sensor & transducer
Capacitive proximity sensors
(detect metal and non-metal)
Inductive proximity sensors
(detect metal only)
Sensor Selection
1. Range and Span 2. Error 3. Accuracy 4. Precision 5. Sensitivity 6. Hysteresis error 7. Linearity 8. Repeatability/Reproducibility 9. Resolution
Strain Gauge
A strain gauge is a device used to measure strain on an object. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate . As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge.
Accuracy
Accuracy is the measure of the difference between the measured and actual values. Example : Temperature sensor, accuracy (+-)2OC. The reading given by the instrument can be expected to lie between (+-) 2OC of the true value.
◦ Oscillator
An electronic circuit for generating ac wave-forms and frequencies from a dc energy sources
◦ Trigger Circuit (Schmit trigger)
Detect oscillation (wave-forms) and change the output
Error
Error is the difference between the result of the measurement and the true value of the quantity being measured Error = measured value - true value Eg : measurement system gives a temperature reading of 25°C when the actual temperature is 24°C, then the error +1°C.
◦ Output Switching Device
Gives the output signal
Diffuse
In optical proximity sensing, also known as diffuse, the light source and the receiver are housed in the same device. Diffused sensors detect objects when the light beam, emitted towards the target, is reflected back to the sensor by the target
Sensing Range
In the diagram shown here, the letter "s" refers to a given sensor specimen's actual switching distance when tested. It is defined as the distance (between the standard target and the sensing face of the proximity switch) at which a signal change is generated.
Sensing Range types
Nominal sensing Distance (Sn) Real Sensing Distance (Sr) Useable Sensing Distance (Su) Assured Operating Distance (Sa)
Piezoelectric transducer
Piezoelectric transducer are a type of electroacoustic transducer that convert the electrical charges produced by some forms of solid materials into energy. The word "piezoelectric" literally means electricity caused by pressure The piezoelectric transducers work on the principle of piezoelectric effect. When mechanical stress or forces are applied to some materials along certain planes, they produce electric voltage.
Displacement transducer
Potentiometer Linear Variable Displacement Transducer (LVDT) Strain Gauge Piezoelectric transducer
Precision
Precision is the ability to reproduce a certain set of reading within a given accuracy.
Repeatability
Repeatability is the ability to reproduce the output signal exactly when the same measured input is applied under the same environmental condition.
Resolution
Resolution is the smallest increment in the measured value that can be detected. Example : Temperature sensor Resolution : 0.01oC
Sensitivity (S)
Sensitivity is the ability of the sensor to respond to changes in the measured input. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C.
sensors
Sensors are device that measure a physical quantity and convert it into signal which can be read by observer / instrument. Sensors are parts of a control system which is responsible for collecting and preparing process status data and passing it to the processor.
Linear Variable Displacement Transducer (LVDT)
The differential transformer is a noncontact displacement sensor. LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. LVDT consists of a cylindrical former where it is surrounded by one primary winding in the center of the former and the two secondary windings at the sides.
Potentiometer
The potentiometer, or pot, is a displacement transducer. This active transducer consists of a uniform coil of wire or a film of high-resistive It has a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
Range and Span
The range of a sensor defines the limits between which input can vary. The span is the maximum value of the input minus the minimum value. Example Load cell for force measurement: Range : 0 - 50 kN Span : 50 kN
Nominal sensing Distance (Sn)
The rated operating distance for which the sensor is designed. It is simply used as a standard reference value. It does not take into account any variations (manufacturing tolerances, temperature, voltage, etc..)
Real Sensing Distance (Sr)
The real sensing distance is measured at the rated voltage (Un) and rated ambient temperature (Tn). 0.9 Sn Sr 1.1 Sn
Hysteresis Error
The sensors can give different outputs from the same value of measured quantity according that value has been reached continuously increasing or decreasing change.
Useable Sensing Distance (Su)
The useable sensing distance is measured at the limits of the variations of the ambient temperature (Ta) and the supply voltage (Ub). 0.9 Sr Su 1.1 Sr
Assured Operating Distance (Sa)
This is the operating zone of the sensor and corresponds to the area in which detection of the standard metal target is certain, whatever the variations in voltage or temperature. 0 Sa 0.81 Sn
Photo Sensor Types
Thru-beam Retroreflective Diffuse
Ultrasonic sensor
Ultrasonic sensors measure distance by using ultrasonic waves. The sensor head emits an ultrasonic wave and receives the wave reflected back from the target. Ultrasonic Sensors measure the distance to the target by measuring the time between the emission and reception.
Stability
ability to give the same output when used to measure a constant input over a period of time.
Photo Sensor
◦ A photoelectric sensor is a device that detects a difference in the light level received from the light source. ◦ There are three major types of photoelectric sensors: thru-beam, retroreflective, and diffused. ◦ Each sensor has its own strengths and can be used in a variety of ways.
Types of proximity sensor
◦ Capacitive proximity sensor ◦ Inductive proximity sensor ◦ Photoelectric sensor ◦ Ultrasonic sensor
Retroreflective
◦ In retroreflective sensing, both the light source and the receiving device are found in the same housing. ◦ The sensor works in tandem with a reflector. ◦ The light emitted from the sensor is aimed at the reflector, which is then sent back to the light receiving element.
Thru-beam
◦ In thru-beam sensing, also known as opposed mode, two separate devices are used to make or break a beam. ◦ One sensor houses the light emitter while the other houses the receiver. A thru-beam sensor detects objects when an object interrupts the light beam between the two sensors.
Principle of operation : Inductive Proximity Sensors
◦ Inductive proximity sensor is an electronics proximity sensor, whichdetects metallic object without touching them. ◦ When there is power supply, the oscillator operates to generate a high-frequency electromagnetic field through the coil. ◦ When a metal object enters the high-frequency field, eddy currents are induced in the surface of the target. This results in a loss of energy in the oscillator circuit; consequently, this causes a smaller amplitude oscillation. ◦ The trigger circuit recognizes a specific change in amplitude and generates a signal that will turn the solid-state output ON or OFF. ◦ When the metal object leaves the sensing area, the oscillator regenerates, allowing the sensor to return to its normal state.
Composition
◦ Oscillator ◦ Trigger Circuit (Schmit trigger) ◦ Output Switching Device
Principle of operation : Capacitive Proximity Sensors
◦ The active face is formed by two metallic electrodes - like an "opened" capacitor. When there is power supply; the electrodes forms an electrostatic field. ◦ As the target approaches the face of the sensor, it enters the electrostatic field. This causes an increase in the coupling capacitance, and the oscilator begins to oscillate. ◦ The trigger circuit recognizes a specific change in amplitude and generates a signal that will turn the solid-state output ON or OFF.
Proximity Sensor
◦A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. ◦ Solid-state electronic devices that are completely encapsulated (filled with epoxy resin) to protect against excessive vibration, liquids, chemicals and corrosive agents found in industrial environment. ◦ A capacitive proximity sensor or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target