ET 438aAutomatic Control Systems TechnologyLaboratory 1Analog Sensor Signal ConditioningObjectives: Use analog OP AMP circuits to scale the output of a sensor to signal levels commonly found in practical control systems. Use OP AMP analog circuits to combine several simulated sensor inputs according to a predefined input signal formula. Produce an error signal using an OP AMP differential amplifier.Theoretical BackgroundSensors used in control systems may produce outputs that are not compatible with the other control elements in the automatic control system. The sensor many not even produce a usable electrical output. The signal level could be so low that the electrical output may be masked by the electrical noise in the environment before it reaches the next stage in the system. Other types of sensing elements, such as thermistors, may produce non-linear outputs to the change in the measured quantities. Signal conditioning circuits amplify, linearize, and scale the output of sensors to match the requirements of the next control device in the automatic control system.It may also be necessary to combine the results of several control inputs into a single analog signal based on some control scheme. A simple combination is the averaging of several widely separated temperature measurements across a large duct that carriesa heated gas. The fluid temperature will vary over the dimensions of the duct, so the average of several temperature probe inputs will give a better estimate of the actual gas temperature than a single probe. Common OP AMP circuits that use the inexpensive LM741 can perform all these functionsThe figures below show OP AMP circuits that can be used to perform analog signal conditioning. Fall 2011 lab38a1r.docFigure 1. Inverting OP AMP Circuit2Figure 1 shows a simple inverting OP AMP circuit. This circuit can be used to scale theoutput of sensors. It is direct coupled, so it can amplify dc signals. The output voltage of the circuit is give by Equation 1.This circuit introduces a sign change in a dc signal level. The values of Rf and Rin can be adjusted to give gains greater than or less than 1. Gains less than one are the equivalent of dividing by a value. (e.g. A gain of -0.3333 = dividing by -3.) One of the limits to this circuit is the input impedance of the circuit. The negative feedback produced by coupling the output voltage to the input current reduces the input resistance of the OP AMP to almost zero, so the input resistance of the amplifier is determined by the value of Rin. To prevent loading effects, Rin should be made as high as possible when dealing with sensors that are voltage sources. A good design rule is to make Rin 10 times greater than the resistance of the previous stage.Figure 2 shows an OP AMP circuit that introduces no sign change in the signal and hasa high input impedance. This circuit is a non-inverting OP AMP amplifier. It has the limitation that it cannot produce gain of less than 1. The gain formula for the circuit is given by Equation 2.Fall 2011 lab38a1r.docV RR - = Vif0-(1)Figure 2. Non-inverting OP AMP amplifier3This circuit can be used to buffer theoutput of a sensor from the load effect ofthe next stage and to introduce a gain ofgreater than 1 to the signal. The valueof input impedance depends on the typeof technology used in the construction of the OP AMP. For the LM741, Ri is between 1 and 2 M. The output impedance Ro is approximately zero. When a gain of 1 is desired but the buffering effect of the high to low impedance is still necessary, the voltage follower circuit in Figure 3 can be used.Fall 2011 lab38a1r.docV )RR + (1 = Vi1fo(2)Figure 3. Voltage Follower Circuit.4The voltage follower circuit has a gain of 1, but has very high input impedance. This reduces the loading effects that stages have on each other. This type of circuit is ideal for couple to sensor arrangement such as the thermistor voltage divider circuits covered in the lecture. The high impedance of the amplifier input will draw little current from the divider circuit, so the voltage drops across the resistor will follow the voltage divider formulas more precisely.When more than one input signal must be combined to produce an output, the invertingsummer circuit of Figure 4 can be employed.The gain formula for this OP AMP circuit is given by the following equation.Fall 2011 lab38a1r.docFigure 4. Inverting Summing Amplifier.5Just as with the single input inverting amplifier, the voltage sense-current summing feedback reduces the input impedance of the OP AMP to approximately zero This means that the input impedance is determined by the input resistors R1 to Rn. For any input, i, the resistance looking into the input will be given byThis is the parallel combination of the remaining resistors added to the input resistanceof the i-th input. For equal values of input resistances in an n input summing amplifier, Equation 4 simplifies to Where, R1 = R2 = R3 ....= Ri-1 = Ri...RnIf all the input resistors on a inverting summing amplifier are equal, then the output will be a scaled average of the input voltages. If the input voltages are the outputs of transducers, then the output voltage will be the analog average of the control variable measurements. When all the input resistors of an n input summing amplifier are equal, the output voltage formula given by Equation 3 reduces toWhere Ri is the value of any of the input resistors and n is the total number of inputs. To create an output that is the exact average of the input values, n(Rf) must be equal to Ri. Values of n(Rf)> Ri will scale the output voltage up while values of n(Rf)< Ri will scale the average value down.A non-inverting amplifier circuit can also simulate the mathematical operation of averaging. The circuit below shows a three input, non-inverting summing amplifier circuit. When the values of resistors R1-R3 are equal the output is a scaled value of Fall 2011 lab38a1r.doc-RV++....RV+RV+RV R- = Vnn332211fo(3)R ...||R || R... ||R || R|| R + R = Rn+1i1-i321i(4)nR + R = Rii(5))V+....+V+V+V( RRn - = Vn321ifo--(6)6the average of the input voltages, V1-V3. The following equation shows that the scaling factor of the averager is set by the ratio of the resistors Rf and R4.Figure 5. Non-inverting Summing Amplifier.3VVVRR1V3214f0(7)When,321RRR The