Sensors and Actuators

1. Sensors and Actuators John Errington MSc

2. 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

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

4. 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

5. 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

6. Actuators • Electrical • Hydraulic • Pneumatic

7. 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

8. 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

9. 32 – 63 mm bore hydraulic cylinder

10. 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)

11. 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.

12. 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.

13. DC Motor characteristics

14. 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

15. 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.

16. 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, and adapt them to portable use from batteries.

17. AC motors:synchronous 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

18. Hydraulic actuators Use pressure in fluids to provide motive force. Hydraulic pumps provide pressures of around 3000 psi (200 kg force per sq cm) so a 10cm piston with an area of 5*5*3.14 = 80cm^2 will provide 16000 kg force. Moving this piston requires hydraulic oil to flow around the system so its not fast.

19. Hydraulic pumps and cylinders

20. Flow control valves

21. Pneumatic actuators • Used when rapid movement is required but with no great force. • Pistons are driven by air pressure • Because air is compressible the position is not precisely controlled.

22. Pneumatic cylinders

23. Pneumatic grippers