivers

A motor driver receives input signals from a controller and converts them to power to drive a motor. A motor driver can be a simple amplifier or it can be an intelligent device that can be configured through software for varying operation parameters. There are three classes of motor drivers available to support the different types of motors used in motion control.

The stepper motor drive receives input signals from the motion controller commanding it to step the motor to a commanded position. The stepper motor drive then applies current to the stepper motor windings in order to move the stepping motor to the next step (increment). This basic operation is known as the Full Step operation of a stepper motor. In this mode, if power is removed from the motor, the stepper motor will not move significantly from its current position due to its inherent holding or detent torque.

A more sophisticated stepper motor driver is capable of applying current to both windings of the stepper motor simultaneously. Proportioning the current of the two windings allows precise control of the position of the motor rotor between detent positions. Using this method known as Microstepping, the motor driver can divide the input step command by 1 to 1000 microsteps. This provides a much higher positioning resolution for stepping motors and minimizes resonance problems inherent to stepper motors over the speed range of the motion device. However, in this mode, if power is removed from the motor, the motor will move to its closest detent or full-step position.

A simple four-phase driver is suitable for basic, low performance applications. But, if high speeds are required, quickly switching the current with inductive loads becomes a problem. When voltage is applied to a winding, the current (and therefore, the torque) approaches its nominal value exponentially (Figure 2). When the pulse rate is fast, the current does not have time to reach the desired value before it is again turned off, so the total torque generated is only a fraction of nominal.

The time required for the current to reach its nominal value depends on three factors: the motor windings' inductance, its resistance, and the voltage applied.

The inductance cannot be reduced, but the voltage can be temporarily increased to bring the current to its desired level faster. The most widely used technique is a high voltage chopper.

If, for instance, a stepper motor requiring only 3 V to reach the nominal current is connected momentarily to 30 V, it will reach the same current in only 1/10 the time.

Once the desired current value is reached, a chopper circuit activates to keep the current close to the nominal value (Figure 3).

Drivers for DC Servo motors simply convert a –10Vdc to +10Vdc analog control signal from the motion controller to a usable current to drive the motor.

Most brushless DC Motor drives are simple amplifiers that convert control signals from the motion controller to a usable current to drive the motor, with the motion controller providing the motor commutation. In some applications, however, the motor drive is an intelligent device that receives an analog input similar to the DC servo motor drive. In this case, the driver must have some internal microprocessing capability and requires feedback from the motor in order to commutate it.

Brushless DC motor drives are available in three basic types. One type accepts a single analog ±10 Vdc control signal (which represents either velocity or torque) from the motion controller and Hall effect signals from the motor, which is needed for commutation reference. Another type accepts two analog ±10Vdc commutation signals from the controller and, therefore, does not require Hall signals from the motor.

Lastly, there are intelligent drives that can “self-commutate” (generate its own sine and cosine commutation signals). These drivers are very flexible and can use the stage’s encoder feedback or Hall effect signals for motor commutation.

Feedback Devices

A feedback device’s basic function is to transform a physical parameter into an electrical signal for use by a motion controller. Common feedback devices are encoders for position feedback, tachometers for velocity feedback, optical or mechanical switches for end-of-travel information, index signals for a fixed reference position, and hall effect sensors for brushless motor phase information.