Displacement and Position Sensors

DISPLACEMENT AND POSITION SENSORS

Displacement sensors are basically used for the measurement of movement of an object. Position sensors are employed to determine the position of an object in relation to some reference point.

Proximity sensors are a type of position sensor and are used to trace when an object has moved with in particular critical distance of a transducer.

1. Potentiometer Sensors

Figure shows the construction of a rotary type potentiometer sensor employed to measure the linear displacement. The potentiometer can be of linear or angular type. It works on the principle of conversion of mechanical displacement into an electrical signal.

The sensor has a resistive element and a sliding contact (wiper). The slider moves along this conductive body, acting as a movable electric contact.

The object of whose displacement is to be measured is connected to the slider by using

• a rotating shaft (for angular displacement)

• a moving rod (for linear displacement)

• a cable that is kept stretched during operation The resistive element is a wire wound track or conductive plastic. The track comprises of large number of closely packed turns of a resistive wire. Conductive plastic is made up of plastic resin embedded dc with the carbon powder. Wire wound track has a resolution of the order of ±0.01 % while the conductive plastic may have the resolution of about 0.1% m.

During the sensing operation, a voltage Vs is applied across the resistive element. A voltage divider circuit is formed when slider comes into contact with the wire. The output voltage (V_A) is measured as shown in the figure 8.4. The output voltage is proportional to the displacement of the slider over the wire. Then the output parameter displacement is calibrated against the output voltage V_A.

V_A = IR_A

As we know that R = ρL / A, where ρ is electrical resistivity, L is length of resistor and A is area of cross section

 V_A = (V_SL_A) / (L_A+L_B)

Applications of potentiometer

These sensors are primarily used in the control systems with a feedback loop to ensure that the moving member or component reaches its commanded position.

These are typically used on machine‒tool controls, elevators, liquid‒level assemblies. forklift trucks, automobile throttle controls. In manufacturing, these are used in control of injection molding machines, wood working machinery, printing, spraying, robotics, etc.

2. Strain Gauges

The strain in an element is a ratio of change in length in the direction of applied load to the original length of an element. The strain changes the resistance R of the element. Therefore, we can say,

∆R/R α ε

∆R/R = Gε

where G is the constant of proportionality and is called as gauge factor. In general, the value of G is considered in between 2 to 4 and there resistances are taken of the order of 100 Ω.

Resistance strain gauge follows the principle of change in resistance as per the equation. It comprises of a pattern of resistive foil arranged as shown in Figure 8.5.These foils are made of Constant an alloy (copper‒nickel 155‒45% alloy) and are bonded to a backing material plastic (polyamide), epoxy or glass fiber reinforced epoxy.

The strain gauges are secured to the workpiece by using epoxy or Cyanoacrylate cement Eastman 910 SL. As the workpiece undergoes change in its shape due to external loading, the resistance of strain gauge element changes. This change in resistance can be detected by a using a Wheatstone's resistance bridge as shown in Figure 8.6. In the balanced bridge we can have a relation,

R₂/R₁ = Rx/R₃

Where R_x is resistance of strain gauge element, R₂ is balancing/adjustable resistor, R₁ and R₃ known constant value resistors. The measured deformation or displacement by the stain gauge is calibrated against change in resistance of adjustable resistor R₂ which makes the voltage across nodes A and B equal to zero.

Applications of strain gauge

Strain gauges are widely used in experimental stress analysis and diagnosis on machines and failure analysis. They are basically used for multi‒axial stress fatigue testing, proof testing, residual stress and vibration measurement, torque measurement, bending and deflection measurement, compression and tension measurement and strain measurement.

Strain gauges are primarily used as sensors for machine tools and safety in automotives. In particular, they are employed for force measurement in machine tools, hydraulic or pneumatic press and as impact sensors in aerospace vehicles.

3. Capacitive element based sensor

Capacitive sensor is of non‒contact type sensor and is primarily used to measure the linear displacements from few millimeters to hundreds of millimeters. It comprises of three plates, with the upper pair forming one capacitor and the lower pair another.

The linear displacement might take in two forms:

(a) one of the plates is moved by the displacement so that the plate separation changes

(b) Area of overlap changes due to the displacement.

Figure 8.7 shows the schematic of three‒plate capacitive element sensor and displacement measurement of a mechanical element connected to the plate 2.

The capacitance C of a parallel plate capacitor is given by, C = ε_rε₀ A/d

Where εr is the relative permittivity of the dielectric between the plates, ε₀ permittivity of free space, A area of overlap between two plates and d the plate separation.

As the central plate moves near to top plate or bottom one due to the movement of the element/workpiece of which displacement is to be measured, separation in between the plate changes. This can be given as,

When C₁ and C₂ are connected are connected to a Wheatsone's bridge, then the resulting out‒of -balance voltage would be in proportional to displacement x.

Capacitive elements can also be used as proximity sensor. The approach of the object towards the sensor plate is used for induction of change in plate separation. This changes the capacitance which is used to detect the object.

Applications of capacitive element sensors

• Feed hopper level monitoring

• Small vessel pump control

• Grease level monitoring

• Level control of liquids

• Metrology applications

• to measure shape errors in the part being produced

• to analyze and optimize the rotation of spindles in various machine tools such as surface grinders, lathes, milling machines, and air bearing spindles by measuring errors in the machine tools themselves

• Assembly line testing

• to test assembled parts for uniformity, thickness or other design features

• to‒detect to detect the presence or absence of a certain component, such as glue etc.

