UNIT 332

What is a hand held programmer?

Also called a terminal programme unit. It is a communication unit which is connected to the PLC, used to program the PLC and interrogate internal memory and data.

What does calibration mean in terms of sensors?

An adjustment or set of adjustments performed on a sensor or instrument to make that instrument function as accurately or error free as possible.

What are the applications of a three phase motor?

- electric train engine - cooling fans - rolling mills - printing machines

What are the applications of a DC shunt motor?

- fans - pumps - small machine tools

What are the applications of a stepper motor?

- hard disk drives - robotics - antennas - telescopes

What are the examples of SENSORS?

- rotational position: tacho, encoder - linear position: LVDT, potentiometer

What are the effects upon the environment if a hydraulic system leaks?

- slip hazards - hazard to health - contamination of the surrounding environment

What are the applications of a separately excited DC motor?

- trains - automatic traction purposes - actuators

What are the applications of a single phase motor?

- vacuum cleaners - fans - washing machines - centrifugal pumps - blowers

What are the typical types of signal conditioning?

- voltage to current - current to voltage - pressure to voltage - pressure to current - analogue to digital - digital to analogue - frequency to voltage - frequency to current - sink to source - source to sink

What type of pneumatic compressors are there?

Centrifugal compressor - air enters the centre of the impeller, centrifugal effects throws air into a volute where it goes to the diffuser resulting in a pressure rise Axial compressor - the inlet area is much greater than the eye of the centrifugal compressor, hence it delivers more flow

What does resolution mean in terms of sensors?

Resolution of a sensor is the smallest change it can detect in the quantity that it is measuring.

What does repeatability mean in terms of sensors?

The ability of a sensor to provide the same result under the same circumstances, over and over again.

What are logic gates?

The basic building blocks of digital systems such as calculators and computers. Decision making process blocks which perform particular logic functions summarised in truth tables. A and B = inputs, Z = output. They can only have one of two values, logic 1 or logic 0.

What is slip?

The difference between the flux (Ns) and the rotor speed (N). Slip speed = Ns - N % Slip = (Ns - N)/N x 100

How do you calibrate a sensor for a 0-10v to 4-20mA converter?

- Apply a known 0v signal to the converter and adjust 'zero' calibration setup until output = 4mA - Apply a known 10v signal, adjust span calibration setup until output = 20mA - Repeat three times to ensure repeatability and reliability.

What is the operation of a frequency to voltage converter?

- D1/C1 form a rectifier circuit. The DC value of C1 is determined by the frequency of the tacho signal. - The higher the frequency, the higher the voltage on C1. - As the voltage on C1 increases, TR1 conducts harder, pulling the voltage at the collector/DC output down to 0v - reducing the potential at the collector and DC output voltage. In summary.... as incoming tacho frequency increases, DC control voltage reduces proportionally.

What types of electrical actuators are there?

- DC/AC motor - Servo/stepper/linear motors - Various rotary to linear drives - Buzzer - Bell - Horn - Incandescent lamp - LED

What are the applications of a micro switch?

- Lifts: control panel switches for floor selection - Timer controls: triggers when timer runs out - Push buttons for alarms: micro switches are used as the buttons for fire alarms, emergency stop controls and emergency door releases - Air conditioning units: detect if door panels are closed, angle of the louvers and are used as reset buttons.

What are the typical sensors of a system?

- Micro switch - Wobble stick - Pressure mat - Inductive proximity - Capacitive proximity - Optical proximity - Thermocouple - Encoder (incremental and absolute)

What types of sequence controller are there?

- PLC - Micro controller (PIC) - Conventional relay logic - Time driven - sequence driven on time - Event driven - wait for something to happen previously

What are the examples of the CONTROL ELEMENT?

- PLC - Microcontroller

What are the applications of a strain gauge?

- Power plants - Ships - Refineries - Automobiles

What are the applications of a light curtain?

- Prevent machine start-up - Form a perimeter guard around a danger zone - Replacing fixed guarding or fencing

What are the applications of an optical proximity sensor?

