Sensor transmitters provide standard, calibrated outputs from a corresponding sensor or transducer.

Basic Information

All sensors convert a measurand (a variable to be measured) into a signal. Sensor output may take many forms depending on the sensor and measurand types, including force, displacement, voltage, or electrical resistance. Because most controllers and displays use input types which differ from sensor outputs, transmitters are used to convert the raw output to a compatible signal. This signal may be analog or digital in nature; the term "sensor transmitter" usually infers the transmission of an analog signal, while purely digital outputs are typically produced by an analog-to-digital converter.

The role of a transmitter in a basic control circuit.

Image credit: Maxim Integrated

Sensor transmitters are primarily used to produce an output with a higher transmission range than the sensor itself can provide. For example, a sensor which outputs a small voltage or resistance value has a short range due to losses inherent in transmission wires.

Calibration and Compatibility

Most transmitter output signals share a linear, proportional relationship with the process variable (or sensor input). There are three terms related to the sensor's process variable and this relationship:

• Range is the difference between the low and high values to be measured. This is always dependent on the physical sensor's range or calibrated range.
• Span is the difference between the range values.
• Zero is the low value of the range. The value itself does not have to be the numeral zero in order to be assigned this label.

For example, a temperature sensor with 5-10 V output may be calibrated to measure values between 800° C and 1200° C. Its range, therefore, would be 800°-1200° C, its span would be 400° C, and its zero would be 800° C. In an example application, this sensor needs to be compatible with control equipment which accepts a 4-20 mA current loop output. In this case a transmitter is required to convert the voltage output to a current loop.

Sensors produce an output proportional to the sensed value. If the sensor detects a temperature value of 1000° C, the midpoint of the sensor range, it will output the corresponding midpoint of its voltage range, or 7.5 V in this case. In order to maintain compatibility with the process variable, the transmitter would then output the midpoint of its current range, or 12 mA.

This process can be described using the formula for the sensor/transmitter pair's transfer function:

where:

H(s) = transfer function

C_m(s) = transmitter output

C(s) = process variable (or sensor) output

The transfer function is simply the ratio between the input and output signals. Transmitters ultimately affect two aspects of the input signal through a transfer function: the size and timing of the signal. The size relationship is measured by gain, while the timing relationship is measured by phase difference. Along these lines, the transfer function can be rewritten as:

where:

K_t = transmitter gain

τ_t = transmitter time constant

s = sensed value

Inputs and Outputs

The most important variable when matching sensors to transmitters is basic compatibility: the sensor's output must be acceptable as an input to the transmitter. Transmitters most commonly accept voltage, current, frequency, or resistance inputs; many devices are configurable to accept two different types.

Transmitters are typically designed for compatibility with a certain sensor type, such as accelerometers, thermistors, flow sensors, thermocouples, and strain gauges.

Device Information

Most sensor transmitters operate as signal converters, accepting an input and transmitting a proportional signal in a differing output. The schematic below shows a typical reference design for a sensor transmitter which converts a voltage input to a 4-20 mA current loop.

This design is common in that it can be broken into three stages, from left to right, on the schematic: an input excitation stage, an input amplification stage, and a signal conversion stage. As shown in the image, signal amplification and conversion is accomplished using a series of resistors, capacitors, operational amplifiers, and transistors.

A typical voltage-to-current sensor transmitter.

Image credit: Analog Devices

Transmitter design varies based on desired inputs and outputs. For example, a device which accepts digital inputs and transmits analog current requires a digital-to-analog converter (DAC) within close proximity to its first stage. The block diagram below illustrates a more complex transmitter design. This device features multiple signal conversions, resulting in the transmission of both an analog current loop signal as well as a concurrent digital diagnostic/control signal.

A "smart" transmitter.

Image credit: Maxim Integrated

Form Factor

Common transmitter form factors are described in the table below.

Table image credits: FLEX-CORE | Measurement Specialties | AGI Ltd | Cole Parmer

Standards

Sensor transmitters may be produced, used, or calibrated according to published standards and specifications. Some relevant standards include:

• BS 6174 Specification for differential pressure transmitters with electrical outputs
• MIL-DTL-7990 Temperature transmitter (electrical resistance)
• MIL-PRF-38230 Pressure transmitter (oil; variable reluctance)

References

Texas A&M University - Sensor Characteristics

Image credits:

Micron Instruments