A scale for weighing for a customer loaded pallets for use in a warehouse environment is being developed for a customer.
The basic sensor is a strain gauge represented by the resistor, Rw, that linearly converts the weight of the pallet into a resistance; for a weight of zero kg the resistance of Rw, is 119 Ω, while its resistance at 999 kg is 127 Ω. The strain gauge is made part of a "bridge" circuit as shown on the left hand side of Figure 1. The principle of the bridge circuit is to generate a signal voltage ΔV(=Va-Vb) that is representative of the weight on the scale, that is, as the weight on the scale changes, so does the resistor value Rw, and therefore so does the voltage ΔV.
The signal voltage ΔV is then amplified by a differential amplifier and interfaced to a digital readout that, by design, will readout the.weight in kilograms equal to the input voltage times 100. That is, when the voltage input to the readout, Vs , is + 1.23 V the readout will display a weight of 123 kg.
The purpose of the resistor combination in the bridge is to allow for "resetting" the bridge output voltage ΔV to zero (i.e. nulling the bridge) using the variable resistor Rz. This will allow compensation for changes in the strain gauge resistance over time.
TASK 1: Considering only the bridge circuit in Figure 1, use analytical circuit analysis techniques to determine the relationship between the bridge voltage AVand the resistances Rw and Rz.
TASK 2: Determine the value of Rw required to null the bridge when the applied weight on the scale is zero kg.
TASK 3: Determine the bridge signal voltage ΔV when the applied weight onthe bridge is 999 kg (assuming the bridge was nulled at zero kg).
TASK 4: Determine the differential amplifier gain, & so that the digital readout displays the weight in kilograms. Why is a differential amplifier needed in this amplifier application?
TASK 5: Provide a design for the differential amplifier indicating how to connect the amplifier lrputs to the bridge points a and b so the voltage to the readout device is of the correct polarity.
TASK 6: When the system was built and tested it was found that the system gives the correct weight at zero kg and 999kg but when a weight of 500kg was placed on the scale the readout weight did not match the real weight. Determine the readout value when the real weight on the scale is 500kg according to the current design and discuss why there is an error at 500kg. If most pallets weigh between 400-500kg, suggest an improvement to the nulling procedure so that the readout accuracy is improved in this applied weight range.
TASK 7: During field testing, it was also found that the display would intermittently display an erroneous weight for a short time before displaying the correct weight again. It was determined that nearby electrical equipment was introducing electrical noise into the bridge circuit at frequencies above 50kHz. It is decided to place a filter between the differential amplifier and the readout to effectively eliminate the effect of the electrical noise. Design a suitable filter that reduces the noise signal by at least 20 dB while the signal voltage, %, is not modified by more than 2%. Using Phasor circuit analysis techniques, provide all analytical analysis details for your design indicating the signal and noise level attenuation your design achieves. Use PSICE to simulate your filter circuit (not the whole system, just your filter) and confirm correct performance of your design.
TASK 8: Finally, the customer asked for a lamp indicator to be included as part of the system. The lamp would light up when a weight of more than 800kg was placed on the scale, alerting the operator of a pallet overweight condition. A control circuit is required to achieve this whose electrical output drives a relay that switches on and off a lamp as shown in Figure 2.
The relay switch output is normally open and becomes closed when 5V is applied to the relay coil. The relay being used has a coil resistance of 200Ω and a coil urductance of l00mH.
BONTIS MARKS: This task is not compulsory but all students that provide a suitable design for the control circuit in Figure 2 will be awarded THREE bonus marks for the unit (i.e. an extra 3% for the overall unit mark). A proper design should contain hysteresis so that the lamp does not flicker between off and on when the weight on the scale is very close to the overweight condition of 800kgs.