- Smartphones: screen brightness adjustment - Smartwatches: measure wearer's heartbeat

What are the applications of a solenoid?

- To power a switch, like the starter in an automobile - Valve, in a sprinkler system

What can be used to program a PLC?

- computer with programming software - hand held programmer

What are the applications of a DC series motor?

- cranes - lifts - aircraft elevator controls

What are the examples of the CORRECTION ELEMENT?

- electric or fluid power motor

What are the examples of the PROCESS ELEMENT?

- gearbox (linear - rack and pinion, rotary - bevel) - heater element

What are the applications of a pressure mat sensor?

- sensors for interactive toys - sensors for health care industry - sensors for interactive multimedia systems

What other valve can be used to operate a single acting pneumatic cylinder?

2/2 valve

What other valve can be used to operate a double acting pneumatic cylinder?

5/2 valve

What is a mechatronics system?

A combination of electrical, electronic, computing and mechanical systems. The mechanical aspects can be considered to be drives, pneumatic and hydraulic systems.

What is an incremental encoder?

A electromechanical device that has two output signals, A and B, which issue pulses when the device is moved. Together, the A and B signals indicate both the occurrence of and direction of movement. It does not indicate absolute position.

What is a DC series motor?

A machine that is used at a place where high torque operating conditions are required. The field winding is associated in series with the armature winding of the motor. This arrangement means the motor is able to produce more starting torque. A series motor has a high starting current which provides a high starting torque. As the speed increases, the armature current, field strength and torque reduce. The speed increases with little load and decreases with a bigger load. If there is no physical load present the motor with self destruct. To reduce high armature current on start up a variable resistor can be placed in series with the armature winding.

How does a push button switch work?

A small, sealed mechanism that completes an electric circuit when you press it. When it's on, a small metal spring inside makes contact with two wires, allowing electricity to flow. When it's off, the spring retracts, contact is interrupted and current won't flow.

Spot welder and robot arm as an example of a controller network.

A spot welder is connected to a robot arm and both have separate controllers. The welder has MASTER control as it is a high level control system with multiple feedback facility to ensure the quality of the weld is constant. The robot arm also has MASTER status as it gives feedback of the position of the welding effector. The two control systems have to communicate and share data simultaneously. This requires transparent communication between both controllers.

What is an encoder?

An electrical mechanical device that converts linear or rotary displacement into digital or pulse signals.

What is a relay?

An electromechanical switch operated by a relatively small electric current that can turn on or off a much larger current. At the heart of the relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it). Relays bridge the gap, making it possible for small current to activate larger ones. This means relays can work either as switches (turning things on and off) or as amplifiers (converting small currents to larger ones.

What does cross talk mean in terms of sensors?

Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel.

What does grounding mean in terms of sensors?

By grounding sensors or connecting them to zero voltage it provides a common point/reference point for a voltage level.

Why are sensors grounded?

By grounding them or connecting them to zero voltage it provides a common point/reference point for a voltage level.

What are the components of a Microcontroller (PIC)?

COMMUNICATIONS PORT - offers a means of downloading and uploading data CPU - processes the data received from the comms port and input port, and outputs the results of the processed program to the output ports ANALOGUE TO DIGITAL CONVERTER - converts the varying input voltage levels into corresponding digital values INPUT/OUTPUT - connect the digital system to the physical environment

What are the basic elements of a closed loop control system?

COMPARISON ELEMENT - compares reference value of the variable condition being controlled with the measured value of what is being achieved and produces an error signal. Error signal = Reference value - Measured value. CONTROL ELEMENT - decides what action to take when it receives an error signal CORRECTION ELEMENT - sends a signal to the process to produce a change which corrects the controlled condition PROCESS ELEMENT - this is what is being controlled MEASUREMENT ELEMENT - produces a signal related to the variable condition being controlled

How does a thyristor work?

Conducts when forward biased. When there is a positive clock pulse at the gate the SCR turns on. Can only be turned off by either removing the supply voltage or by reducing the anode to cathode current externally (switch)

How does a capacitive proximity sensor work?

