Motion Control and System Engineering Considerations
- Wednesday, 01 February 2012
Drawing the Dividing Lines
The criteria for selecting the dividing lines within the system are: 1) minimize the number of interfaces; 2) with the intentional interfaces being well defined, understood, and testable with understood margins; and 3) a suppression of inadvertent interfaces both as they are affected by other systems (victims) and as they affect other systems (aggressors). To the ability of the system team, the unintentional interfaces must be located and either eliminated or converted to intentional interfaces with testable margins. Unintentional interfaces are the most common root cause of integration problems when attempting to combine otherwise functional subsystems.
To meet these criteria, the subsystem boundaries need to be consistent for the electronic, mechanical, and software teams. Such common boundaries help enable interdisciplinary teams to work to the same goals. Critical system resources shared between different subsystems often leads to problems at integration time as the reality of the number of accidental interfaces within these shared resources is painfully learned. Sharing processors between multiple subsystems with hard, real-time requirements can reveal subsystems that work well individually but may become erratic or nonfunctional at all when integrated. Unintended interfaces in software include priorities and latencies where fixing or modifying one subsystem can break another subsystem. This leads to the need for much deeper regression testing following even a minor change as all of the subsections sharing the resource need to be verified.
Connecting the Parts Together Use common and well-defined media and protocols within the system. Select robust interfaces: for example, RS-485 supports higher speeds and longer runs than RS-232, and RS-485 also tolerates larger signal ground shifts. CAN is also differential with similar benefits, and has controlled latency and deterministic priority along with support for small packets and built-in error detection. Ethernet is both fast and isolated, but special protocols and hardware may be needed to implement fast signaling with guaranteed low latency.
When designing mechanical interfaces, consider using six-degree-of-freedom mounts, also called kinematic mounts. When combined with software alignment, this approach can simplify assembly and maintenance, allowing a part to be removed and remounted without realignment. The resulting mechanical interface is more robust, clearly defined, and testable with the tolerance analysis done earlier in the design process.
Motors are a common electronic-tomechanical interface, while their controllers are involved in the software-toelectronic interface. Their selection depends on many issues including life expectancy, tolerance over their expected life, contamination requirements, heat considerations, allowable noise levels, safety requirements, motion requirements, and the variability of the load. DC brush motors are very cost-effective and are available with a wide range of speed and torque capabilities, but they can generate EMI from the arcing at the brushes, and produce contaminating dust from the wearing of their brushes. They also have relatively short life expectancies.
Many systems have requirements that will preclude the use of DC brush motors in some or all axes. Brushless motors overcome these problems and may be operated either commutated or non-commutated. Operated in noncommutated mode, brushless motors include stepper motors, synchronous motors, and induction motors. These same styles of motors, when operated with feedback and commutating controllers/ drivers, become servos in position or variable-speed applications. Commutated brushless motors include both two- and three-phase brushless configurations. High-pole-count motors — called stepper motors when operated open loop — become hybrid servos when commutated and operated in closed-loop operation. Figure 1 shows an example hybrid servo.
The selection of motors used in positioning applications, such as the cranes or pumps seen in many medical applications, usually come down open-loop steppers, or closed-loop DC or brushless servos. DC brush motors must be properly encased to eliminate the contamination from the wear of their brushes, and their life expectancy typically ranges from tens of hours to a few thousand hours of operation without maintenance. Open-loop stepper motors are relatively inexpensive, and many are designed for long life, but they may generate enough heat to significantly increase sample evaporation or cause other thermal issues within the system. Stepper motors can also be acoustically loud and cause mechanical vibrations in the system, according to how they are driven, and they may lose position due to strong resonances and varying load conditions. Stepper motors do have a very high torque constant, allowing for direct drive or low gear/pulley ratios for many loads.
Closed-loop brushless motors in general have higher performance and higher cost. The number of pole pairs of the motor trade speed for torque, with more speed for a lower number of poles and more torque for a higher number of poles. Stepper motors and other high-pole-count motors can be paired with feedback and commutating drives to operate as high-pole-count servo motors — the combination referred to as “hybrid servo motors” or “direct drive motors” by various vendors. Proper commutation and closedloop control of these motors eliminates the low- and mid-frequency resonances and minimizes the power usage, while greatly improving their performance, reducing acoustic noise, and eliminating lost steps.
Many applications have loads with higher torque requirements from friction, load, or inertia components. The torque requirements may be met by a combination of belts and pulleys or gears. Lower torque motors require higher gear ratios, while higher torque motors may be able to direct drive their loads, or may be able to drive with a single pulley/belt or gear stage. Single pulley/belt systems can handle up to about 7:1 or 8:1 ratios with low cost, good damping, and quiet operation, often with a substantial cost reduction. Pulley systems (with appropriate materials) can provide long life with little chance of the type of contamination that follows seal failure on a gear head.