sensors31
TRANSCRIPT
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Sensors
BADI Year 3
John Errington MSc
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Sensors
• Allow a design to respond to its environment – e.g. a line following robot may use photosensors to detect a paint line on the floor.
• Enable systems to determine when the required action has been completed; for example a trip switch will indicate that a lift has reached the desired floor.
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Trip switches
Perhaps the most common sensor used in industry is the trip switch or limit switch – a simple microswitch in an industry-sealed case with a rugged mechanical activation system such as a lever + roller as shown above
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Strain gauges
Also extremely widely used, strain gauges measure the amount of extension or compression experienced by the material it is fastened to. This gives a measure of the strain, and can thus be used to measure deformation or applied load.
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Optical sensors
• Photodiodes and phototransistors• Integrated photoswitches• Light dependent resistors• Diode arrays (1d & 2d)• CCTV & digital cameras• Low light sensors e.g. photomultipliers
• No moving parts so inherently reliable BUT• Susceptible to dirt and changes in ambient light
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Photodiodes
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OptoSwitches
Sensors comprising a light source, photodiode and amplifier
Diffuse scan – detects reflected light
Through scan – detects when beam is interrupted
Hall effect sensor also shown here detects when a ferrous metal breaks the magnetic field
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Proximity sensors
Types:• Mechanical• Inductive• Capacitive• Ultrasonic• Optical
Application etc.• Contact sensing• Ferrous only 1cm• Metals only 1cm• 6m range solid/liquid/powder• Through beam or • Reflective • Clean environments only
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Non-contact sensors (1)Inductive proximity sensor
Coil inductance increases as iron / steel object (S ) gets closer, because lines of magnetic flux can flow through the iron, making the effective path shorter.
Capacitive proximity sensor
C1
C3
C2
Capacitance increases as metal object (P) gets closer because additional capacitance paths C2 & C3 are added and increase in value as the separation reduces. C1 is always present.
S P
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Non-contact sensors (2)
Other non-contact sensors use sound (ultrasonic) or electromagnetic waves (light, microwaves, etc.) to gather information about the distance between the sensor and a surface.
Microwave sensors are reliable in dirty industrial environments but exposure of personnel must be prevented.
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Proximity switchesCapacitive, Ultrasonic Inductive
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Rotary and linear position
• Potentiometric (sliding contact)
• LVDT
• Encoders
Main issues: operating conditions, reliability, analog or digital output
Contact or non contact - is wear important?
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Magnetic pick-ups
Often used to detect engine timing by counting teeth on the starter gear.
Robust, reliable non-contact
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Rotary shaft encoder
Gives a digital output indicating the angular position of a shaft. They often use a disk with marks like a bar code, that are read by optical photodetectors inside the encoder.Robust, reliable non-contact.
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Absolute rotary encoder
An absolute encoder has a number of binary outputs that indicate the switch shaft's absolute rotational position referenced to some spot on the switch's body. For example, a three bit absolute rotary encoder will divide the rotational position into eight sectors. Absolute encoders are available in a variety of style with a variety of resolutions.
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Incremental rotary encoder
An incremental rotary encoder won't tell you where the shaft is positioned. It will only tell which direction the shaft is being turned and how fast. Incremental encoders have two outputs called phases. Each outputs a square wave. Turning the shaft one direction causes one phase to lead the other by 90 degrees. Reversing the direction will cause the other phase to lead. The frequency of the output is proportional to the rotational speed of the shaft.
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Rotary position sensors
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Linear position
• Linear Variable Differential Transformer
• Other (cheaper!) linear position sensors use a resistive track and slider but this is less precise and reliable due to wear of the track
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Potentiometric sensors
+
-
Resistive track may be wire wound on a former, metal film or graphite on substrate L
x
Vs
Vout
The voltage picked off is proportional to the position of the sliding contact
Vout = Vs * x / L
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Open loop control
• Simple and easy to implement
• No feedback of controlled parameter
• No certainty that parameter is at desired value
• Change in system parameters such as load will result in a deviation from the desired value
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Servo systems
Measure output variable
Compare to desired value
Amplify the difference
Apply to correct the error
Result: servo system can adjust for changes in load, amplifier gain etc and still give good compliance with required value