Consists of a high frequency oscillator and a sensing surface formed by two metal electrodes. When an object comes near the sensing surface, it enters the electrostatic field of the electrodes and changes the capacitance of the oscillator. The oscillator circuit starts oscillating and changes the output state of the sensor when it reaches a certain amplitude.

What are controller connections (networking)?

Controllers can be networked together so each cell can have an individual controller. Once the first cells operation is complete, the cells controller communicates to the next cell for it to take over control and forward the desired operation to the next stage/process.

What is the advantage of distributed control in a mechatronic system?

Conventional control systems rely on centralised control with all actions being distributed from one point. Distributed control gives each area autonomy therefore, if one part of the system fails, the rest will continue to work.

What is the advantage of routine condition monitoring of a mechatronic system?

Conventional control systems rely on planned maintenance whereas mechatronics systems use routine condition monitoring which means maintenance costs are cheaper as the physical condition is monitored and parts aren't changed at a set period.

What is the advantage of networking in a mechatronic system?

Conventional control systems reply on dedicated hard wired discrete I/O. With the use of networking mechatronic systems it vastly cuts down cabling and therefore, cost.

What is the advantage of using software logic in a mechatronics system?

Conventional control systems use hard wired relay logic whereas mechatronics systems use software logic therefore, faults are easily found and reprogrammed, rather than tracing hard wired points to find and rectify the fault.

What is the advantage of using integrated control in a mechatronics system?

Conventional control systems use sequence control whereas mechatronics systems use an intelligent sensor and integrated control which gives more production flexibility and shorter down time.

What prime movers - electrical actuators are thers?

DC MOTORS - separately excited: field and armature are excited by different sources - shunt: field winding in parallel with armature - series: designed for large currents, field and armature in series - compound: combination of shunt and series SERVO MOTORS - variation of gear-head motor coupled with a potentiometer to give feedback on the motors position - by pulsing the RC circuit, you can set the motors position in a range of 0 to 180 degrees - three wires = power, ground and digital control line STEPPER MOTORS - move in a series of steps - centre shaft has magnets mounted around it - has no commutator - coils surrounding shaft are switched using current which attracts rotor - 5-6 wires, 1 for eahc coil and 1-2 for ground - 2 coils need to be on for maximum torque

How does a wobble stick?

Designed for delicate and precise manipulations of samples and materials in ultra high vacuum (UHV) systems. The shaft of the wobble stick can be guided by the operator in a smooth and controlled fashion. The wobble stick may be controlled in a linear and angular motion as well as continuously rotated.

What is the control of a double acting pneumatic cylinder?

Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high pressure air. A 4/2 way directional control valve is used.

What is an absolute encoder?

Feedback devices which provide speed, position information by outputting a digital word or bit in relation to motion. Unlike incremental encoders that output a continuous stream of present pulses, absolute encoders output unique words or bits for each position.

What type of hydraulic pumps are there?

Gear pump - cheap but becomes troublesome at high operating speeds and pressures Vane pump - can be used as a variable-delivery pump Axial piston pump - high efficiency and can be used at higher hydraulic pressures than gear and vane pumps

What gearing can be used?

Gearing can be used to convert the rotary motion into linear motion. SPUR AND HELICAL GEARS - used for the transmission of rotary motion between parallel shafts BEVEL GEARS - used for the transmission of rotary motion between shafts which have axes inclined to one another RACK AND PINION - transforms either linear motion to rotational motion or rotational motion to linear motion GEAR TRAINS - used to transfer and transform rotational motion

How does a 5/2 directional control valve work?

Has five connection ports - inlet, two ports that connect to device being connected and two exhaust ports. The two states of the valve are: - the inlet connects to port 2 while port 4 vents through exhaust port (5). - the inlet port connects to port 4 while port 2 vents through exhaust port (3).

How does a pressure mat sensor work?

Have a normally open contact which closes or shorts when a person walks or stands on the mat. This completes the circuit. The contact opens or breaks as the person moves off the mat.

What mechanical actuators are there?

