1 . (a) A temperature control process has a first-order response with a step response time of 12 seconds. in selecting a temperature sensor for the control system for the process, for what maximum value of sensor time-constant would you be looking?

(b) If your requirement is for a sensor suitable for a heavy industrial application, with a linear characteristic, working in temperatures up to 300°C, what type of sensor would you recommend? Draw the symbol for the sensor and show its interfacing circuit.

2. In a flow control loop in a process for which you are specifying a flow sensor, it is undesirable to insert any sensing device into the actual flow because of interference with the flow. Because of the corrosive nature and high pressure and temperature of the flow, only stainless steel pipes can be used anywhere in the system. The liquid is non-conductive. Recommend a flow sensor, explaining its operating principle.

3. A request has been received from an external customer for recommendations for linear position control sensors to be used in an explosive environment. For this reason unsafe electrical contacts must be avoided. The sensors will be installed in locations that are hazardous to access except when the plant is shut down annually for maintenance. The span required is 5 cm. The sensor case must not permit any ingress of dust or of water.

State what type of position sensors you would recommend and give your reasons. Sketch both the recommended sensor and circuit, and explain its operation. Comment on how signal conditioning requirements can now be met. Identify the durability required in respect of dust and water ingress.

4. A requirement to sense glass bottles leaving an automatic washer on a production line conveyor belt has been notified to you. The output from the sensor is to be of binary form, i.e. logical I when the part is present and logical 0 when no part is present. It is also necessary to ensure that a change of sensor state occurs closer to the sensor when a bottle approaches than when it moves away. The switching distance will lie between 5 and 20 mm. The production line is for food products and the environment is clean and maintained at a temperature of 15°C. Bottles will pass the sensor at the rate of 10 every second. There is no requirement for protection from contact with objects but it must be proof against water drips.

Recommend a suitable sensor and justify your recommendation. Sketch a diagram that shows the functional blocks within the sensor and explain each function.

You may refer to the tables in Lesson MSP - 3 - 7.

Would the recommended sensor be suitable if a change was made from glass to plastic bottles?

5. (a) In which type of memory is the executive program that controls the PLC located?
(b) How is loss of a loaded program prevented when the PLC is switched off?
(c) Is opto-coupling found only on the inputs to a PLC?

6. A machine tool in a manufacturing cell is driven by a 3-phase a.c. motor controlled by an inverter. Before the inverter receives a RUN signal from a PLC, the following conditions must be met:
(a) The workpiece is to be in its initial position in the chuck. (WPI)
(b) Lubrication pressure is to be 5 bar or above. (LUB)
(c) No movement is to have been detected in the cell for 1 minute. (SAFE)
(d) The pre-start inverter self-test routine is to be satisfactory. (TEST)
(e) The motor must be stationary, i.e. the motor speed signal is to be zero. (RPMO)
(0 The manufacturing cell door is to be shut. (DOOR)

If the above conditions are met the PLC is to output a RUN signal to the inverter. The output is to be from a rung dedicated to outputs.

The memory element on this rung is to be activated by memory cell MC 1. The output rung is also to control an output to the manufacturing cell door bolt solenoid.

The memory cell MC 1 is to remain on until one of the following occurs:
(a) Motor speed exceeds 1000 rpm. (RPM1000)
(b) The emergency stop button is pressed. (EMER)
(c) Lubrication pressure falls below 5 bar. (LUB)
(d) A signal indicating the end of machining is received. (END)

When memory cell MC I goes high, the following are to occur:
(a) MC 1 is to receive a hold in supply.
(b) The inverter is to receive a RUN signal. (RUN)
(c) The manufacturing cell door is to be locked by a solenoid-operated bolt. (LOCK)
Draw a ladder diagram to represent the software program that should be installed in the program memory of the PLC. Identify the elements on the rungs by the abbreviations given.

7. (a) An internal customer has asked you to list the factors that you would take into account in selecting a rotary electrical actuator for a mechatronic application. Provide the list for them.

(b) A stepping motor in use for a mechatronic application is to have its acceleration time increased and its deceleration time decreased. How would these adjustments be implemented?

(c) It has been decided to improve the resolution of stepper motors used in your company's products. How can this be achieved most cost-effectively?

(d) What are the specification names given to the following stepper motor characteristics?
(i) The maximum load torque that can be placed on the motor shaft without loss of steps.
(ii) The maximum torque that can be applied to a powered-up motor without shaft rotation.

(iii) The fastest stepping rate that can be applied to a motor accelerating from rest, without loss of steps.
(iv) The slowest step frequency to which the motor will accurately respond.
(v) The range of frequencies between (iii} and (iv) above.
(vi) The greatest load that the motor can cope with for a given stepping rate, and accelerate to its operating speed without loss of steps.

8. (a) Explain, with the aid of a sketch, how the piston of a hydraulic ram can be mechanically linked to an input servo valve to obtain a negative feedback servomechanism. Describe how a position demand is met.

(b) Assume that you are employed by a company that manufactures hydraulic servomechanisms. You have been asked to recommend an electrical method of providing position feedback rather than the mechanical method used at present. The demand signal has been chosen to be a d.c. voltage. Cost-effectiveness and reliability are essential.

Examine position sensor options and select a cost-effective and reliable solution. Justify your choice.
Sketch a closed-loop block diagram of the modified hydraulic servomechanism, identifying the sensor, its signal conditioning and the form of error detector. You can assume that the servo valve will be operated by a progressive solenoid that receives its operating current from a voltage-to-current converter.