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Test Systems Move Out
Adding motion control to a test system requires planning.
By Jon Titus -- Design News, October 11, 2004
Many electronic-test sequences require movement: Think of positioning a printed circuit board (PCB) in a test fixture and then pressing it against a bed of needle-like probes that connect the PCB to test instruments. Although engineers often understand how motors work, they may not know how to integrate motion control with other portions of a test system.
Almost all motors connect to mechanical devices such as X-Y stages, robotic arms, and other devices that interact with a device under test (DUT). After outlining a test application that requires motion, system designers must choose the right type of motor. As designers ponder motor choices, manufacturers can provide applications assistance, which may include drawing on their experience automating test apparatus.
After selecting a motor type, test-system designers must look at driving circuits, or "drives," that generate the voltages or currents that cause a motor's shaft to move. In effect, a motor's associated drive circuits amplify signals produced by a computer interface. A drive's output signals cause a motor to move in a specific direction with a given velocity and acceleration, all preset under software control.
Feedback loop
Each type of motor relies on unique drive circuitry. A small stepper motor, for example, requires a drive that produces several synchronized high-current square waves.
To ensure proper operation, many drives require motor-position information, often supplied by a quadrature encoder, as part of a control loop. The feedback loop ensures the motor moves properly when energized and it holds the energized motor's shaft in place once it reaches its destination. Brushless ac motors require current feedback to ensure proper torque output during shaft rotation. An ac brushless motor also requires current commutation—the control of currents in the motor's coils. Commutation applies the optimum sine-wave currents in the coils to move the motor's shaft with the correct velocity and torque at a given time. (This type of control requires sophisticated intelligence in the drive.) To simplify motor-drive connections and to ensure compatibility, designers often procure motors and drives from the same supplier.
Many drive circuits accept voltage signals from an interface board that plugs into a PC bus such as PCI or an instrumentation bus such as PXI. Some drives communicate with a host controller or PC using serial protocols such as CANOpen or IEEE 1394 (Firewire). Designers also can choose from basic drives that offer fundamental motion operations or "smart" drives that directly accept motion commands and set up complicated motion sequences. Smart drives are expensive, and system designers often can perform the same operations—at lower cost—using software on a host controller or PC.
Find total cost
Motion-control software can come from several sources: 1. Drive vendors supply programs that control motors, usually through a set of proprietary commands. 2. Third parties sell general-purpose test software that incorporates motion-control routines and algorithms. 3. Several independent vendors offer Visual Basic or C/C++ motion-control libraries 4. Integrators can provide custom code tailored to specific needs.
The resulting application programs causes a motor to move something from point a to point b. Developers usually have complete control over characteristics such as a specific trajectory, motion profile, velocity, acceleration, and so on. (Motion characteristics go beyond the scope of this column.)
After engineers analyze their needs and hardware and software choices, they must evaluate a system's total cost, which includes more than the cost of motors, drives, software, and so on. Total cost includes program-development time, training, maintenance, and so on. And engineers must make tradeoffs. If equipment includes unneeded functions and capabilities, it may cost less to build or buy a bare-bones controller and customize it for a system. But, if engineers take the customization route, they must budget added development costs.
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| All in One: Automating motion in a test system requires a motor, drive electronics, a computer to control the drive, and intelligent software to put motion commands into effect. In this case, the motor produces a linear motion. |
Custom vs. standard
Although off-the-shelf components may at first cost more than a customized system, standard components can save money: They're easy to repair, replace, and upgrade without reengineering hardware or rewriting code. The chosen development path often depends on how many systems a company plans to deploy. As a rule of thumb, for 100 or more systems, the custom-controller route pays off. But, if you expect to need only a few systems, think about taking the standard-product path.
Whether you develop a custom controller or buy off-the-shelf components, in all likelihood the resulting motion-control system will interact with other electronic equipment. In a packaging system designed for the beverage industry, for example, the system controller had to manage 50 digital I/O lines, three motors, and three cameras. That type of system calls for real-time control of devices and careful timing of measurements. Such systems often rely on a real-time operating system.
Even when an application calls for only a simple stand-alone motion-control system, invariably someone will want to add machine-vision or electrical-measurements capabilities.
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