Hydraulic - uses incompressable fluid - has a positive action or motion - faster response than pneumatic Pneumatic - air is readily available - no recycling is necessary - non-flammable - negligible change in viscosity

What is hydraulics?

Hydraulics is moving and operating equipment with oil under pressure. It is any system working on fluids under pressure. Require a complete circuit from pump to pump to circuit the hydraulic oil. A hydraulic pump, drives a prime mover by forcing hydraulic fluid around a system.

How does an impeller flow sensor work?

Impeller flow sensor probes are equipped with light weight helical impellers. The impellers contain magnets which actuate hall-effect switches within the probe to detect the impeller rotation. The impeller rotation is proportional to flow rate. A hall effect sensor varies its output in response to a change in the magnetic field.

What feedback sensors can be used?

Incremental encoder - used for the measurement of angular velocity and position Tachogenerator - used to meausure angular velocity Potentiometer sensor - linear or rotary displacements are converted into potential difference

How does an optical proximity sensor work?

Infrared light is emitted from the IR LED emitter. The beam of light hits the object and gets reflected back at an angle. The reflected light reaches the light detector and the sensor in the light detector determines the position/distance of the reflective object.

How does a snap action limit switch work?

Involves a rapid motion of contacts when changing between closed and open states. Only small amounts of force are required from the actuator to initiate the snap action. As the actuator is depressed it reaches the operating point. The common contact accelerates away from its free position with the normally closed contact towards the normally open contact. The acceleration is caused by the spring mechanism of the switch.

How does a solenoid work?

It is a coil of wire wrapped around a piston, often made of iron. A magnetic field is created when an electric current passes through the wire. The magnetic field of an activated solenoid has positive and negative poles that will attract or repel material sensitive to magnets. In a solenoid, the electromagnetic field causes the piston to either move backward or forward, which is how motion is created by a solenoid coil.

What are the application of a snap action limit switch?

Levelling and safety switches in vending machines, arcade buttons and to detect paper jams or other faults in photocopiers.

What type of pneumatic actuators are there?

Linear - single acting cylinder - double acting cylinder Rotary - vane type - air motor

How does a thermocouple work?

Made up of two dissimilar metals joined together to form two junctions. One junction is placed on the surface or in the environment that is being measured - the 'hot' junction. The other junction remains at a known constant temperature - the 'cold' junction. By changing the temperature at the hot junction, voltage will flow through the loop. The voltage can be converted into a temperature reading using a formula.

What is the difference between an OR gate and XOR gate?

OR gate = one input or the other need to be true for the output to be true. Both inputs true means output will be true. XOR gate = one input or the other need to be true for the output to be true. Both inputs true means output will not be true.

How does a PLC operate?

POWER SUPPLY UNIT - supplies low voltage power to the CPU and I/O cards MEMORY - RAM contains the program and data that is executed and processed by the CPU CENTRAL PROCESSING UNIT - Interrogates the inputs, compares them to the program and updates the outputs every cycle of operation

What is the importance of signal conditioning?

Plays an important role to achieve signal at a particular level when there are many inputs or outputs between the PLC controller and the outside world. Signal conditioning also isolates the system from possible electrical hazards such as high voltages.

What are the applications of an absolute encoder?

Provide a unique position value or data word at every point of rotation representing the 'absolute' position of the encoder.

How does a two stage reciprocating compressor work?

STAGE 1 - Air is compressed to medium level in the cylinder/piston. Moves a high volume of air at a low pressure. COOLER - Air is cooled back to a much lower level, making the compressor more efficient and reducing stress. STAGE 2 - The air is compressed further to the end pressure, which is the smaller cylinder/piston. It moves a lower volume of air but at a higher pressure.

What is the control of a single acting pneumatic cylinder?

Single-acting cylinders use one air port to allow compressed air to enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the home position when the air pressure is removed. A 3/2 way directional control valve is used.

What are the applications of a thermocouple?

Temperature measurement for: - Kilns - Gas turbine exhaust - Diesel exhaust

What is a soleoid?

