Training Stuff

How to Choose Compensation Resistors

release date:2017-02-07 23:53:21


During the production of high accuracy transducers, a series of compensation should be done in order to improve the specifications of transducers. Mainly to compensate sensitivity temperature coefficient, sensitivity, zero balance and zero temperature shift, the following is an introduction to each compensation methods and choice of compensation resistors:
(1) Sensitivity temperature compensation (i.e. elastic modulus compensation): usually adopts RNF, RBF series fixed (or combined) compensation resistor. When the transducers environmental temperature changes, the elastic modulus of the spring element and the strain
gages' factor will also be changed correspondingly, and the sensitivity of the transducer is changed accordingly, which cause the measuring errors. Therefore, high accuracy transducers need to compensate this error. The method is as follows: To connect the compensation resistor in series into the supply excitation circuit, using the characteristic that the resistance will be changed by temperature, and the direction is just opposite to the transducer sensitivity changes, to counteract the drift caused by temperature changes, thereby to reach the compensation purpose. The compensation resistance value can be calculated by the formula bellow:

Rm≈[(S1-S2)*Rin]/{[1+αc*(T1-T2)]*S1-S2}

Rm refers to the resistance value of compensation resistor, S1、S2 refer to temperature T1 and T2 transducer sensitivity respectively, Rin refers to the bridge input resistance when temperature value is T1 ;αc refers to the resistance temperature coefficient of the compensation resistor (Resistance temperature coefficient of the RNF series compensation resistor is 5.5×10^-3 /℃ , resistance temperature coefficient of the RBF series compensation resistor is 4.3× 10^-3 /℃ ). Sensitivity S=E0 /V( Eo refers to the bridge output voltage, V refers to the supply excitation voltage). Generally, for steel transducers you can choose RNF series 20Ω compensation resistance, for aluminum transducers you can choose RNF series 32Ω compensation resistance. The specific compensation resistance value should be confirmed through the test, and to adjust according to the transducers' accuracy.

(2) Sensitivity compensation: You can adopt RCF series compensation resistor or thinner wires with lower resistance temperature coefficient. Because the spring element material, machining difference and gage factor combine together (≤ 1% ), transducers sensitivity decentralization would be larger. In order to increase the interchange of transducers, during transducers manufacturing, generally to design the sensitivity a little bit higher than the standard value intentionally, then during machining to adjust it into the standard value according to the
test result. The specific method is: To connect the compensation resistor with smaller resistance temperature coefficient into the excitation circuit, to lower down the real excitation voltage of the transducers, so as to decrease transducer's sensitivity. The compensation resistance value can be calculated by the formula bellow:

Rc ≈(S1-S2)/S0*R

Rc refers to resistance value of the compensation resistor, S1 、S2 refers to the real sensitivity before connection and standard sensitivity after adjustment respectively, R refers to input resistance of the bridge.

(3) Zero balance compensation: Usually to connect a RCF compensation resistance or manganin varnish wrapped wire with lower resistance temperature coefficient into one of the arms in the bridge, to make the transducer's strain gage bridge output close to zero without load, so as to decrease the measuring error and easy for zero adjustment of the indicator. Usually we use frictional structure, cutter structure and short connection compensation resistors that can adjust bridge zero neatly and easily. Resistance value of frictional compensation resistor could be adjusted through polishing grids with abrasive; Resistance value of cutter compensation resistor could be adjusted through cutting the connection grid; Resistance value of short connection compensation resistor could be adjusted by short connecting the grids.

(4) Zero temperature compensation: Usually to decrease the temperature effect on zero output through connecting RNF compensation resistor or varnish wrapped pure copper wire, or varnish wrapped nickel wire with larger resistance temperature coefficient to one of the arms in the bridge. Transducers output is almost zero with no loads, when transducers temperature changes, one side, spring element, bonding adhesive and strain gages will expand or shrink for different extent to cause strain gage resistance changes. Another side, sensitivity grid material resistance temperature coefficient will also cause the strain gage resistance changes. All of these factors will affect transducer's zero output, even to adopt self-temperature compensation strain gages and full bridge connection, due to dispersion of the strain gage temperature performance, the output zero will also be changed more or less, so it needs to be compensated. The specific method is: first to test transducers temperature performance, after you get the rule of the compensation resistance and zero temperature drift, then to adjust the corresponding bridge arm compensation resistance value according to the transducer temperature zero drift value. The compensation resistance value can be calculated by the formula bellow:

Rt=|R*(U2-U1)|/|250*αc*Uin*(T2-T1)|

Rt refers to resistance value of compensation resistor; R refers to bridge resistance; Uin refers to the excitation voltage;αc refers to resistance temperature coefficient of the compensation resistor; U2 , U1 refer to zero output voltage at temperature T2 , T1 . The compensation of zero
temperature we often use the compensated wire or compensated resistance with the structure of frictional type, cutter type and short cnnection type.

The theory of zero temperature compensation is similar with the zero balance compensation, but it needs to be accomplished during the simulated temperature field.