Analog Sensors

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3.2.2 Analog

Analog sensors produce an output that is proportional to a measured property. There can often be offsets and linear errors associated with analog sensors that must be taken into account when using the measurements, and calibration to a known standard is often required.

3.2.2.1 Pressure, Force, Flow and Torque

Force can be measured using a variety of devices. One common element in measuring the amount of force exerted on an object is a strain gauge. The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as superglue. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

A strain gauge can be configured in a variety of physical packages and used to measure force or weight. Strain gauges are attached to load cells and used at multiple points to weigh objects accurately. Vibration and acceleration can also be determined using strain gauges.

For measurements of small strain, semiconductor strain gauges, so called piezoresistors, are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.

Flow of liquids or gases can be measured in a number of ways. A rotary potentiometer (resistive element) is often used when attached to a vane which turns in the fluid or gas. Other flow sensors are based on devices which measure the transfer of heat caused by the moving medium. This principle is common when using microsensors to measure flow. Flow meters are related to devices called velocimeters that measure velocity of fluids flowing through them. Laser-based interferometry is often used for air flow measurement, but for liquids, it is often easier to measure the flow. Another approach is Doppler-based methods for flow measurement. Hall effect sensors may also be used, on a flapper valve, or vane, to sense the position of the vane, as displaced by fluid flow.

Commonly, torque sensors or torque transducers use strain gauges applied to a rotating shaft or axle. With this method, a means to power the strain gauge bridge is necessary, as well as a means to receive the signal from the rotating shaft. This can be accomplished using slip rings, wireless telemetry, or rotary transformers. Newer types of torque transducers add conditioning electronics and an A/D converter to the rotating shaft. Stator electronics then read the digital signals and convert those signals to a high-level analog output signal, such as +/-10VDC

3.2.2.2 Color and Reflectivity

As described in the digital section previously, various colors of LED light reflect from different colored materials with varying intensity. This property can be used to sample the amount of light returning to a receiver and determine color. Combinations of reflected red, green and blue light can be analyzed to determine shades and hues to separate items of different color properties.

This same property can be used to determine the relative distance of an object from the sensor. As an object moves farther away the amount of light received by the sensor becomes less. This property of reflectivity can be used to scale a distance measurement when using the same target.

For more accurate determination of color a CCD (Charge Coupled Device) is used to capture a colored region. CCDs react to photons and when a filter called a Bayer Mask is placed over the CCD it becomes a color sensitive device. Red, blue and green again are the operative colors for color CCDs. CCDs are also used to create black and white images that can be converted to a scale for intensity measurement.

3.2.2.3 LVDTs

Linear Variable Differential Transducers, or LVDTs are a type of electrical transformer used for measuring linear displacement. The transformer has three solenoidal coils placed end-to-end around a tube. The centre coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube.
An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary. The frequency is usually in the range 1 to 10 kHz.
As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change. The coils are connected in reverse series, so that the output voltage is the difference (hence “differential”) between the two secondary voltages. When the core is in its central position, equidistant between the two secondaries, equal but opposite voltages are induced in these two coils, so the output voltage is zero.
When the core is displaced in one direction, the voltage in one coil increases as the other decreases. This causes the output voltage to increase from zero to a maximum. The output voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to a maximum, but its phase is opposite to that of the primary. The magnitude of the output voltage is proportional to the distance moved by the core (up to its limit of travel), which is why the device is described as “linear”. The phase of the voltage indicates the direction of the displacement.
Because the sliding core does not touch the inside of the tube, it can move without friction, making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the LVDT to be completely sealed against the environment.
LVDTs are commonly used for position feedback in servomechanisms, and for automated measurement in machine tools and many other industrial and scientific applications.

3.2.2.4 Ultrasonics

Ultrasonic sensors transmit sound pulses at a high frequency and evaluate the echo received back from the sensor. Sensors calculate the time interval between sending the signal and receiving the echo to determine the distance to an object.

Ultrasonic sensors are often used for distance measurement but are common in liquid and tank level applications. The technology is limited by the shapes of surfaces and the density or consistency of the material, for example foam on the surface of a fluid in a tank could distort a reading.

3.2.2.5 Distance and Dimensions

Photoelectric sensors, proximity switches, LVDTs, ultrasonics, and encoders can all be used to measure distance and dimensions. In addition there are laser based devices that can be used similarly to photoelectrics, rows of LEDs or lasers that can measure dimensions based on the number of beams broken or the amount of light received, and CCD based devices that can measure distances accurately. Techniques using precision tooling and physical contact with the target are also commonly used.

3.2.2.6 Thermocouples and Temperature Sensing

There are a variety of devices that can be used to measure temperature. One of the most widely used is the thermocouple. A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor and can also be used to convert heat into electric power. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; System errors of less than one kelvin (K) can be difficult to achieve.

Any circuit made of dissimilar metals will produce a temperature-related difference of voltage. Thermocouples for practical measurement of temperature are made of specific alloys, which in combination have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges, and to resist corrosion. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires, which are less costly than the materials used to make the sensor. Thermocouples are standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; a few applications would include temperature measurement for kilns, measurement of exhaust temperature of gas turbines or diesel engines, and many other industrial processes.

The most common type of thermocouple in use is the K thermocouple (Chromel-Alumel). This covers temperature ranges from -200 to 1350 degrees Celsius. It is inexpensive and available in a variety of styles. J thermocouples (Iron-Constantan) are less popular than K due to their lower temperature range of -40 to 750 degrees Celsius. Other types include E, N, B, R,S, T, C, M and Chromel-Gold/Iron.
Thermocouples are not linear devices and the voltage curve must be linearized in the input instrument. Temperature loop controllers contain linearization algorithms for the most common types of thermocouples. Selection of the thermocouple type can be made by setting dipswitches or software parameters.
One note on thermocouple polarity: there is a polarity labeled + and – for connection to input terminals. Counter to the common thought that the red wire is positive in many DC circuits, red is always the negative lead for thermocouples. Not every thermocouple pair has a red wire, but when using the ANSI (American National Standards Institute) color code the red lead will always be negative.

Thermistors are a type of resistor with resistance proportional to its temperature. Thermistors are usually made of a ceramic or polymer material. They have a high precision over a limited temperature range.

RTDs, or Resistance Temperature Detectors also change resistance proportionally with temperature, but are made of pure metals. They are useful over a wider temperature range than thermistors but are less accurate. RTDs and thermistors may both be used with standard analog inputs and an excitation voltage because of their linearity, unlike thermocouples which must use a special input to linearize the signal.

Infrared thermocouples or infrared temperature sensors are used as non-contact methods of sensing temperature. They use the thermal emission from the target to scale temperature to a readable value.

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