IN COURSE EXAMINATION

Transcription

1 IN COURSE EXAMINATION Code and Name of Course: ECNG 3032 Controls and Instrumentation I Date and Time: Monday 27 th October 2008 Duration: 2 hrs INSTRUCTIONS TO CANDIDATES: 1) This paper has 6 pages and 6 questions. Answer ALL questions. 2) Please fill in your student ID number CLEARLY on the answer book. 3) The marks per question and the marks per part are indicated. 4) Use a pen for all answers except drawings. 5) Programmable calculators are not permitted. Oil drilling operators often use a heavy liquid, called mud in their work. Water-based drilling mud consists of bentonite clay (gel) with additives such as barium sulphate and calcium carbonate (chalk). The mud is pumped under pressure into the well holes being drilled for a number of reasons, for example to lubricate the drill bits and counteract the oil and gas pressure in the well and prevent blowouts. A particular manufacturer of mud requires the temperature, flow and pressure of the liquid and associated machinery to be monitored. The pump motor temperature is monitored by resistance temperature detectors, while the mud temperature is monitored by type K (chromel-alumel) thermocouples. The thermocouples have an output voltage of 40μV/ C. The flow rate is measured by using a flow meter whose principle of operation depends on differential pressure measurements. Q1 The particular flow meter recommended measures the differential pressure across a restriction placed in the fluid flow. Two possibilities are the orifice plate and the venturi flow meters. Briefly discuss the significant features of each type including their principles of operation, advantages and disadvantages and the positioning of the pressure sensing points in the meter. (10 marks)

2 Q2 The mud has the following characteristics: density 2750 viscosity kg m, Pa s. An orifice plate flow meter is inserted in a 0.15 m diameter pipe to measure the mud flow, but to guarantee turbulent flow through the meter the flow has to maintain a Reynolds number of Calculate the minimum flow rate required in m 3 s. (10 marks) Q3 The output of the flow meter is conditioned by a differential amplifier, similar to the one shown in Figure Q3. All of the resistances used are accurate, meaning that they operate at their nominal value, except R F, which has a value of nominal plus 5%. R1 = R2 = 20kΩ and RF = RG = 400 kω. Derive expressions for the differential and common-mode gains and hence calculate the in circuit CMRR of the differential amplifier under these conditions. (20 marks) R 1 R F V diff R 2 R G Figure Q

3 Q4 In a suggested upgrade, it is recommended that the differential amplifier of Q3 be replaced by an AD524B instrumentation amplifier, adjusted for the same gain, whose datasheet is given on page 5. If the measurement situation is expected to be susceptible to a 75mV 2Hz coherent interference input signal from another channel, calculate the overall amplitude error, %RTI produced by the instrumentation amplifier. Use the maximum values given for the appropriate parameters in the data sheet. The signal frequency is 1Hz and the amplifier bandwidth is 1000kHz. The circuitry is expected to operate in an environment whose temperature varies from 25 to 45 C, while the temperature of the drilling fluid varies from30 to 300 C. The maximum amplifier output voltage is 5V and the pressure transducer output impedance is 1kΩ. (30 marks) Q5 A 4-pole 2Hz, LPF is now installed after the amplifier to remove the interference. Using the SNR values with and without the filter as well as the error contribution of the filter itself, comment on the usefulness of the LPF in this application. (20 marks)

4 Q6 Figure Q6 is a schematic representation of the thermocouple wiring which monitors the mud temperature. The distance from the hot junction to the first zone box is 0.5m. The distance between the two zone boxes is 3m and the distance from zone box 2 to the control room is 25m. a. Where would you locate reference junction and why?; and b. Identify the wires A to F and justify your answer. (5 marks each) Type K A B C D E F CONTROL ROOM Zone Box 1 Zone Box 2 Figure Q

5 VS = ±15 V, RL = 2 kω and TA = 25 C, unless otherwise noted. AD524 All min and max specifications are guaranteed. Specifications shown in boldface are tested on all production units at the final electrical test. Results from those tests are used to calculate outgoing quality levels. Table 1. AD524A AD524B Parameter Min Typ Max Min Typ Max Unit GAIN Gain Equation (External Resistor Gain Programming) 40,000 40,000 1 ± 20% 1 ± 20% R G R G Gain Range (Pin Programmable) 1 to to 1000 Gain Error 1 G = 1 ±0.05 ±0.03 % G = 10 ±0.25 ±0.15 % G = 100 ±0.5 ±0.35 % G = 1000 ±2.0 ±1.0 % Nonlinearity G = 1 ±0.01 ±0.005 % G = 10, G = 100 ±0.01 ±0.005 % G = 1000 ±0.01 ±0.01 % Gain vs. Temperature G = ppm/ C G = ppm/ C G = ppm/ C G = ppm/ C VOLTAGE OFFSET (May be Nulled) Input Offset Voltage μv vs. Temperature μv/ C Output Offset Voltage 5 3 mv vs. Temperature μv Offset Referred to the Input vs. Supply G = db G = db G = db G = db INPUT CURRENT Input Bias Current ±50 ±25 na vs. Temperature ±100 ±100 pa/ C Input Offset Current ±35 ±15 na vs. Temperature ±100 ±100 pa/ C Rev. F Page 3 of 28

6 AD524 AD524A AD524B Parameter Min Typ Max Min Typ Max Unit INPUT Input Impedance Differential Resistance Ω Differential Capacitance pf Common-Mode Resistance Ω Common-Mode Capacitance pf Input Voltage Range Maximum Differential Input Linear (VDL) 2 ±10 ±10 V Maximum Common-Mode Linear (VCM) 2 G G V 12 V VD 12 V VD 2 2 Common-Mode Rejection DC to 60 Hz with 1 kω Source Imbalance V G = db G = db G = db G = db OUTPUT RATING VOUT, RL = 2 kω ±10 ±10 V DYNAMIC RESPONSE Small Signal 3 db G = MHz G = khz G = khz G = khz Slew Rate V/μs Settling Time to 0.01%, 20 V Step G = 1 to μs G = μs NOISE Voltage Noise, 1 khz RTI 7 7 nv/ Hz RTO nv Hz RTI, 0.1 Hz to 10 Hz G = μv p-p G = μv p-p G = 100, μv p-p Current Noise 0.1 Hz to 10 Hz pa p-p SENSE INPUT RIN kω ± 20% IIN μa Voltage Range ±10 ±10 V Gain to Output 1 1 % REFERENCE INPUT RIN kω ± 20% IIN μa Voltage Range ±10 ±10 V Gain to Output 1 1 % Rev. F Page 4 of 28