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Calibration of a mV/V Wheatstone Bridge TransducerDaytronic manufactures instruments which will condition (power and amplify) mV/V (millivolt per Volt) Wheatstone bridge sensors. Their product includes DC Voltage excited (powered) units and AC Voltage excited units. AC Voltage models from Daytronic will end in “78” and are typically used for rotary torque sensors involving transformer coupling to the measurement bridge. Either series of units will calibrate the transducer basically by the same method(s)…. 2-point, Shunt or calculated which is sometimes referred to as absolute calibration. Within certain products, multiple calibration points can enhance the system accuracy by increasing the 2-point method to multiple points. For instance, 5 points or 10 points. This method follows the basic 2-point method with the added span/force points being established in a memory lookup table for load or torque curve correction.How does a mV/V Transducer work?Most Load cells and torque transducers utilize a Wheatstone bridge circuit to convert a mechanical strain into an electrical output signal. The strain gauges themselves are typically glued or welded to a mechanical member with known characteristics and are “flexed/strained” to the load or stress being exerted. This micro-flex elongates the measurement’s gauges in a known manner to create a resistive transformation resulting in an electrical change in the output signal with the given excitation power to the bridge. The basic Wheatstone bridge circuit consists of four resistive elements which are connected into a diamond shaped configuration as shown below with B & D terminals providing the excitation power and the A & C terminals providing the resulting millivolt output signal. Typical DC power to the bridge is 5 or 10 VDC and AC power units being approximately 3 VAC RMS. Higher voltages are seldom used due to heating effect to the bridge with most vendors recommending less than 20 Vdc.If the gauges are equal in resistance and are applied to the stress member perfectly, the bridge network will be balanced and represent an output signal of electrical zero when the transducer is in its “relaxed” position. Not being a perfect world, there most likely will be a zero “offset” which needs to be trimmed with the signal conditioning electronics or with additional adjustment resistors to obtain a zero value. In most cases this zero position will be your first adjustment in the calibration process. 230759020320Four - 350 Ohm foil type gauges with a gage factor of 2 or 4 resulting in a 2,3, or 4 mV/V nominal transducer sensitivity. Higher sensitivities, > 4 mV/V are typically semiconductor gages which have higher signal output and are more sensitive for the strain measurement. Transducers may also contain additional circuits for temperature compensation to aid in the measurement stability.00Four - 350 Ohm foil type gauges with a gage factor of 2 or 4 resulting in a 2,3, or 4 mV/V nominal transducer sensitivity. Higher sensitivities, > 4 mV/V are typically semiconductor gages which have higher signal output and are more sensitive for the strain measurement. Transducers may also contain additional circuits for temperature compensation to aid in the measurement stability. Calibration ProcessDaytronic instruments refer to several calibration methods. 2-Point, Shunt, Calculated (absolute) and Multipoint linearization. Most widely used method is the 2-Point since most mV/V transducers are linear by design and the Full-Scale (FS) range is verifiable with a known load value that authenticates end to end operation of the system which include the Transducer, Cabling, Connections and Signal Conditioner.2-Point The 2-Point method refers to the “zero” or “relaxed” position (Point 1) sometimes referred to the as balance or offset position. Point 2 will be the transducer’s Full Scale (FS) value and is the position within the force slope which will set the transducer’s gain value to convert the transducer’s millivolt output to a usable engineering unit’s value or voltage/mA output value. The zero point is easily understood since it is the “No Load” position of the transducer. The Full-Scale value can be the rated capacity value of the transducer or can be the working or nominal range of the load needed for the application measurement. Many instances the transducer is over-rated for the task, so if you are using a 5,000 Lb. load cell to measure a 2,500 Lb. application, logically the second point would be 2,500 Lbs. Whichever value used, this input needs to be a known, stable force to adjust the Full Scale point for calibration. This is done using a transfer standard or known dead weight. In some cases, it can be a simulated millivolt input, but care needs to be taken since the load cell is ungrounded electrically and inducing a ground into the bridge circuit can cause an unknown offset. First set your zero (Point 1). Once completed then apply the known load and adjust the Full-Scale value (Point 2). Repeat these two points for verification and readjust as needed. If the load measurement is going to be compression & tension (or CW a& CCW for torque), there maybe a “symmetry” adjustment within the signal conditioner to trim the opposite polarity of the transducer (Point 2’s inverted polarity). Symmetry