The generic term for a coil of wire used as an electromagnet. It also refers to any device that converts electrical energy into mechanical energy using a solenoid. The device creates a magnetic field from electric current and uses the magnetic field to create linear motion.

What is a closed loop control system?

The output does have an effect on the input signal, modifying it to maintain an output signal at the required value. The initial setting will be for a particular flow/pressure/speed and this is maintained by feedback. They have the advantage of being relatively accurate in matching the actual to the required values. They are more complex and so more costly with a greater chance of breakdown as a consequence of the greater number of components.

What is an open loop control system?

The output from the system has no effect of the input signal. They have the advantage of being relatively simple and consequently low cost with generally good reliability. However, they are often inaccurate since there is no correction for error.

How does a 4/2 directional control valve work?

There are four connection ports - inlet, two ports that connect to device being controlled and a common exhaust port.

How does a 3/2 directional control valve work?

There are three connection ports - inlet, outlet and exhaust. The two positions are open and closed. When the valve is open, air flows from the inlet to the outlet. When the value is closed, air flows from the outlet to the exhaust. Non-actuated = normally closed.

What are the applications of an impeller flow sensor?

Used for low flow applications or when high measurement accuracy is required.

What are the applications of an incremental encoder?

Used in positioning and motor speed feedback applications which includes servo/light industrial oe heavy-duty applications.

What are the applications of a wobble stick?

Used mainly to operate in vacuum mechanisms such as: - adjustment probes - variable orifices - screwdrivers - to tightly lock sample holders in their dedicated position

How does a TRIAC work?

When MT2 gets a postive voltage with respect to MT1 and the gate gets a positive trigger then the left SCR of the TRIAC completes the circuit. If the polarity of the terminals change and a negative pulse is applied to the gate then the right SCR completes the circuit. When the gate current it removed, the TRIAC switches off. A minimum holding current must be maintained at the gate to keep the TRIAC conducting.

What is boolean algebra?

When programming logic terms will be used. Typical digital logic terms are as follows: AND . NOT - OR +

How does a micro switch work?

Reacts to environment changes by opening or closing a set of switching contacts to control an appliance. When the micro switch senses movement from its surroundings, a plunger is pressed or released to open or close a circuit.

What are the applications of a capacitive proximity sensor?

- high/low tank level - material present/ absent - dry tank

What are the applications of a servo motor?

- robotics - conveyor belts - camera auto focus - solar tracking system

What are the applications of an inductive proximity sensor?

- Metal detectors - Traffic lights - Car washes

How does a light curtain work?

A photoelectric conductor projects an array of synchronised, parallel infrared light beams to a receiver unit. When an opaque object interrupts one or more of the beams in the sensing field, the control logic of the light curtain sends a stop signal to the guarded machine.

What does deadband mean in terms of sensors?

A region of pressure where a change in pressure produces no change in measurement output of control signal.

What types of electropneumatic elements are there?

ACTUATORS - pneumatic cylinders - rotary actuators - lamps/buzzers PROCESSORS - logic elements - pneumatic/electric converter - relays CONTROL ELEMENTS - solenoid actuated directional control valves - relays SENSORS - limit switches - push buttons - proximity sensors ENERGY SUPPLY - compressor - pressure regulator - AC or DC power supply - receiver - air service equipment

How can you control the speed/action of a cylinder?

Connect a flow control valve to either the inlet or outlet?

What is a separately excited DC motor?

Stator contains field windings and rotor contains armature windings. There is a separate supply provided for excitation of both field coil and armature coil. The separate supplies mean that any change in armature current has no effect on field current. The motor can operate above base speed. Direction of rotation of motor can be reversed by reversing voltage. The field current is constant as it is supplied with a constant voltage.

How does a relay work?

1) The input circuit ha no current flowing through it until a sensor or switch turns it on 2) When a small current flows in the input circuit, the electromagnet is activated which produces a magnetic field 3) The electromagnet pulls the metal bar in the output circuit closed, allowing large currents to flow 4) The output circuit operates a high current appliance such as a lamp or electric circuit

What is a stepper motor?

