Digital counter with Dual Channels and Programmable Functions
Using a digital counter for event reporting:
In its simplest form, a digital counter is a device used to count the number of times an event has occurred and to display the results digitally. Programmable digital counters can be used to measure frequency, rate, speed, flow, total, volume, time, position and more, all without a change of hardware.
The basic counter uses a dual input signal conditioner board, which accepts inputs from proximity switches with PNP or NPN output, TTL or CMOS logic, magnetic pickups, contact closures, low-level outputs from turbine flow meters down to 12 mV, and high-level AC line inputs up to 250 Vac. The dual channels can be used independently, such as one channel for rate and the other channel for totalizing, or be used in combination, where one channel detects the leading edge of a pulse and the other channel detects its falling edge. Such combinations are used in applications such as timing the width of a single electrical pulse. Pulse rates can be up to 1 MHz.
Digital counter applications with a dual-channel pulse input signal conditioner
A basic application of a digital counter is totalizing. In its simplest form, this can be reporting the total number of pulses or momentary contact closures; however, digital scaling is often required so that the displayed reading reflects a scale factor and an offset. For example, the displayed reading can be in gallons, and that reading may need to be scaled so that each gallon corresponds to 47.863 pulses from a turbine flow meter. The displayed reading may also need to be offset so that it reflects an incremental value, such as the number of gallons added since last reset.
The availability of two channels A and B also allows mathematical combinations of these channels. For example, two flows can be added for total volume (A+B), two flows can be subtracted for net volume (A-B), two flows can be divided for chemical mixing (A/B), or 1 can be subtracted from the ratio (A/B-1) to control elongation of material compressed by rollers (draw).
A digital counter can also count periods of a built-in quartz crystal oscillator to display time. This allows timing with sub-microsecond resolution. The raw count can then be scaled to display time in decimal seconds, minutes or hours, or in the familiar HH:MM:SS clock format. Timing accuracy will be an error of only one two parts per million (PPM).
A digital counter can also count periods of a built-in quartz crystal oscillator to display frequency in Hz or rate in engineering units such as meter/second, gallons/hour, RPM, etc. A counter determines frequency by timing an integral number of periods over a specified gate time, and then taking the inverse of period. Rate is obtained by multiplying the input by a scale factor. The inverse period approach allows greater accuracy and faster update times than conventional meters which count signal pulses over a time interval. AC line frequency may accurately measure 50.0000 or 60.0000 Hz in a few line cycles with 0.0001 Hz resolution. 1000 Hz signals may be measured to 0.01 Hz resolution at up to 25 readings per second. Fast response is ideal for alarm and control applications.
Many other digital counter applications are possible with the dual-channel pulse input signal conditioner, such as duty cycle, phase, power factor, and more. All this is achieved by a basic panel-mounted counter, avoiding the need for a more expensive specialized instrument.
Digital counter applications with a quadrature signal conditioner
A digital counter with the quadrature signal conditioner accepts the A and B quadrature signals from linear encoders and shaft encoders to provide a highly accurate, scaled display of position, length, or angle in engineering units, such as ft, cm or degrees, or speed in engineering units such as meters/second. The A and B quadrature signals are 90° out of phase, and their phase relationship determines whether up counts (+) or down counts (-) are counted. The meter totalizes the counts and then scales the total in software for display and control. A zero index signal, or Z signal, may be added as a third input to the A and B signals.
Compared to other electronic methods of determining position or speed, quadrature counters provide readings that are exceptionally accurate and stable, even in the presence of mechanical vibration.
Digital counter applications with a voltage-to-frequency signal conditioner
A digital counter with the voltage-to-frequency signal conditioner converts a 4-20 mA, 0-1 mA or 0-10V analog signal into an internal frequency, which is then processed into a totalized count or a rate, both in appropriate engineering units. A classical application is totalizing the output of a 4-20 mA flow transducer to display totalized flow or volume in units such as liters or gallons. Rate in engineering units such as gallons/minute can also be displayed. In these applications, the voltage-to-frequency approach offers the additional benefit of exceptional reading accuracy, which is based on the counter's highly stable quartz crystal oscillator.
Features to look for in a programmable digital counter
In addition to basic functionality as a counter, look for programmable features that will allow the instrument to function as a rate meter and timer. Look for programmability to change units of measure, to add offsets, and to count up to a preset or to count down to zero. Look for a dual-channel pulse input signal conditioner so that two channels can be combined or rising and falling pulse edges can be processed individually. Look for counting rates up to 1 MHz and sub-microsecond timing capability. Also look for ease of use. In addition to front panel programming, the manufacturer should also offer Windows based setup software that runs on a PC to connected to the instrument.
Look for options that will allow the digital counter to work as part of a larger system and to be interfaced via an Ethernet network. A state-of-the-art counter should offer a choice of plug-in boards for communications, control, and network use. For example, serial interface boards include Ethernet, USB, RS485 and RS232. Additional boards can provide a 16-bit, isolated analog output, and dual or quad relays for alarm or control. Look for other features that you may need, such as a red or green LED display, and large digit heights. Digits up to 8" high is offered for a viewing distance up to 300 feet, for example across a factory floor.
About the Author
Barry Crowther has been working closely with Laurel Electronics Inc., who have been in the electronics business for many years. For more information about Laurels Electronic Inc. and Digital Counter please visit http://www.laurels.com/blog/
Tell others about
this page:
Comments? Questions? Email Here