POSITION SENSORS ch1 Length measurement Sensors and Measurement

POSITION SENSORS ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Types of position sensors • Sensing presence • Sensing Position ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Sensing presence • • Limit switches Photoelectric limit switch Magnetic detection Inductive proximity ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

• Switches – simplest form of digital displacement sensor • many forms: lever or push-rod operated microswitches; float switches; pressure switches; etc. A limit switch ch-1 Length measurement A float switch Sensors and Measurement Instrumenattion Systems II

• Opto-switches – consist of a light source and a light sensor within a single unit • 2 common forms are the reflective and slotted types A reflective opto-switch ch-1 Length measurement A slotted opto-switch Sensors and Measurement Instrumenattion Systems II

Proximity Sensing: • Proximity detectors are electrical or electronic sensors that respond to the presence of a material ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Inductive proximity sensors ____________ – coil inductance is greatly affected by the presence of ferromagnetic materials – here the proximity of a ferromagnetic plate is determined by measuring the inductance of a coil Inductive proximity sensors ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

When to Use Mechanical • Where physical contact is possible • Where definitive position is required • In operation-critical or safety-critical situations • Where environment conditions preclude the use of optical or inductive sensors ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

• Optical Proximity Sensors Optical Proximity • Consist of a light source (LED)Sensors and light detector • (phototransistor) • Modulation of signal to minimize ambient lighting • conditions • Various models: 12 -30 V DC, 24 -240 V AC, power • Output: TTL 5 V, Solid-state relay, etc. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

When to use an Optical • • Pros – Non-contact, no moving parts, small. – Fast switching, no switch bounce. – Insensitive to vibration and shock – Many configurations available • Cons – Alignment always required – Can be blinded by ambient light conditions (welding for example) • – Requires clean, dust and water free, environment ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Application • • Stack height control/box counting Fluid level control (filling and clarity) Breakage and jam detection And many others… ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

When to use Ultrasonic • • Provide range data directly: • Level monitoring of solid and liquids • Approach warning (collisions) • Can (usually) work in heavy dust and water • • Ambient noise is potentially an issue ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

• Resistive Position sensors Ø Mechanically varied resistance (potentiometer) Linear and Angular potentiometer Ø Resistivity change (Strain gage) • Capacitive • Inductive LVDT (linear voltage differential transformer) • Reluctance Resolver and Synchro • Piezoelectric • Pulse (encoder) Linear and Rotary encoder ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

POTENTIOMETERS ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Resistive sensor : potentiometer • A potentiometer converts a linear or angular displacement into a corresponding change in ouptut voltage : – Object whose displacement is to be measured is connected Vi to the moving contact l x Vo – Output voltage is linearly related to the displacement, x – DC power • range~l • mechanical wear • Example application : positioning of robotics; e. g. in artificial limbs ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Linear Position ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Shaft Angle With Potentiometer • shaft angle proportional to voltage Vs • voltage Vs changes because of the change in resistance • simple application of Ohm’s law • current is constant ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Angular Displacement - Potentiometer ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

POTENTIOMETERS ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Resist ivy change : strain gauge • A strain gauge is a displacement sensor whose resistance changes in response to a change in length caused by an applied force Consider a wire with length, l and cross sectional area, A : F A l F – With no force applied : – With an applied force, the length increases by Dl. – Since the total volume of the material remains constant, the change in area is given as : and the change in resistance as : ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Resistive sensor : strain gauge • Gauge factor, G, is given as : and may be taken as the sensitivity of the sensor • For metal wire gauges (constantan), G ~ 2, while silicon semiconductor gauges have higher sensitivity, G ~ 200 • Bonded strain gauges have folded wires bonded to a semi flexible backing material, with unbonded gauges having flexible wires connected between fixed and movable frames fixed plate movable plate • a ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Strain Gauge • resistance varies with the amount of stretching (strain) • flexure can be measured with a strain gauge • force can also be measured ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Bridge circuits • Often the sensitivity of the basic sense element is very low; e. g. a strain gauge may produce a maximum change in resistance of only ~1% over the full operating range. It is important that this change can be accurately detected. • Sensors are often sensitive to more than one quantity; e. g. the resistance of strain gauges depends on both strain and temperature. Therefore, if the temperature of the gauge changes it produces a change in resistance which may be interpreted as a change in strain. • Two and more active sense elements can be used in a bridge circuit to increase the overall sensitivity of the sensor system. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Wheatstone bridge • The basic Wheatstone bridge circuit is shown : – The output voltage is given as : R VB + R 1 2 + by voltage divider on each leg. Vo – The bridge is used in 2 ways – either as a null detector or as a device that reads a difference voltage directly. • For sensor circuits, the difference voltage due to deviation of one or more resistors in the bridge from an initial value is usually measured. R 3 ch-1 Length measurement R 4 Sensors and Measurement Instrumenattion Systems II