A brushless, synchronous motor which divides a full rotation into a number of steps. Steps per revolution: 12, 24, 72, 144, 180 and 200 result in stepping angles of 30, 15, 5, 2.5, 2 and 1.8. Electromagnetic stator coils are surrounded by a soft iron or magnetic rotor shaft. The rotor and stator have poles which may be teethed. When the stators are energised, the rotor aligns itself with the stator or moves to have a minimum gap with the stator. The stators are energised in sequence to rotate the stepper motor. There are three types : permanent magnet stepper variable reluctance stepper hybrid stepper motor

How does an inductive proximity sensor work?

A coil is wound around an iron core within an electromagnetic field to form an inductive loop. When a ferromagnetic material is placed within the eddy current field, the inductance changes. The proximity sensor detects this change and produces an output voltage.

What is a servo motor?

A rotary actuator or motor that allows for precise control in terms of the angular position, acceleration and velocity. It has certain capabilities that a regular motor does not have therefore, it uses a regular motor alongside a sensor for position feedback. Servo motors work on the principle of pulse width modulation (PWM) which means its angle of rotation is controlled by the duration of pulse applied to its control pin. A command input is given according to the position of the shaft. If the feedback signal differs from given input, an error signal alerts the user. This error signal is amplified and applied as the input to the motor, hence the motor rotates. It is a closed loop feedback system.

How does a strain gauge work?

A sensor whose resistance varies with force. It converts force, pressure, tension, weight etc into a change in electrical resistance which can then be measured. When external forces are applied to a stationary object stress and train are the result.

What does sensitivity mean in terms of sensors?

A sensors sensitivity indicates how much the sensor's output changes when the input quantity being measured changes.

What is a DC shunt motor?

The field windings are connected in parallel to the armature winding. Both windings expose to the equal voltage power supply. This motor maintains an invariable speed with any kind of load. It has a low starting torque. The field current and magnetic field remain constant and do not change with motor speed. Torque only varies with changes in the armature current. Start-up torque is slightly less than series motor. A DC shunt motor will not accelerate to destruction with the absence of a physical load as field current does not vary with speed.

How does a three phase induction motor work?

The stator of the motor consists of overlapping windings offset by an electrical angle of 120 degrees. When the primary winding or stator is connected to a three phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed. An emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor. The rotor speed may not reach the synchronous speed of the stator. If the speed becomes equal, there would be no relative speed and no emf induced in the rotor therefore, no current would flow and torque would not be generated. Consequently, the rotor cannot reach the synchronous speed. The difference between the synchronous speed and the rotor speed is called slip.

How does a single phase induction motor work?

When a single phase AC is applied to the stator winding of a single phase induction motor, the alternating current starts flowing through the stator or main winding. This alternating current produces an alternating flux called main flux. The main flux links with the rotor conductors and cuts the rotor condutors. Emf gets induced in the rotor and current starts flowing. The rotor current produces its flux called rotor flux. The two fluxes produce the desired torque which is required by the motor to rotate. There are 4 types: split phase capacitor start capacitor start and run capacitor start capacitor run

How does a single acting hydraulic cylinder work?

When you feed hydraulic oil into it through the inlet port, the piston extends and exerts a force. Pressurised fluid can be provided by hand pumps, electric pups or air driven pumps. When the work is done, the fluid should be released back into the tank. A valve can be fitted to the pump to do this. There should always be a way of returning the cylinder.

How does a solid state relay work?

While the sold state relay and electromechanical relay are fundamentally similar in that their low voltage input is electrically isolated from the output that switches and controls the load. Electromechanical relays have a limited contact life cycle, can take up a lot of room and have slower switch speeds (especially large power relays and contactors). Solid state relays have no such limitations and have o moving parts that wear out.

How does a double acting hydraulic cylinder work?

You need a four way directional control valve to operate a double acting cylinder. If you pressurised the bottom of the ram you will see the piston extends. By reversing the four way valve and pressurising the reduced bore end, the piston can be retracted.