Automated Industrial Load Measurement System - AU Journal

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Fig. 1 Theoretical approach to the measurement idea written in various languages (e.g., C, C++, FORTRAN, Assembly etc.). In this case, assembly language has been chosen for its capability of having greater control over the machine, i.e., the PC (Abel 2001) and for the familiarity of the authors with this language. Hardware The main hardware components of the system are discussed in considerable detail in this section. Strain Gauge (SG) As discussed in the introductory part of this paper, load cell is the main sensor used for the measurement system. The load cell is primarily made up of strain gauges. V ref R R R D Fig. 2. Construction of a strain gauge (Johnson 2003) Construction of strain gauge is shown in Fig.2. It basically consists of resistive elements in a Wheatstone bridge configuration. R The nominal values of the resistances are equal under no-load conditions. Thus the voltage output from the circuit is zero in this setup. The resistance of R D changes in a linear manner with the force acting on it. The sensitivity of this bridge to strain can be found by considering the equation for bridge offset voltage. Suppose R 1 = R 2 = R D = R, which is the nominal (unstrained) gauge resistance. Then the active strain gauge resistance will be given by (Johnson 2003): R A = R (1+ΔR/R) And the bridge off-null voltage will be given by: ΔV = V S [R D /(R D +R 1 ) – R A /(R A +R 2 )] Finally, the expression of output voltage is given in terms of strain as below: ΔV = - (V S /4) GF (Δl/l) This voltage is the output, which is fed to an instrumentation amplifier for signal amplification. Instrumentation Operational Amplifier There are many instances in measurement and control systems in which the difference between two voltages needs to be 24
conditioned. A good example is the Wheatstone bridge where the offset voltage ΔV = V a – V b is the quantity of interest. Since the load cell is working with the same principle as the Wheatstone bridge, the ideal approach was to feed the load cell voltage output to a differential instrumentation amplifier. An ideal differential amplifier will provide output voltage with respect to ground that is some gain times the difference between two input voltages (Boylestad and Nashelsky 1999). V out = G (V a –V b ) where, G is the differential gain and both V a and V b are voltages with respect to ground. Such an amplifier provides useful operation in control and measurement. There are a number of op-amp circuits for a differential amplifier. The circuit uses two pairs of matched resistors. But if the resistors are not well matched, the Common Mode Rejection (CMR) will be poor. Normally the input impedance for such circuits is not very high and it is not the same for two inputs. For this reason, voltage followers are often used on the input to provide high input impedance. Such a device is called a differential instrumentation amplifier. It is just a differential amplifier with high input impedance and low output impedance. Many IC manufacturers package instrumentation amplifiers in a single IC. Instrumentation amplifier IC INA125P from Burr Brown was used for this purpose, as it offers all the aforementioned advantages. V X = V ref (a 1 2 -1 + a 2 2 -2 + a 3 2 -3 + …….+ a n 2 -n ) where, V ref is the ADC reference voltage, V X is the input analog voltage and n is the number of bits. In this case, the 8-bit ADC 0804 IC has been used. This 8 bit digital information will be sent to a PC for conversion to weight or a load value using the appropriate software. It is noteworthy that there is an inherent uncertainly in the input voltage producing a given ADC output, and this uncertainty is given by: ΔV = V ref 2 -n Therefore, it is very important to take this into account when using an ADC in industrial instrumentation and control systems, and in defense sector applications, where 100% accuracy is required. PC Parallel Port Interface The parallel port is the most commonly used port for interfacing applications involving data transfer to and from the PC (Hall 1992). This port allows the input of up to 9 bits or the output of 12 bits at any given time, thus requiring minimal external circuitry to implement many simpler tasks. Analog-to-digital Converter (ADC) An analog to digital converter or ADC is the next stage after the instrumentation amplifier. An ADC takes analog voltage as an input and provides a digital output. This digital information is in binary format consisting of 0s and 1s only. The ADC finds a fractional binary number that gives the closest approximation to the fraction formed by the input voltage and reference. The following is the conversion equation (Boylestad and Nashelsky 1999): Fig. 3. LPT port connection diagram The port is composed of 4 control lines, 5 status lines and 8 data lines. It is on the back of a PC as a D-Type 25 Pin female connector. 25
Page 1: AU J.T. 10(1): 23-28 (Jul. 2006) Au
Page 5 and 6: Fig 6 Screen shot of the user inter

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