4. Linear variable differential transformer (LVDT)

Linear variable differential transformer (LVDT) is a primary transducer used for measurement of linear displacement with an input range of about 2 to 400 mm in general. It has non‒linearity error 0.25% of full range. Figure 8.9 shows the construction of a LVDT sensor. It has three coils symmetrically spaced along an insulated tube. The central coil is primary coil and the other two are secondary coils. Secondary coils are connected in series in such a way that their outputs oppose each other. A magnetic core attached to the element of which displacement is to be monitored is placed inside the insulated tube,

Due to an alternating voltage input to the primary coil, alternating electro‒magnetic forces (emfs) are generated in secondary coils.

When the magnetic core is centrally placed with its half portion in each of the secondary coil regions then the resultant voltage is zero. If the core is displaced from the central position as shown in Figure 8.9 say, more in secondary coil 1 than in coil 2, then more emf is generated in one coil i.e. coil 1 than the other, and there is a resultant voltage from the coils. If the magnetic core is further displaced, then the value of resultant voltage increases in proportion with the displacement. With the help of signal processing devices such as low‒pass filters and demodulators, precise displacement can be measured by using LVDT sensors.

LVDT exhibits good repeatability and reproducibility. It is generally used as an absolute position sensor. Since there is no contact or sliding between the constituent elements of the sensor, it is highly reliable. These sensors are completely sealed and are w widely used in Servomechanisms, automated measurement in machine tools.

A rotary variable differential transformer (RVDT) can be used for the measurement of rotation. Readers are suggested to prepare a report on principle of working and construction of RVDT sensor.

Applications of LVDT sensors

• Measurement of spool position in a wide range of servo valve applications

• To provide displacement feedback for hydraulic cylinders

• To control weight and thickness of medicinal products viz. tablets or pills

• For automatic inspection of final dimensions of products being packed for dispatch

• To measure distance between the approaching metals during Friction welding process

• To continuously monitor fluid level as part of leak detection system

• To detect the number of currency bills dispensed by an ATM.

5. Eddy current proximity sensors

Eddy current proximity sensors are used to detect non‒magnetic but conductive materials. They comprise of a coil, an oscillator, a detector and a triggering circuit. Figure 8.10 shows the construction of eddy current proximity switch.

When an alternating current is passed through this coil, an alternative magnetic field is generated. If a metal object comes in the close proximity of the coil, then eddy currents are induced in the object due to the magnetic field. These eddy currents create their own magnetic field which distorts the magnetic field responsible for their generation. As a result, impedance of the coil changes and so the amplitude of alternating current. This can be used to trigger a switch at some pre‒determined level of change in current.

Eddy current sensors are relatively inexpensive, available in small in size, highly reliable and have high sensitivity for small displacements.

Applications of eddy current proximity sensors

• Automation requiring precise location

• Machine tool monitoring

• Final assembly of precision equipment such as disk drives

• Measuring the dynamics of a continuously moving target, such as a vibrating element,

• Drive shaft monitoring

• Vibration measurements

6. Inductive proximity switch

Inductive proximity switches are basically used for detection of metallic objects.

Figure 8.11 shows the construction of inductive proximity switch. An inductive proximity sensor has four components; the coil, oscillator, detection circuit and output circuit. An alternating current is supplied to the coil which generates a magnetic field. When, a metal object comes closer to the end of the coil, inductance of the coil changes. This is continuously monitored by a circuit which triggers a switch when a preset value of inductance change is occurred.

Applications of inductive proximity switches

• Industrial automation: counting of products during production or transfer.

• Security: detection of metal objects, arms, landmines.

7. Optical encoders

Optical encoders provide digital output as a result of linear / angular displacement. These are widely used in the Servomotors to measure the rotation of shafts. Figure 8.12 shows the construction of an optical encoder. It comprises of a disc with three concentric tracks of equally spaced holes. Three light sensors are employed to detect the light passing through the holes. These sensors produce electric pulses which give the angular displacement of the mechanical element e.g. shaft on which the Optical encoder is mounted. The inner track has just one hole which is used locate the position of the disc. The holes on the middle track offset from the holes of the outer track by one‒half of the width of the hole. This arrangement provides the direction of rotation to be determined. When the disc rotates in clockwise direction, the pulses in the outer track lead those in the inner; in counter clockwise direction they lag behind. The resolution can be determined by the number of holes on disc. With 100 holes in one revolution, the resolution would be,

360° / 100 = 3.6°

8. Pneumatic sensors

Pneumatic sensors are used to measure the displacement as well as to sense the proximity of an object close to it. The displacement and proximity are transformed into change in air pressure. Figure 8.23. Shows a schematic of construction and working of such a sensor. It comprises of three ports. Low pressure air is allowed to escape through port A. In the absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the port B. However when an object obstructs the low pressure air (Port A), there is rise in pressure in output port B. This rise in pressure is calibrated to measure the displacement or to trigger a switch. These sensors are used in robotics, pneumatics and for tooling in CNC machine tools.