Sensors and Actuators John Errington MSc. Sensors and Actuators Sensors produce a signal in response to a change in their surroundings e.g. Thermostat.

Sensors and Actuators

John Errington MSc

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

Sensors

• Convert one type of energy to another

• Mainly used for providing an electrical signal related to some physical parameter

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

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

Actuators

• Electrical

• Hydraulic

• Pneumatic

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

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

32 – 63 mm bore hydraulic cylinder

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)

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.

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.

DC Motor characteristics

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

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.

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.

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.

6 – 90W 0.25 – 3 kW2 – 8W

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.

Hydraulic pumps and cylinders

Flow control valves

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.

Pneumatic cylinders

Pneumatic grippers