Question

An historic bridge is showing signs of deterioration. Until renovations can be performed, it is decided that only cars weighing less than 1600 kg will be allowed across. To monitor this, a four-pad weighing system is designed. There are four independent voltage signals, one from each wheel pad, with 1 mV = 1 kg. Design a circuit to provide a positive voltage signal to be displayed on a DMM (digital multimeter) that represents the total weight of a vehicle, such that 1 mV = 1 kg. You may assume there is no need to buffer the wheel pad voltage signals.

Answer 1

We know that any four wheeler vehicle exerts its own weight to the ground through its four individual wheels uniformly. That means if a four wheeler car has weight of W kg-wt then all the individual wheels exerts W/4 kg-wt force to the ground.  

based on this analogy we have to collect those output voltages from those individual pads and then add to get the total output voltage to calculate the total weight of the car considering the conditions 1mV = 1kg.


Now to make an adder to sum the input analog voltages we can use an Operational Amplifier circuit, Let all the input voltages are , as given that there is no need to buffer the wheel pad voltage signal so we can consider the voltage gain factor of the OpAmp as unity... Therefore all the input resistances should be equal and also the feedback resistance also should be same, say R.

Now the corresponding circuit diagram is shown below:

This above Op Amp based summing amplifer circuit shows the output voltage

As here the feedback resistance, connected from the o/p to the inverting i/p is same as all the resistances R so the amplification gain is


At the inverting terminal of he OpAmp is taken as virtual ground as because the voltage difference is zero initially b/w inverting and non-inverting terminals and as the non-inverting terminal is grounded initially also.

Do not forget as because of we introduce those voltage inputs at the inverting terminal the output signal will be inverted but no matter if you change its polarity further using a capacitor or introducing the i/p at the non-inverting terminal, you can get the same polarity output . These above arrangements are enough to measure the all total voltage at the output using a DMM to calculate the weight of the car!