is not needed or adjusted if the transducer is used in one direction. If the signal conditioner can activate a Shunt across the bridge, this can be done after the 2-Point calibration process as a convenient means to re-verify calibration. Activate the Shunt when the transducer is in the relaxed position. Record this value as a convenient means to re-verify calibration.ShuntShunt refers to the calibration method of placing a known resistor value across one arm of the Wheatstone bridge to simulate a known load. Typically, this would be a 59K or 60K Ohm resistor between Negative Excitation and Negative Signal at the transducer or at the signal conditioner connections. The Shunt can be a different resistor value and used across other arms of the bridge depending on signal magnitude and having a positive or negative output signal result. Most Daytronic instruments will do both polarity directions which enables the measurement to have a symmetrical calibration adjustment (CW/CCW or Tension/Compression). For the Shunt calibration, it follows the same path as the 2-Point whereas you establish the “zero” or “balanced” point (Point 1). Then simulate the Full-Scale value (Point 2) – by activating the Shunt. The Full-Scale Shunt value is derived by referencing the transducer’s calibration sheet for the output load which the Shunt represents. In the Calibration Data sheet example below - this is a 59K Ohm which equates to 1.45420 mV/V which represents 2,430 Lbs. simulated force. Set the zero position, then activate the Shunt resistor and adjust the Full-Scale slope value to the simulated force value. Repeat for verification. If symmetry adjust is needed, activate the Shunt in the opposite direction (Shunt negative) and trim as needed. Shunt verifies electrical connections are in place, but not mechanical aspects of the measurement. If another value of Shunt in used, increasing the resistance will decrease the output result. A useful formula that can be used if the calibration sheet does not list the shunt condition is:836295000CalculatedA calculated (absolute), calibration is performed using the Full-Scale value as a mathematical point calculated from the values contained on the calibration sheet of the specific transducer. When working with a linear transducer the equation Y = mX + B is used to determine the gain or slope of the measurement to convert to the user’s engineering unit’s measurement or output signal level for the application. Whereas the Y is your engineering units result (or amplified output voltage), m is your gain factor, X is the signal output of the transducer and B is the zero-offset value. In the example calibration sheet below these terms would be represented using the formula …. 5000 lbf = 167.09 * 29.9236 + 0.0049. Per the Calibration Data Sheet these values would be the Full-Scale Calibration Factor (2.99236 mV/V) which equates to Full Scale range of 5,000 Lbs. and the negative offset value which needs to be “offset” in the positive direction. The gain or slope, which is calculated at 167.09, will achieve the Full-Scale engineering result of 5000 lbs. in this example.Since every transducer is constructed with varying gage alignments, the “Catalog” specification of the load cell typically refers to the Nominal millivolt per volt value whereas the Actual Calibration values for this specific load cell are an exact value on the calibration sheet and should be used. Calibration of the zero point is done with the relaxed load cell (Point 1) as in the 2-Point method or the zero can be a calculation of the Zero Load Balance value on the Calibration sheet. The second point will be the Full-Scale calculated value. Doing the absolute calculated method will set the calibration to the data sheet without regard to cabling or mechanical offsets of the transducer. MultiPointMost Wheatstone bridge transducers are linear by design; however, some applications may need to curve fit the transducer to improve the application measurement accuracy. Depending on the instrument, this can be done with a known look up table and entered in “line by line” as in - known mV/V electrical values to the known Engineering Units values. In some cases the multiple calibration points can be created using multiple dead weight stimulus values at each load point and “forcing” the load value on the conditioner to represent the Engineering Units value as shown in the example Look Up table below by using 500 -pound increments. -16700542926000Catalog Data sheet example Calibration Sheet example33147003175 400000 Look Up table Example for Multi-Point Calibration-12319017526000Depending on the need when performing a multi-point calibration, it is recommended to list the values within an Excel type table and perform a “paper” calibration to ensure linearization accuracy and the value points are correct. Care needs to be taken between calibration points to verify there is adequate signal change between calibration points to produce the required engineering resolution for the table.For example, if you require a 25.0 lb. change between points and there is only 5.0 lb. of actual change, the instrument’s Analog to Digital circuit may not have enough step to step gain in its amplification circuit to resolve the gain. Wiring – Cabling connectionsThe basic Wheatstone bridge transducer will have 4 connections…. 