Under Control


Philosophers, and would-be philosophers, look to the meanings of words---sometimes even technical terms. We've talked about real time before. It refers to tasks or functions executed so fast that their feedback guides task completion. Real-time processing is fast, robust and predictable.

Real-time control models a process to act upon it. Data coherency from sensors and presentation of the values via the effecters is important. The sampling and hold system must fit within the performance envelope of the system designer's requirements.

A little appreciation for the theory is in order. Sampling theory says we can reproduce analog wave forms by sample and hold, if appropriate conditions are met. Each baud must contain at least one valid sample of the amplitude. Generally, engineers use the rule of thumb that 4 samples per cycle (or 2 samples per baud) will adequately reproduce the wave form without much processing.

Simply oversampling---with the idea that more is better---seldom enhances accuracy. The benefits of frugality can be seen in the compact disk (CD) player. Clearly, CD's provide the best reproduction sounds available. It's only the single transformation (in DDD disks) of the analog wave form to digital that can introduce error. Digital transmission prevails throughout the rest of the reproduction system.

Several factors influence good analog-to-digital conversion, which is essential to any measuring. The most obvious is sample rate. I must be fast enough to satisfy the theoretical requirements. Less obvious is the need for predictability of the sample rate. Otherwise, unwanted modulation products emerge.

Another requirement is to establish digital conversion accuracy, which can vary from 8 to 21 bits, with the typical range from 12 to 16 bits. Ten bits has an unknown value of one part in a thousand, Twenty bits is accurate to one part in a million. To put that in perspective, consider that temperatures here in New Hampshire range from 20 degrees Fahrenheit below zero to 110 degrees above. A range of 130 degrees. The body is sensitive to about a degree, or one part in a hundred. Eight bits of conversion accuracy should satisfy home thermostat requirements for anyone except my wife.

Conversion is meaningless unless the held sample is representative of the "instantaneous" value being measured. If the digital conversion takes significant time and the signal changes during that time, errors result. the sampling aperture must be such that the capture time is within the error budget of the process. This is an oft-neglected, major contributor to error.

In sum, a designer must consider the aperture, sample rate, sample coherency, and number of bits. What has all this to do with computerized process control? Plenty. In a classic small, real-time system, the designer must consider similar elements. Quick response ensures that the system is sampled consistent with theory. Interrupts must be grouped and prioritized so that the aperture of capture is within the change limits of the desired system. Software error budgeting must bear in mind the number of events, the change rate of sensed data, and the service for each event. Real-time operating systems take all this into account. too often, novices assume any operating systems take all this into account. Too often, novices assume any operating system can be real time. Disasters result.

The PLC, however, has a scan time of about 5 million seconds independent of change. The PLC operating system ensures consistency of product, independent of stimulation. Predictably, many military computers use real-time operating systems or modified common operating systems. To use a modified, PC-based operating system, you must know---a priori---the system requirements.

In any industrial application, programming types, operating systems and I/O structure must be defined. Real-time considerations should be part of each systems analysis. We, in the business, tend to become enamored with buzzwords Distributed systems have better real-time performance locally, but penalize overall system performance. Centralized systems penalize local-node performance with attendant superiority overall. Classical string languages and operating systems have excellent interfaces and database performance with marginal consistence control aspects. Using appropriate tools heterogeneously is best for now.


As appeared in Manufacturing Systems Magazine March 1994 Page 90

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