Motion control amplifier choices
Selecting the right amplifier for a motion control application can make a big difference in both the cost and performance of the final system. Generally, two choices exist: buy an off-the-shelf amplifier, or design your own.
The amplifier-one of three basic elements of a motion system along with the controller and motor-converts output signals from the controller into higher power motor coil voltages that drive various motor types. Controller and amplifier (sometimes even the motor) can be combined into one unit.
Servo motor amplifiers accept aired current (measured by a Hall-sensor or a dropping resistor). Other functions performed by the amplifier can include short-circuit and over-temperature protection, torque control, and commutation.
Cost and convenience are the main tradeoffs. An off-the-shelf unit
involves no detailed knowledge of amplifier design. Designing your own
amplifier-particularly at low power levels-is not very difficult with
all-in-one H-bridge chips; and the cost savings can be tremendous.
Off-the-shelf, or design your own
An amplifier from stock is the most common design choice. Off-the-shelf units are available from numerous domestic and international sources, offering a wide variety of features and ranging from a cell-phone-sized block to an entire 19-in. rack.
Power output also varies from just a few watts to multiple kilowatts. Most off-the-shelf servo and step-motor amplifiers use switching drive methods to lower heat output. Some specialized units provide linear (transconductance) amplification necessary in applications that cannot tolerate electrical noise generated by switching power transistors.
It is common to purchase the amplifier and motor from the same company, but sometimes ‘mixing and matching’ from different vendors can offer better performance at lower cost.
The most popular choice when designing an amplifier is to buy an integrated H-bridge amplifier chip. These provide many functions found in box amplifiers, including switching logic, shoot-through protection, over-current and over-temperature sensing, etc. However, typical power output is limited, ranging from 1 to 3 A continuous (2-8 A peak).
This solution allows the amplifier to be designed into the same card that holds the motion controller. It saves space, but also makes machine service easier without interconnecting wires from controller to amplifier. In addition, this option is less expensive than an off-the-shelf amplifier-saving $100 or more per axis. Integrated H-bridges are available from several vendors, including Alegro, National Semiconductor, Siemens, STMicroelectronics, and Unitrode
As an alternative, amplifier design can begin by using discrete components. With this approach, MOSFETs (metal-oxide semiconductor field-effect transistors) or other switching chips are connected into an H-bridge or half-bridge configuration. However, the designer is responsible for all the ‘higher order’ functions such as shoot-through protection, charge pump, and over-current sensing. Designing in these extra functions can add substantial complexity to the project, but this option has the advantage of offering power output levels as high as needed (as determined by the switching ICs chosen and heat sinking). MOSFETs and other discrete power chips are made by International Rectifier, IXYS, Motorola, Texas Instruments, Siemens, STMicroelectronics, Unitrode, and others.
While most amplifiers designed today use digital switching methods (due to their power efficiencies), some applications require a linear power amplifier. It is important to remember, that for the same amount of applied power, a linear amp is much less efficient than a MOSFET switching amp. Typical efficiencies are in the 60-80% range, compared to over 90% for switching chips. This is why heat sinking becomes an important consideration with linear amplifiers.
When designing your own chip-based amplifier, H-bridge chips can simplify the task. However, to run at higher power levels using discrete MOSFET drivers requires substantial knowledge; it can still be worth the effort, if the application’s volume and price sensitivity warrant it.
|Chuck Lewin, president of Performance Motion Devices Inc. (Lexington, Mass.).|