2 for power (+ & - Excitation) and 2 for the returned signal (+ & - Signal). In some applications, when longer cable lengths are needed, there may be provision for + & - Sense lines for the Excitation regulation to compensate for wire resistance change due to the added length and for temperature variations. Daytronic also adds a CAL Sense line which is used to sense the Shunt calibration line when the Shunt method is used. When constructing an interface cable to the signal conditioner it is recommended to use: Twisted pair, shielded cablePair the Excitation with each other and Signal pair together. If sensing is used, pair them together as well. Cal Sense line will be separate.Overall shielding of the cable24 AWG - low capacitance type cable Below is a cable illustration using a 7-wire hookup to the transducer. Note the color wiring is for the Western Regional Code hookup for a strain gage transducer. It is recommended for cable lengths greater than 20 feet; the sense lines be used as shown. If the Sense lines are not connected at the transducer, then they need to be connected at the instrument to regulate the Excitation properly and close the circuit for CAL sense, if Shunt is utilized. Seven wire Wheatstone bridge connectionCommon review considerationsAC mV/V strain gage conditioners (ending in “78”) can be used with DC bridges. DC mv/V strain gage conditioners cannot be used with AC strain gage transducers. AC strain gage conditioners are commonly used for in-line rotary torque transducers which use AC transformer coupling to interface to the rotating member of the transducer.Accuracy is largely dependent on the sensor but will typically be specified as percentage of Full-Scale …. ± 0.25% or ± 0.1% of it rated Full Scale range. Signal conditioners / meters will typically add ± 0.02% to the overall measurement accuracy equation.In general, when a torque or load cell transducer is damaged, its zero / balance point will shift to a larger offset value. This is due to the deformation of the metal gage member being “bent” or the gage becoming un-glued resulting in the transducer becoming non-linear. Most test and measurement transducers are rated at 150% of Full-Scale range – static condition before permanent damage is done. Refer to the specific transducer’s data sheet for details.On installation, over-torqueing the mounting of the transducer or having a “shear” weight or shaft imbalance due to uneven mounting of the transducer’s measurement axis will cause damage to the transducer.Impact peak loading at or above the rate transducer’s range can damage the transducer over time and should be avoided. Use of a “star bridge” in troubleshooting a Wheatstone bridge transducer is extremely helpful, especially in AC excited transducers. A star bridge simulates a balance bridge by using four resistors of equal value in the following arrangement shown below. Use four - 180 to 250 Ohm resistors for a 350 Ohm bridge simulation. The specific resistor value is not as important as having all four resistors of the same value to create a balanced bridge. left147320020000With the star bridge in place at the end of the transducer cable or at the signal conditioner should result in a near “zero” output of the meter or conditioner. Using “Shunt” (note the CAL Sense line is tied to + Signal for this purpose) will provide a change in output of a specific magnitude depending on the Shunt value and gain settings of the conditioner. Both zero and Shunt positions should be responsive and stable. If the measurement is non-linear, drifts or the signal conditioner cannot gain the return signal enough or there is a large zero offset (> 5%) in the relaxed position - double check your cable connections and ensure the Sense lines are connected properly. Reversing the bridge’s Excitation and the Signal leads will appear correct at zero but due to temperature compensation circuits will result in an incorrect measurement.If signal polarity needs to be reversed in an AC system (compression/tension – CW/CCW), switch the ± Signal lines since the phasing (as mentioned below) is derived from the Excitation sense lines.When troubleshooting or monitoring a DC mV/V excited transducer, use a 5-digit (or better) voltmeter since the output signal will be in the millivolt area (±0.0010 VDC).When troubleshooting or monitoring an AC mV/V excited transducer, using an Oscilloscope is recommended for visualizing the excitation and signal waveform to ensure the signal is symmetrical and not distorted. If an Oscilloscope is not available, using a 5-digit multimeter in AC RMS mode can provide useful amplitude and frequency measurements. Typical excitation frequency - in an AC strain gage system is 3 to 5 KHz at around 3 VAC RMS as measured between ± Excitation.In AC mV/V transducers there is a commonly used “phase” adjustment which is used to align the excitation phase to the returned signal phase. This adjustment compensates for capacitance in the cable, transducer windings and armature. Different rotary torque manufacturers will have a phase offset in the area of ± 16 to 35 degrees. Not using the “phasing” adjustment will create non-linear errors in the measurement which may exceed 3%. Refer to the conditioner’s manual for details. ................
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