Telemetric microcontrolled data acquisition system

Abstract

The project involved the design and evaluation of a telemetric data-acquisition system to interface 3 remote water-level measuring pressure-sensors, to a central display unit The telemetry medium chosen was a, BS 6346, 1.5 mm, 12 core, steel armoured, PVC insulated, service cable. It is very suited to the environmentally harsh open air conditions found at Ta' Qali reservoirs. 300 metres of this cable were laid at Ta' Qali for system evaluation purposes (refer to thesis, page 6, fig. 1.1 ). The transducer chosen for water level measurement was the MPXlOOAP piezoresistive pressure sensor(refer to thesis, page 174). This transducer has a sensitivity of 0.6mV/kPa and a linearity of.0.05% overfull scale range of 0to100 kPa. This range is the equivalent of 0.00 to 10.00 metres of water. Telemetry Transmitter This millivolt output cannot be accurately and securely transmitted over 300 metres of cable. Hence the transducer signal was firstly amplified and then converted into a frequency modulated square wave. The square wave signal has a range of 1,000 to 5,000 Hz, corresponding to 0.00 to 10.00metres of water, and a constant amplitude of12Volts. Due tot he millivolt values an accurate and thermally stable instrumentation amplifier was constructed, having an input impedance of 1x1012 ohms, a thermal coefficient of 50 ppm°C and a gain range of 90 to 2000. The voltage to frequency conversion circuitry includes a 9400CJ which has a linearity of 0.01% and a high thermal stability of 25-40 ppm°C The output square wave was then amplified and buffered by typical op-amp circuits, ready for connection to 2 cores of the telemetry cable. (Refer to thesis, pages 23 and27). The transmitter circuitry receives power al a range of 7 to 9 volts smoothed DC over the telemetry cable and converts this volt level to + 5 volts and ±.12volts. The + 5 volts powers the pressure transducer and the .12volts powers the signal conditioning circuitry. (Refer to thesis, page 31). Field Study In order to investigate the viability of utilising a BS 6346 service cable as the telemetry medium, afield exercise was carried out at Ta' Qali. Two persons took part in this exercise. The apparatus was the following: •Trio FG-271 signal generator. •Intercom pair. •Beckman industrial circuit mate 9020, 20Mhzdual trace oscilloscope. •Polaroid CR-9 land oscilloscope camera. One operator, one intercom and the signal generator were located at one end of the cable. These second operator, one intercom, the oscilloscope and the polaroid camera were located at the other end. Communication between the two stations was maintained through the experiment via the intercom pair. The results (refer to pages 11 through 15 of the thesis), show that a 12 volt peak to peak, square wave signal in the range 1000 Hz through 5000 Hz, can be transmitted over the service cable with minimal distortion. Telemetry Receiver The frequency modulated square wave signal received via the cable, is offset and then amplified to .±..12 volts by op-amp circuits. It is then clipped to ±1.4 volts by a diode circuit. This is done to remove any distortion introduced by capacitive coupling in the telemetry cable. (Refer to pages 13, 14 and 33, 34 of the thesis). This clean signal is then fed to a demodulator circuit, which responds to 10-6000 Hz nominal, built around the accurate 9400CJ. The output voltage from this circuit, though linear, has a ramp form. In order to prevent aliasing in the ADC and to improve the linearity, a simple low pass filter was constructed. This improved the linearity to around 0.01 % (Refer to pages 36 through 40 of the thesis). This volt signal was then offset and the span was trimmed so that the range 0.00 to 10.00 metres of water would correspond to 0.00 to 2.50 volts. The accuracy of the volt signal arriving at the data-acquisition unit can be guaranteed up to 0.05% over this range as limited by the transducer itself. Data-Acquisition Unit The data-acquisition unit converts 0.00 to 2.55volts into 0.00 to 10.20 metres of water and displays this value on a four by 7-segment red LED display arrangement. The data acquisition unit has 3-channels. The 3 analogue inputs are multiplexed and an accurate 8-bit ADC converts these inputs into their corresponding digital values in turn. The resolution is 10.20/255 = 0.04metres. This corresponds to an accuracy of 28 x100% = 0.4% for the analogue volt signal. If the full accuracy of 0.05% of the analogue signal had been required then an 11 bit ADC would have been used and this would have given a resolution of0.005 metres over 10.200 metres of water. The data-acquisition unit contains a Texas instruments, TMP77C82 microcontroller. This microcontroller supports the 8-bit TMS 7000 machine language. The microcontroller was programmed to control the multiplexing, analogue to digital conversion and conversion cycle timing operations. It multiplies theADCoutputby4, converts this result to binary coded decimal and then feeds this data to the relative 7-segment displays for each channel. (Refer to thesis, page 64, figure 4. 7). The inclusion of a microcontroller on the data-acquisition unit makes it very flexible. For example, it can be easily connected to a modem board via its expansion connector. The microcontroller can then be reprogrammed to transmit the data to auto-answering modem units. This can be done with very little alteration of the original program. Software development for the microcontroller was carried out on an JBM-PC compatible AMSTRAD PC1512. The program was then downloaded via an A.RT. APP-1 eprom programmer to the microcontroller.