Compensation resistor designation and selection method

High accuracy transducers not only need high accuracy strain gauges to achieve their specifications but also need series compensation and adjustment resistors. ZEMICs R series compensation resistors is a kind of bondable adjustable compensation resistor. The resistors can be used to improve a transducers specifications like output sensitivity, temperature effect on sensitivity, zero output, zero output drift due to temperature and other technical specifications. In addition it has several other advantages; the resistors can be easily installed by easy bonding, they are easy to adjust, the temperature change is the same as the temperature change of the spring elements material due to the Self-temperature compensation and can reach very high compensation accuracy.

 

Choice in compensation resistors
During the production of high accuracy transducers, a series of compensations should be done in order to improve the specifications of transducers. This is done mainly to compensate the temperature effect on sensitivity , the overall sensitivity, zero balance and the temperature effect on zero balance. Following is an introduction to the possible compensation methods and a guide to choose the correct compensation resistor: 

1. Temperature effect on sensitivity compensation (i.e. Elastic Modulus Compensation) usually adopts RNF or RBF series compensation resistors. When the transducers environmental temperature changes, the elastic modulus of the spring element and the strain gauges gauge factor will also change accordingly. This causes an error in measurements. This is the reason that for high accuracy transducers, this has to be compensated. The method is as follows: To connect the compensation resistor in a serial connection with the input excitation circuit. The compensation resistor will change with the same temperature effect characteristics as the transducer will but in opposite direction which will therefore compensate the change back to a minimum.
To know what compensation resistance value is necessary, this should be calculated with the following formula:

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

Wherein Rm refers to the resistance value of the compensation resistor, S1 and S2 refer to the transducers sensitivity at temperature values T1and T2 respectively. Rin refers to the input resistance of the bridge when the temperature value is T1. αc refers to the temperature resistance coefficient of the compensation resistor. These coefficients are as follows: For the RNF resistors: 5.5 x 10⁻³/℃ and for the RBF resistors: 4.3 x 10⁻³/℃ . Further, S (transducer sensitivity) is calculated as follows: S=E0/V in which E0 is the bridge output voltage and V is the supply excitation voltage.
Generally spoken, steel transducers usually use a RNF series 20 Ω compensation resistor. For aluminium transducers a RNF series 32 Ω compensation resistor is usually chosen. The specific compensation resistance value however, should be confirmed by testing and adjusting the compensation resistor according to the transducers' accuracy.

2. Sensitivity compensation usually adopt RCF series compensation resistors or thin wires with a lower resistance temperature coefficient than the transducers. Because the spring elements material differences, process variation and gauge factor dispersion combined together (which is usually lower than 1%), the dispersion between transducers sensitivity would occur and therefore make it harder to interchange transducers. During the production of transducers, the sensitivity of transducers is generally a little bit higher than intended. So that at the end of the process it can be adjusted to the correct value according to test results. The specific method is as follows: The compensation resistor with smaller resistance temperature coefficient into the excitation circuit with the intention to lower the real excitation voltage of the transducer will be connected into the input circuit. This way the sensitivity of the transducer is decreased. The compensation resistance value can be calculated with the following formula:

Rc ≈ ((S1-S2 ))/(S1 ∙R)

Rc refers to the resistance value of the compensation resistor. S1 and S2 refer to the sensitivity before adjustment and the sensitivity after adjustment. R refers to the input resistance of the bridge.

 

3. Zero balance compensation usually adopts RCF compensation resistors or varnished wrapped wires with lower resistance temperature coefficient. They will be applied into one of the arms of the bridge to make sure the transducers strain gauge bridge output is as close to zero as possible without any load applied. In this way measuring errors can be prevented and zero adjustments by indicators are easy to perform. Usually, polishable, cutable and length adjustable compensation resistors are used. This way, the bridge zero can be easily and neatly adjusted. Resistance value of polisable compensation resistors can be adjusted by carefully polishing the grid with an abrasive. Cutable compensation resistors can be adjusted by carefully cutting the grid on designated places. The resistance value of length adjustable compensation resistors can be adjusted by changing the length of the resistor.
A small example of how this works is shown below:

 

Figure 4: Zero balance compensation

 

When strain gauge R1 and R3 receive compressive strain and R2 and R4 receive tensile strain and the zero temperature output is positive (The positive output is the difference between the zero output at normal temperature and at higher temperature), terminal F-G should be having an increased resistance. For polishable compensation strain resistors this means the grid should be polished, for cutable compensation resistors, the grid should be cut to increase the resistance and for a length adjustable compensation resistor the length should be increased. Meanwhile it is important to keep an eye on the bridge output. The intention is to get the bridge output to zero. When too much is polished, cut or the length too long, another adjustment has to be made. To return the value back to zero the same procedure should be executed on terminal E-F. This will compensate the other way until the zero balance reaches zero again. Please not that this adjustment method can't be performed unlimited times. Eventually the resistance will break or no cutting can be done or the length adjustable resistor will be too long.

 

Basic drawing of transducers wiring compensation

R1 ~R4 ------ Strain Gauges
Rt ------ Zero Temperature compensation resistor
Ra ------ Adjustable Zero output resistor
Rm ------ Temperature sensitivity compensation resistor (or elastic modulus resistor)
Rc ------ Linearity compensation resistor
V ------ Excitation voltage
E0 ------ Bridge output (or measuring output)