Controlling motors is a crucial element of automated manufacturing.
One control method uses traditional “clang-bang” motor starters, which are essentially on-off switches for high currents. For applications that require precise position and speed control of the motor, including the ability to hold full torque at zero speed, motion control, or servo control systems are employed.
Motion control systems are usually closed loop. The system includes a controller (computer), amplifier, motor and encoder. The controller contains the motion profile and sends commands to the amplifier. The amplifier also known as a drive, takes the low-voltage control signal and outputs the profile in sufficient current to drive the motor. Encoders are feedback devices attached to the motor that tell the controller the speed, direction and, sometimes, position of the motor’s rotor.
Pick a controller
Brad Smith, regional applications specialist, Delta Computer Systems (www.deltamotion.com), defines three types of controllers: dedicated-purpose motion controllers designed for a particular application, such as injection molding machines; general-purpose, “freely programmable” motion control computers; and general-purpose motion controllers with pre-programmed functions and flexible bus interfaces.
A dedicated motion controller typically offers a complete standalone solution for a specific operation, making it easy to implement basic motion functionality. However, these systems are very restrictive and can generally only be modified or expanded by the manufacturer.
“Freely programmable” motion controllers (essentially non-programmed computers) offer low hardware cost, but require more initial development and long-term support costs. Although freely programmable general-purpose controllers have the advantage of a high degree of flexibility—virtually any type of control scheme can be implemented—the downside is that the machine designer must shoulder the burden for all of the design maintenance.
Pre-programmed general-purpose controllers offer a good mix of flexibility and ease of use, and can be an ideal compromise for hydraulic system designers. The controller manufacturer develops and maintains the functions, allowing the machine designer to concentrate on machine control issues.
Chris Radley, senior product line manager at Rockford, Ill.-based Danaher Motion (www.danaher.com) offers several tips for selecting the correct motion control system.
“Just as a chain is only as strong as its weakest link,” he states, “so it is with the components that make up a machine or system. Motor and drive selection will have a significant impact on overall machine cost, size, reliability and throughput. There are a number of actions that designers can take to ensure that the components they specify will result in maximum reliability and productivity.”
Radley says to first define the machine structure and motion performance needs based on mass, force and motion profiles. It is important to consider that existing designs may not be able to handle higher performance levels offered by newer technology.
His next tip is to partner with a reputable supplier early in the design process. Choose one who has breadth of expertise, engineering resources and available product.
Third, design engineers should use the resources that manufacturers have available, such as software that can help solve motion system requirements and identify components that will meet those requirements. They can also work closely with the manufacturer’s application engineers, who can help with intangibles that aren’t necessarily covered in a software package.
Matching components is the next important design step, according to Radley. Mismatched motors and drives will result in underperformance and/or overheating. The drive should be specified at the same time as the motor, for each axis of motion. When that is not possible, or if it is overlooked during motor selection, it is vital to make certain that the drive specified later complements the motor and application needs.
Gary Mintchell, [email protected]