X-Y-Z: Analyzing Amplifiers
Have you ever played the gossip game? You tell someone that you saw Arnold Schwarzenegger at a restaurant, with instructions to pass it on. Twenty versions later, you discover that you’re co-starring with "Ah-nold" in "Terminator III."
The amplifier in a motion control system works something like that, but without corrupting the message. The amplifier takes a low-voltage command signal from the controller and amplifies it into electrical power to drive the motor.
Amplifiers have three operating modes: torque, velocity and position. Some amplifiers support only one mode; others support all three. Depending on the motor, amplifiers can also perform commutation.
Amplifiers generate current for the motor in two ways. Linear amplifiers produce a constant output current. Pulse-width-modulated (PWM) amplifiers generate a current that switches between high and low output levels. Most amplifiers, particularly those with power ratings above 100 watts, use the PWM method, which minimizes power losses. Linear amplifiers are more common in low-power applications.
"PWM amplifiers have a much higher power capability for a given volume than linear amplifiers. PWM amplifiers are also less expensive," says Ron Rekowski, director of product marketing for the Motion Control Div. of Aerotech Inc. (Pittsburgh).
Linear amplifiers are required when it’s important to minimize radiated noise. For instance, a high-resolution ultrasonic inspection system may have a high-gain transducer that is sensitive to electromagnetic interference (EMI). By decreasing EMI, a linear amplifier can increase the in-position stability of the machine’s gantry.
Linear amplifiers are also useful for driving very small, brush and brushless motors. PWM amplifiers can become unstable with these low-inductance motors.
Thanks to improvements in microprocessors, some controller functions, such as servo loop control and trajectory generation, can now be performed by the amplifier. Removing this computational overhead from the controller frees up processor bandwidth, enabling the controller to scan I/O and process programs more quickly. It also improves reliability and simplifies troubleshooting.
According to Jan Bosteels, product manager for Advanced Motion Controls (Camarillo, CA), putting intelligence into amplifiers is analogous to the mainframe vs. PC revolution 20 years ago. "In the past, the central controller generated all the trajectories and closed all the position loops. The amplifiers were like dumb terminals," he says. "Now, position loops are closed in the drive, and some trajectory generation is done in the drive, and the controller is becoming like a server with only top-level responsibilities."
When specifying a motion control system, engineers should match the amplifier with the current and voltage required by the motor. Engineers should also consider the amplifier’s peak and continuous current ratings.
The peak current is the maximum output current of the amplifier over a short interval. The amplifier cannot sustain this output for long. It is only delivered during the acceleration and deceleration of the motor.
The continuous rating is how much current the amplifier can deliver at a constant level. For example, if a motor must hold an object in place against gravity, it must generate a continuous force. The maximum continuous force it can provide is the continuous current rating Arial the motor torque constant. A better example of applying the continuous current rating is to look at the root-mean-square (RMS) current required for a series of moves. This is done by examining the motion profile (acceleration time, constant speed time, deceleration time and dwell time) and calculating the RMS value of the current. This will determine if the amplifier can produce enough power for the application.
Another key piece of information for selecting an amplifier is the electrical specifications of the motor. "Most PWM amplifiers have a minimum load inductance requirement to keep the current loop stablethe inductance filters some of the effects of the switching," Rekowski says. "For linear amplifiers, you need to know the resistance of the motor to calculate the amount of power dissipation in the amplifer transistors."