Machine Design

Timers and Counters

Self-contained machines such as fryers for fast-food restaurants, cook-and-hold ovens, and bakery equipment usually employ a simple automatic controller. A typical controller includes one or more timer functions and, occasionally, a counter to turn the process on and off at programmed intervals in response to some external event.

Timers and counter controls can be either electromechanical or solid state. They control a set of relay contacts commanded by settings dialed or keyed into their front-panel controls.

In general, single-function electromechanical devices are used when the cost factor is more important than high accuracy. One reason is that these devices are less accurate than a solid-state unit. Their setting resolution is limited by the analog setting mechanism they use. Electromechanical timer costs are typically in the $50 to $60 range. Solid-state devices often run from $70 to $400 each.

As a general guideline, electromechanical devices are adequate when the timing accuracy can vary by ±1% of full scale. Digital solid-state timers are preferred when better accuracy is needed. The setting resolution of a digital device is, in fact, the time value of the least-significant digit. If the timing range of the device is 0.01 to 99.99 sec, settings can be increased or decreased in increments of 0.01 sec.

Another important factor is repeatability or the degree to which the timer can repeat the same time setting. Solid-state timer accuracy has improved immensely from the early days of resistor/capacitor networks. One reliable technology uses the ac line frequency as a timing base. Repeat accuracy with this technology is constant to the line frequency. Another technology uses an internal quartz oscillator as its timing base.

A typical repeat accuracy for a solid-state device is on the order of 0.003% or 35 ms, whichever is larger. Typical setting accuracies are usually ±0.1% of the setting or 50 msec, whichever is larger.

Industrial counters are mechanical, electromechanical, or electronic instruments that totalize a series of events and actuate contacts at predetermined counts. Most counter inputs accept either voltage pulses or simple switch closures. Some counters also have a control input used to start (enable) or reset the counter either to a preset number or to zero.

If the counter can add and subtract events from a total, it may also have an up/down input that controls the direction of the counts received. Some models also provide separate inputs for up-count events and down-count events.

Counter output can be a simple numerical display (count totalizer), a switching action (predetermining counter), a coded electrical signal, or a printout. Most counters have a dial, thumbwheels, or keyboard for entering the count number. Once the counter reaches the predetermined count, the output switch contacts transfer. Some counters accept no further counts at this point until reset through the reset/enable control input.

Electronic counters use all solid-state components and digital counting elements, hence they can accept high-speed inputs to provide counting rates faster than mechanical and electromechanical counters. These devices generally use logic-level, electrical pulse, or switch-contact inputs to provide counting speeds on the order of 10 kHz. Many include LED or neon-lamp displays that indicate the predetermined count as well as the totalized count. Most have CMOS logic circuitry and include a stand-by battery.

Computerized counters generally contain single-chip microcomputers that provide predetermining and totalizing counter functions for several simultaneous operations. Most devices include solid-state memory to retain data in the event of power failure and accept unidirectional and bidirectional signals for up/down counting.

Most computerized counters have many of the same characteristics as electronic timers, including high-speed counting, remote or automatic reset, and pushbutton (or keyboard) reset. Additional capabilities include prescaling (multiplying input signals by a constant factor to match flow metering, machine drive ratios, and so forth) and prewarning, where a secondary control input is switched to provide a warning signal that the predetermined count will soon occur.

TAGS: Controllers
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