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Cork Institute of Technology
Bachelor of Science (Honours) in Electrical Power Systems – Award
(NFQ – Level 8)
Summer 2007
Automation
(Time: 3 Hours)
Instructions Answer four questions
Examiners: Mr. R. Daly Prof. E. McQuade Mr. M. Hennessy
Q1 (a) Show by means of a neatly drawn, fully labelled diagram, the internal circuitry of an emergency stop safety relay complying with EU safety requirements. 10 marks
(b) Show how a double-pole, emergency stop push-button is connected to the relay unit in (a) in order to comply with Category 4 safety requirements and how a direct on line starter with redundancy is connected in the safety circuit. Explain fully, the operation of the circuit. Contact numbering to be used throughout for reference purposes. 9 marks
(c) Explain clearly, by means of the diagram references in (a & b), what is meant by the terms circuit monitoring and redundancy? 6 marks
Q2 (a) Compare in terms of temperature v output change, the outputs from: (i) Themocouple (ii) PT100 RTD (iii) NTC Type Thermistor 6 marks
(b) At what temperature is the resistance of the TD100 = 100Ω? 1 mark
(c) A thermistor has a resistance of 4714Ω at 15O^ C and a ß constant of 3891 O^ K. At what temperature will its resistance be 1310Ω? 6 Marks
Table 1
Temperature Resistance Ω
T 1 = 60 O^ C
T 2 = 56 O^ C
T 3 = 50 O^ C
R 1 = 746.
R 2 =?
R 3 = 1081
(d) Use table 1 above to interpolate the resistance R 2 of the thermistor at 56 OC. 4 Marks
(e) The resistance changes from the thermistor above are to be converted to a 0 to 10V for connection to an analog input module in a PLC. The resistance range for the thermistor is from 101kΩ (-40OC) to 14.5Ω (180OC). Draw a circuit diagram to make this conversion suitable for an analog input impedance of 1kΩ. 8 Marks
Q3 (a) Describe with the aid of a diagram, the IEEE-754 binary representation for a 32-bit floating-point number. 5 marks
(b) Convert the decimal value 7879.4375 to (
n x 1.f) format and determine its 32-bit floating-point binary representation. Show all the steps involved. 12 marks
(c) Show the valid range of negative and positive values in 2
n x 1.f format that can be supported in 32-bit floating-point format in accordance with IEEE-754. Use your calculator to convert these to decimal values in 10
n exponent format and write down the complete decimal range. 6 marks
(d) How are the values 0 and infinity represented in the system? 2 marks
Q4 (a) Show by means of a block diagram, the essential elements of a PLC analogue input module designed for 4-analog inputs. (^) 6 marks
(b) Fig. 1 shows how a pump controlled by a variable-speed controller maintains the level in a large silo. The 0 – 10V speed reference to the controller input is derived from a PLC analog output. A pressure/current transducer having an output signal range of 0-20mA for a pressure range of 0 to 6 bar is used to determine the level of liquid in the silo. This signal is connected to an analog input in the PLC. Given that the analog modules have 15-bit resolution, plot a graph showing all the input and output ranges and from it derive the linear mathematical function for the control required in (c) below. 9 marks
(c) The pump speed is controlled by the 0 to 10V analog output ( PQW 320 ) such that when the analog input ( PIW 304 ) to the PLC is 0mA, the pump speed is 1150 r.p.m, and when the analogue input is 20mA the pump speed is 200 r.p.m. Between these two limits, control is inversely proportional to the analog input current. Devise for a Siemens S7-300 PLC, a suitable program using the mathematical functions required to control the speed of the motor. 10 marks
Q5 (a) Give a brief account of the construction and operation of an incremental encoder,
and explain the functions and application of the outputs A, B, and Z. 8 marks
(b) Fig.2 shows a conveyor system used to measure the length of components. The components must meet given tolerance limits for the measurement to be accepted. Those outside the limits will be diverted onto another conveyor as rejects. The output from the encoder is connected to a high-speed counter input on a Siemens S7- PLC. Using the data in Fig.2, calculate: (i) The measurement resolution for the encoder. 4 Marks (ii) The length of the components if the input count is 6366. 4 Marks (iii) The max conveyor speed (m/s) if the counter has a max frequency of 40kHz. 4 Marks
(c) Explain why an interrupt is used when the measured length is completed and its value is to be stored in memory for further use. 5 marks