Bridge sensor circuits • Sensor bridges normally consist of 4 identical sensor elements. Consider the case when only one of these is sensitive to the quantity to be measured (other sensors are ‘dummy’ sensors) : VB + R R R • R+DR + Vo For example, for a max 1% change in resistance, the output voltage is ~0. 0025 VB; i. e. 2. 5 m. V for a 10 V supply. The range of Vo in this case is 0 – 2. 5 m. V, which can be accurately detected using a voltmeter with FSD of 2. 5 m. V. Consider the alternative of measuring the absolute change in resistance ! ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Advantages of bridge circuits • Increased measurement accuracy • Elimination of ‘noise’ effects on sensor output; i. e. if a sensor is sensitive to changes in both temperature and strain, if 4 identical elements are used with only one of them subjected to the strain, the temperature effect is cancelled R R VB + R • Possibility for increased sensitivity : R +DR VB + R ch-1 Length measurement R R+DR + Vo Sensors and Measurement Instrumenattion Systems II R+DR + Vo

Disadvantages of bridge circuits • Linearity error : R +DR VB + R R R+DR + Vo E. g. For maximum DR = 1%, a linearity error of 0. 5% of the FSD output voltage range is calculated. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Zero bridge linearity error • Zero-linearity error is achieved by having pairs of sensors with equal and opposite response characteristics : R–DR R +DR R –DR R VB + R + R+DR V o VB + R –DR R+DR + Vo • Advantages of increased accuracy, increased sensitivity and immunity to noise / temperature are maintained. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Inductive and Capacitive • Inductive sensors use change in local magnetic field to detect presence of metal target • Capacitive Sensors use change in local capacitance caused by non-metallic objects • • Generally short ranges only • • Regarded as very robust and reliable ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Inductive and capacitive • inductive: for detection of steel, chrome-nickel, stainless steel, brass, aluminum, copper parts • capacitive: for detection of steel, water, wood, glass, plastics ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Capacitive displacement sensor • A capacitive displacement sensor produces an output signal due to the change in capacitance caused by varying the distance between a fixed and a movable plate : A movable plate d fixed plate • Example application : measuring patient respiration rate or patient movement by having several sensors placed underneath them on a bed. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Inductive displacement sensor (LVDT) I • A linear variable displacement transformer (LVDT) is a 3 coil inductive transducer. – Mutual inductance between the coils is changed as the position + of a high permeability rod is S 1 vout(t) moved between them : + vin(t) P – ferrite rod – + S 2 – – Secondary windings are connected in series opposition – When the rod is centred, equal secondary voltages are induced, s. t. the output voltage is zero Sensors and Measurement ch-1 Length measurement Instrumenattion Systems II

Inductive displacement sensor (LVDT) ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Inductive displacement sensor (LVDT) II + + vin(t) P – u u S 1 – + S 2 vout(t) – As the rod moves from the centre, as shown, k 1 increases, while k 2 decreases – vout(t) is linearly related to the change in position of the rod AC voltage source – • range is limited by extent of coils • no moving parts Example application : repetitive displacements in robotics ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Linear Variable Displacement Transformer (LVDT) • moving iron core changes properties of transformer • iron core position changes primary/secondary voltage ratio • difference in phase is measured and tranformed to a voltage ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

• The major categories are: inductive: for detection of steel, chrome-nickel, stainless steel, brass, aluminum, copper parts • capacitive: for detection of steel, water, wood, glass, plastics ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Encoder • A device used to convert linear or rotational position information into an electrical output signal. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Absolute position encoders – a pattern of light and dark strips is printed on to a strip and is detected by a sensor that moves along it • the pattern takes the form of a series of lines as shown below • it is arranged so that the combination is unique at each point • sensor is an array of photodiodes ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Incremental position encoder – uses a single line that alternates black/white • two slightly offset sensors produce outputs as shown below • detects motion in either direction, pulses are counted to determine absolute position (which must be initially reset) ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Other counting techniques • several methods use counting to determine – position two examples are given below • Inductive sensor ch-1 Length measurement Opto-switch sensor Sensors and Measurement Instrumenattion Systems II

ENCODERS ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

INDUSTRIAL APPLICATIONS of ENCODERS ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II

Reluctance RESOLVERS • A resolver is a rotary transformer that produces an output signal that is a function of the rotor position. ch-1 Length measurement Sensors and Measurement Instrumenattion Systems II