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By Lee Hudson
By acquiring the parameter and control-state data that is the basis for control-loop feedback, industrial sensors, including transducers, supply a particular kind of intelligence for process-control systems.
Sensors convert a measured mechanical parameter to an electrical signal that is fed into a signal conditioner, which modifies the output signal for further processing, such as into a readout, machine controller or computer. (See graphic 1).
While process instrumentation output today is predominantly analog, including that expressed in voltages and currents, digital output is gaining in preference. There are reasons for this over and above the fact that our world is increasingly digitized. Discrete signals are less susceptible to potential interference, which leads to higher quality and more reliable measurement values. This also satisfies the requirements of increasingly complex systems.
How long will it stand?
For years, analog output and associated process logic controllers (PLCs) have been process-control industrial mainstays. Popular applications relating to position measurement include automation tasks such as dimensional gauging (“go/no/go” inspection of complex mechanical parts), servo-loop positioning, valve-control monitoring and process-control applications.
In these applications, output signals can be voltage (3V, 5V, 10V, +/-5V, +/-12V, +/-15V), current output (4 – 20 mA, 0 – 20 mA) or a combination to accommodate a wide range of controller input requirements.
While voltage outputs are sufficient for many industries, excessive noise generation and voltage drops in specific environments can cause issues for downstream controls. To mitigate this analog voltage signal behavior, the market place has adopted the 4-20mA current output as its sensor signaling standard. This very robust signal allows users to run longer cables from a sensor to the control system with less worry of EMI or noise being induced on the signal. Recalibration to correct for voltage drop due to cabling impedance is also unnecessary.
Analog outputs are generally necessary in environments where digital electronics cannot survive, such as in high-temperature environments. In these cases, an analog signal must be initially generated by an environmentally-resistant sensor for conversion to digital at a benign location.
However, analog output has limitations. Signal can be degraded due to the distance of the cable from the sensor and the operating environment. Certain output signals can experience signal loss or generate noisy signals, which reduces the output’s accuracy. Excessive noise generated from environments such as oil fields and turbines can cause incorrect results and non-linearity errors as analog systems become more complex.
Many of today’s more complex process-control systems require communications that can only be delivered by digital output. In these applications, additional signal-conditioning and data-acquisition equipment is needed to transform the transducer’s output into a useable numeric format for use in spreadsheets with other measured results of other calculations.
Address analog limitations
When sensor signal transmission is critical and networked communications are desirable, digital communications serve as a more reliable and cost-effective alternative to analog output.
Digital outputs accurately reflect sensor output and are not subject to cascading errors. With greater immunity to noise, digital systems have a greater capacity to control errors and provide a complete signal from sensors into computer software and other network programs.
In networked communications, digital communications allow for daisy-chaining multiple signal conditioners on one bus line and reduce I/C cards, wiring, footprint and installation costs.
One digital network connection can replace multiple analog control-wire connections between the process controller and power panels. This configuration is especially useful in applications with a large number of inputs or if each input, or certain ones, must be sampled frequently. A digital RS 485 output supports “multi-drops” where hundreds of devices can be on one bus line.
Similar to the aforementioned assortment of analog signal options, digital transmission is available in a variety of communication protocols and languages — e.g., Modbus, CANbus & Ethercat. The added flexibility of these digital systems contributes to effective system integration, while also supporting the prospect of an intuitive interface.
Analog sensor, digital signal
While some engineers may believe that the only reason analog signals have not disappeared is because the engineering world has not agreed upon a single way of powering and communicating, analog communications and control-loop process-control systems will continue to be used for many years. Selection between analog and digital output depends on a variety of factors including resolution, reliability, environment, redundancy, type of sensors and costs.
It is also the case that digital output can be derived from any standard sensor using an A/D converter or signal conditioner designed to provide a RS-485 digital signal.
For example, a Macro Sensors signal conditioner interfaces with wide range of AC linear variable differential transducers (LVDT), rotary variable differential transducers (RVDT), and VR half-bridges and offers customers the option of using either a 4-20 ma analog signal or a digital signal RS-485. Both outputs can be used for different requirements, or either, based on user preferences. With the use of the RS-485 port, a host computer is able to retrieve measurement data, receive operational status, do remote calibration, and perform hot swap reconfiguration. Synchronization to other signal conditioners is accomplished by a daisy chain connection to a synchronization bus. One unit will assume the Master function based on DIP switch priority setting. If a fault should occur, the next highest priority unit will take over as Master.
A comparison of the attributes of analog vs. digital signals can be found at www.diffen.com/difference/Analog_vs_Digital.
Lee Hudson is an application engineer at Macro Sensors.
Pennsauken, N.J.-based Macro Sensors makes linear, rotary, spring-loaded, free core and custom position sensors. It says its core LVDT technology is proven as a long-term reliable electro-mechanical device. The company’s origins lie in the work of Harman Schaevitz, widely recognized as the pioneer developer of LMDT technology and who founded Schaevitz Engineering in 1945.