Chapter 9 - Encoders, Transducers, and Advanced Sensors

Solution:

First, find the resolution of a 10-bit encoder, which is 360 degrees / 210 = 0.35156 degree. Then convert 0011100101G to binary using the method described previously. This will result in the binary number 0010111001. Now convert this number to its decimal equivalent, which is 185, and multiply it by the resolution to get 185 x 0.35156 = 65.04 degrees.

9-11. Linear Displacement

Potentiometer

A slide potentiometer is the simplest of all linear displacement sensors. It’s principle of operation is simply that we apply a voltage to a resistor and then move a slider across the resistor. The voltage appearing on the slider is proportional to the physical position of the slider on the resistor. Generally, in most displacement sensing linear resistors, the resistive element is made from small wire. This makes the unit more durable and less sensitive to variations in temperature and humidity. These are called slidewire potentiometers. The most common applications for slidewire potentiometers are in XY plotters and chart recorders.

Linear Variable Differential Transformer (LVDT)

The linear variable differential transformer, or LVDT, is an old technology that is still very popular. The device operates on the principle that the amount of coupling between the primary and secondary of a transformer depends on the placement of the transformer core material. Consider the LVDT shown in Figure 9-33. Identical coils (equal numbers of turns and equal size wire) L1 and L2 make up the primary of the transformer, with L3 being the secondary. All three coils are wound on a non-metallic tube. L1 and L2 are connected such that the same alternating current passes through both coils, but in opposite directions. This means that the magnetic fields produced by L1 and L2 will be equal but opposite. At the exact center between L1 and L2, the magnetic fields from the two coils will cancel producing a net field of zero. Therefore, the induced voltage in coil L3 will also be zero.

A high permeability iron slug is positioned inside the tube and a rod from the slug connects to the moving mechanical part to be sensed. As long as the slug remains centered, the flux coupled from coils L1 and L2 into L3 will be equal and opposite and will produce no induced voltage in L3. However, if the slug is moved slightly to the right, the permeability of the slug will lower the reluctance for coil L2 and increase the magnetic flux

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

Ultrasonic

The ultrasonic distance sensor works in the same manner as the ultrasonic proximity sensor. However, instead of a discrete output, the sensor has an analog output that is proportional to the distance from the sensor to the target object. The ultrasonic distance sensor has the same advantages and disadvantages as the ultrasonic proximity sensor.

Glass Scale Encoders

If we were to unwrap the glass disk of a rotary encoder and make it a straight narrow glass scale, we could easily have an encoder that would, instead of producing angular position information, produce linear displacement information. Like rotary encoders, glass scale encoders are available in both incremental and absolute versions. Their principles of operation are identical to that of their rotary counterparts. Since they are capable of detecting linear movements as small as 0.0001" or less, they are commonly used in applications that require extreme accuracy and repeatability such as numerically controlled mill and lathe machines.

Magnetostrictive Sensors

The magnetostrictive linear displacement sensor is a relatively new technology that has been perfected for making precise measurements of linear motion and position. The system utilizes two fundamental physical properties: a) whenever a current is passed through a conductor resting in a magnetic field, there will be a mechanical force produced, and b) sound waves travel through a solid material at a predictable velocity.

Consider the cutaway drawing of a magnetostrictive position sensor shown in Figure 9-34. The sensor unit consists of a sensor element head, waveguide, and sensor element protective tube. The sensor element head contains the electronic circuitry to operate the system. The waveguide is fundamentally a length of steel wire. For this application, it must be both conductive and elastic (much like a piano string). The sensor element tube is conductive and provides a protective shell for the waveguide. External to

(and separate from) these elements is a toroidal permanent magnet. The magnet is generally attached to the mechanism that moves, while the sensor element head, waveguide, and tube remain stationary. The waveguide tube is inserted through the hole in the toroidal magnet.

In operation, the sensor element head generates an extremely short current pulse that is applied to the waveguide. This pulse will cause a magnetic field to be induced around the waveguide. This magnetic field will interact with the stationary magnetic field produced by the toroidal magnet causing a very short mechanical pulse to be produced on

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Chapter 9 - Encoders, Transducers, and Advanced Sensors

the waveguide. This is much like plucking the string of a guitar; however, in this case the mechanical force is a torsional (twisting) force. This mechanical pulse causes a torsional wave to begin traveling down the waveguide toward the sensor element head. The system works much like sonar, except that we have a transmitted electrical signal that produces a mechanical echo.

The sensor element head contains a mechanical transducer that measures torsional motion of the waveguide and converts it to an electrical pulse. Then the electronic circuitry in the sensor element head measures the elapsed time between the current pulse and the returning mechanical pulse. This time is directly proportional to the distance the permanent magnet is located from the sensor element head. The pulse delay is then converted to an analog voltage which appears on the output terminals of the sensor.

Figure 9-34 - Magnetostrictive Position Sensor, Cutaway View

(MTS Systems Corp.)

The electronic circuitry is generally calibrated to have an output range of 0 to 10 volts. Zero volts corresponds to a distance of zero (i.e., the magnet is located at the sensor head end of the tube) and 10 volts corresponds to the full length of the tube. Sensors are available with various length tubes. As an example, assume we are using a 24" sensor. In this case, 10 volts = 24", and all positions from 0" to 24" are scaled proportionally.

As an application example, consider the arrangement shown in Figure 9-35. This shows a cutaway view of a hydraulic cylinder on the right and a magnetostrictive sensor on the left. The shaft of the hydraulic cylinder has been bored to make room for the sensor’s waveguide tube. The toroidal permanent magnet is fastened to the face of the piston inside the cylinder. As the hydraulic cylinder extends or contracts, the magnet will

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