Embedded control

Oct. 1, 2009
Most motion systems have at least some embedded control. Now evermore-powerful data processing and chip manufacturing is being leveraged — so increasingly sophisticated firm and hardware is being designed for embedding.

Noun: Application-specific software and custom-integrated circuits specifically designed and implanted to generate commands that bring forth the best possible performance from the machine to which they're devoted.

The strength of embedded motion control is that it reduces cost and the number of components needed to power machinery. Second, embedded chips are more easily configured to motor types, position ranges, and speed profiles than traditional systems.

Consider the AMIS-30623 driver, made by ON Semiconductor, Phoenix. This single mechatronic chip has both controller and interface to microstep connected motors. The device receives instructions through the bus, and drives motor coils to position. Its host microcontroller unit (or MCU) detects blocked rotor, stall, and end-of-run conditions without switches, Hall sensors, or encoders. The MCU is programmed to calculate rotor position, dynamically adjust current or speed, and prevent lost steps — to eliminate endstop noise, and boost accuracy without lots of components.

Other times, individual motion components carry embedded ID chips that communicate with the larger system. For example, in some servomotors from B&R Industrial Automation Corp., Roswell, Ga., all relevant mechanical and electrical data is stored on the motor's encoder chip. In-field setting adjustments are eliminated, because as soon as the encoder is connected to the drive and power, the motor (B&R's 8LSA, for example) is automatically identified. The motor sends ratings and limits to the servodrive, which automatically determines current limits and control parameters required for optimal motor operation. Figure 1

Similarly, embeddable cameras from National Instruments Corp., Austin, are preloaded with software for deploying machine vision and inspection without traditional programming. Some of their cameras — with help from a DSP coprocessor — can execute pattern recognition and run algorithms four times faster than comparable systems, without software changes.

In the same way, the company's servo and stepper modules for NI CompactRIO embeddable controls connect directly to fractional-hp dc servomotors, solenoids, and other actuators. At least one is an H-bridge module — which allows application of voltage in two directions, to run motors forwards or backwards.

Physical and software controls can also be embedded in robots. These usually work in concert with other portions of a larger distributed motion-control system through IEEE 1394 or other bus. Adept Technology, Inc., Pleasanton, Calif., embeds SmartServo-based controls in their robots. Amplifiers placed inside the robots further reduce robot footprint.

A different approach is to develop all firmware on a PC and then load it onto embeddable motion-controller chips once prototyping is finished. Software from The MathWorks Inc., Natick, Mass., lets designers deploy such code generated for realtime execution on embedded microprocessors, microcontrollers, and DSPs. The software can integrate peripheral devices and realtime operating systems with algorithms created through models or a language subset called Embedded MATLAB, without requiring designers to write runtime code and low-level drivers. Figure 2

Today: Integrated chips do away with pieces-parts. For example, pulse-width-modulated ICs from Cirrus Logic Inc. — Apex Precision Power SA57 and SA306-IHZ — simplify designs and drive fractional-hp dc motors with supplies to 60 V. They're suitable for automation, robotics, and positioning.

These control motor current on a cycle-by-cycle basis, to allow the power system to partner with the processor or DSP in realtime for each motor phase. This way, overall current doesn't have to be limited. Mirrors in each driver leg monitor current in all phases; should current exceed safe limits, the drivers shut down outputs for the rest of the switching cycle and reset them at the next cycle start. Cycle-by-cycle current limiting also allows motor soft starts by handling the inrush of startup current.

A Magellan motion control processor combined with development software from Performance Motion Devices Inc. improves OEM machines and reduces time to market. The processor is already the core motion controller in blood analyzers, wafer handlers, and 3D printers. One to four-axis chipset versions control a variety of dc motors. The Pro-Motion and C-Motion software has a programming library with C/C++ routines — and all code required to control the processor in an application. The software also allows application-specific C/C++ coding that's combinable with library source code. Datasheets are at www.pmdcorp.com.

Look for a followup on embedded controls next month, when we cover application specifics and talk with Tony Lennon, MathWorks manager for automation, about the role of development software in the field. Figure 3

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