August 2009

A matter of translation and manipulation

In a typical measurement system, signal conditioning is the equipment located between the transducer and the display.

As its name suggests, this equipment conditions the transducer signal into a form usable by the display. In some transducers, the signal conditioning electronics is in the body of the transducer.

Other transducers have some or all of the signal conditioning built into an in-line unit located near the transducer but away from any extreme test environment.

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The most common situation is to have the transducer mounted directly on or very close to the test object. Cable runs of tens to hundreds of feet are used to carry the transducer's output signal to the signal conditioning equipment, which is located in a control room that also houses the display equipment and the human system operators.

For some transducers, the cable also carries the excitation voltage that the transducer needs and the signal conditioner provides. The main advantage of this type of system is its flexibility. 

A strain gauge amplifier can have adjustable excitation voltage, adjustable amplification, adjustable zero setting, built-in bridge completion resistors, adjustable filter settings, and multiple outputs. This type of signal conditioning can work with strain gauge transducers from different manufacturers and with almost any type of display. The main disadvantage of this type of system is long cable runs are required between the transducer and signal conditioner, and these are susceptible to noise pickup.

The most common electrical output from signal conditioning equipment is a DC voltage range that is linearly proportional to the transducer's input range. A typical DC voltage range is 0 to 10 volts, where 0 volts is the output from the signal conditioner when the transducer input is zero, and 10 volts is the output when the transducer input is at full scale.

This DC voltage can connect to a wide variety of display equipment, which can convert the DC voltage level into an equivalent number of input units that the transducer is sensing. A strain gauge pressure transducer with an input range of 0 to 1,000 psig may have a full-scale output sensitivity that is 2 millivolts per volt.

This means the output from the transducer bridge at an input of 1,000 psig will be 2 millivolts for every volt of bridge excitation voltage. If the bridge excitation voltage is 10 volts then the output from the transducer bridge will be 20 millivolts at an input of 1,000 psig.

At 0 psig, the bridge will be balanced, and the output will be 0 millivolts. Since the transducer is linear, the bridge output will be 10 millivolts at an input pressure of 500 psig. To provide the 0-to-10-volt signal, the signal conditioning equipment must include an amplifier that has a gain of 500.

At a full-scale input of 1,000 psig, the amplifier will multiply the 20-millivolt bridge output signal by 500 to produce the required signal conditioner output of 10 volts. An input pressure of 500 psig produces a bridge output of 10 millivolts, which will amplify by a factor of 500 to produce an output signal of 5 volts. An input of 0 psig will produce an output of 0 volts.

In this example, the signal conditioning equipment consists of a DC power supply that has a stable, regulated output of 10 volts and a voltage amplifier with a gain of 500. In practice, these units would reside in a single module, along with adjustments for amplifier gain, amplifier zero, and input and output connectors for cable connections to the transducer and display.

The adjustable amplifier gain and zero are provided to calibrate the system so 0 psig produces exactly 0 volts and 1,000 psig produces exactly 10 volts. This is required since no transducer has an output that is completely ideal. The strain gauge bridge may not be completely balanced at 0 psig, and the output at 1,000 psig may not be exactly 20 millivolts due to physical variations in the construction of the transducer and variations in the strain gauges and their installation.

This voltage amplifier is one of the simplest forms of signal conditioning, and it is likely to be a building block within more complex signal conditioners. It is easy to see how a few integrated circuit chips and a few discrete components can add to the transducer housing to make a transducer with built-in signal conditioning.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Basics. The primary source for this article is Fundamentals of Test Measurement Instrumentation, Keith R. Cheatle, ISA Press, 2006

Terminology

Transducer converts one type of energy or physical attribute to another. Some commercial examples are a TV that converts electric signals to images, a light bulb that converts electricity to light, or a loudspeaker that converts electric signals to sound. In the InTech world of manufacturing automation, the transducer might be a thermocouple that converts temperature (input) to voltage (output). The output then goes to the signal conditioner for manipulation and subsequent display as say, 312°C.