Sensors and Actuators

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Sensors and Actuators

• John Errington MSc

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Sensors and Actuators

• Sensors produce a signal in response to a change
  in their surroundings
• e.g. Thermostat turns heater on or off in
  response to temperature
• Actuators produce a physical change in their
  surroundings in response to an input signal
• e.g. electric drill starts to turn when power is
  turned on

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Sensors

• Convert one type of energy to another
• Mainly used for providing an electrical signal
  related to some physical parameter

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More about sensors

• May be on/off threshold sensing
• Examples trip switches, thermostats
• Or proportional
• Examples temperature sensors, load cells
• Output may be
• a switch open / closed
• a voltage or current
• a digital signal

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Actuators

• Devices that bring about a physical change e.g.
  movement or heating) in response to a control
  signal.
• When talking about actuators we usually use the
  term to mean motors things that produce a
  movement

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Actuators

• Electrical
• Hydraulic
• Pneumatic

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Electrical Actuators

• Very widely used as easy to control. Available in
  a huge range of power ratings
• Motors (producing rotation)
• Linear Motors (movement along a rail or track)
• Solenoids
• Muscle wires
• Heaters

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Hydraulic Actuators

• Slow but very powerful, used for moving or
  transferring heavy loads. Liquids used are
  virtually uncompressible so they provide precise
  control of position, limited only by the
  flexibility of hoses etc.
• Examples
• Hydraulic rams found on JCB loaders etc.
• Hydraulic motors used for turning bridges

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32 63 mm bore hydraulic cylinder
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Pneumatic Actuators

• Pneumatic actuators are capable of very fast
  responses. As gases are compressible they lack
  the precise positional control and enormous power
  of hydraulics.
• They include
• Pneumatic cylinders
• Air blast
• Motors with very high speeds (100,000 rpm)

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Power for hydraulic and pneumatic systems

• Hydraulic and pneumatic systems often rely on
  electrical systems compressors and pumps as a
  source of power. Hydraulic systems mounted on
  vehicles frequently use the vehicles engine to
  drive the hydraulic pump.

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DC motors

• DC Motors are widely used in mobile robots. They
  are able to work from a battery supply, so no
  external power supplies are required. DC Motors
  are robust and reliable, well suited to these
  applications.

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DC Motor characteristics
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Application

• DC motors are employed in many industrial
  applications, because they are so versatile
  they can provide a wide range of speeds and
  starting torques.
• Methods of speed control are simpler and less
  expensive than for AC motors

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Why use a DC motor

• AC motors, pneumatic and hydraulic systems need
  trailing wires cables or pipes that can be a
  disadvantage when working in confined spaces.
• DC motors allow free operation from internal
  power sources e.g. batteries and are more easily
  controlled.

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AC motors

• AC motors are even simpler, more robust and
  reliable than DC motors.
• Recent advances in power conversion, control
  techniques and microcomputers are helping AC
  drives to become more competitive.

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AC motorssynchronous and induction motors
Synchronous motors work at a constant speed
depending on the supply frequency. They are
easily stopped. Induction motors are powerful
used for lifts, lathes bench drills, power
tools etc.
2 8W
6 90W
0.25 3 kW
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Hydraulic pumps and cylinders
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Flow control valves
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Pneumatic cylinders
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Pneumatic grippers