A force transducer is a sensor that transforms the physical parameter – force, load, weight, into an equivalent analog electrical voltage signal that is proportional to the magnitude of the force. Force transducers are also called load cells.
There are different types of load cells, each with its unique mode of operation and construction.
- pneumatic load cells
- hydraulic load cells
- strain gauge transducers
- piezoelectric transducers
The pneumatic and the hydraulic load cells are force balance devices that determine the size of the force by the pressure it exerts on a transmitting medium like fluids or gases. Hence, the load cell portion is not really a transduction device. A pressure transducer is, however, fixed to the pressure output of the load cell to give the reading. This makes the whole system bulky and expensive.
The piezoelectric transducer uses the piezoelectric effect that makes a material generate electric charges when subjected to a stress. This device, however, can only support dynamic loading conditions because the device works like a capacitor. This capacitor has a large internal resistance, causing rapid charge decay over a period of time.
The most commonly used force transducer is the strain gauge load cell, as it has a moderate combination of the good characteristics of the force balance devices and the piezoelectric transducer. The remainder of this article describes its electrical connections.
The Internal Electrical Setup of the Strain Gauge Load Cell
The major element of this transducer is the strain gauge, which is a length of a flexible conductor – metallic or semiconductor – attached or micro-machined to a substrate layer that may be polyester or any non-conducting material.
Together, the conductor and the substrate form a flexible component. The strain gauge is then bonded to an elastic structural element like a beam that makes up the force transducer. The bonded strain gauge is fixed such that its long length lies along the direction of application of the force to be measured.
There are designs that use multiple strain gauges with each attached to different parts of the structural element. The gauges could also be fabricated on a single substrate and then placed on a central location on the structure as shown in Figure 1 below. In general, multi-gauge systems have improved sensitivity because the number of strain gauges is directly proportional to sensitivity.
When a force is applied to the loading point of the transducer, a stress is exerted on the elastic structural member and that structure deflects. In this way, the structural member acts as a primary sensor. This deflection in turn creates a local strain along the length of its body.
The strain gauge is bonded around the region of maximum strain on the body of the structural member. The developed strain causes the strain gauge to deform in geometry, causing variations in electrical resistance.
This variation in electrical resistance is then measured and simply displayed by a sensitive voltage meter, or further manipulated to calculate the magnitude of the applied force. A Wheatstone bridge is most commonly used to manipulate these changes in resistance. The Wheatstone bridge is a voltage divider that has four arms as shown in Figure 2 below.
Various configurations of the Wheatstone bridge exist, based on the number of strain gauges present in the arms of the bridge. These are discussed below.
Quarter Bridge Configuration
In a quarter bridge configuration of the Wheatstone bridge, the change in resistance across a single strain gauge is measured. The strain gauge is simply fixed into one arm of the Wheatstone bridge (as Figures 2-3 show) and it is therefore no different from a resistor; in fact, it is a variable resistor.
It should be noted that even though the strain gauge changes in resistance, it normally has a base resistance value. To simplify the design, the resistors in the other arms of the bridge are made equivalent to the strain gauge resistance at zero strain.
The half-bridge configuration consists of two strain gauges fixed into two alternating arms of the Wheatstone bridge. The strain gauges can be mounted on the structural element at high strain location to measure axial or bending strain. Its output is twice as sensitive as the quarter-bridge’s output. See Figure 4 below.
The full-bridge configuration has four strain gauges fixed into the four arms of the Wheatstone bridge. They are connected such that two strain gauges measuring the compressive strain are fixed on alternating arms while the remaining two strain gauges measuring the tensile strain are fixed into the other two arms of the bridge. One is depicted in Figure 5 below.
Design Considerations to Mitigate Error
The electrical resistance and the voltage output sensitivity of a strain gauge vary with temperature, introducing measurement errors. Therefore, Wheatstone bridge designs should account for the effects of these temperature changes on resistivity. Some designs use strain gauge materials that are temperature self-compensating. Other designs make use of dummy gauges, especially in the quarter-bridge configuration. In this case, the dummy gauge is placed in the arm opposite the strain gauge. Still other designs use a temperature compensated multi-strain gauge bridge configuration to account for temperature shifts.
Electrical Wiring of Force Transducers
Intuitively, looking at the underlying Wheatstone bridge connection of a strain gauge transducer, one would see that there are, at a minimum, four electrical wires connected to the device. That is, by default, it is a four-wire system.
In the four-wire system, there are two wires for power supply to the Wheatstone bridge: the positive and negative input terminals. The other two wires are the signal output terminals of the bridge, likewise positive and negative. This wiring system is shown in Figure 6.
Most textbooks and articles show the lumped parameter models of the strain gauge Wheatstone bridge and its connections. The lumped parameter model ideally assumes that the connecting wires to all these terminals on the Wheatstone bridge have zero resistance, hence a zero voltage drop.
However, in practice, the distributed parameter model is more accurate. Here, the connecting wires actually have resistance to the flow of current; there are voltage drops across the wires. Since the connecting wires have resistance, their length, cross-sectional area, and resistance variation with temperature must be considered in the bridge’s design.
The distributed parameter model is the motive behind a six-wire strain gauge system. The six-wire strain gauge load cell has the normal power supply/excitation and signal output terminal wires. Additionally, positive and negative sense terminals exist. The sense terminal wires are connected at one end to the same nodes as the power supply wires, shown in Figure 4 below.
The other end of these sense terminals is connected to the input port of an amplifier; the output of the amplifier is then connected to the power supply terminals of the Wheatstone bridge load cell.
In this way, the actual voltage powering the load cell is detected and the amplifier makes the necessary adjustment to keep the voltage supply at the desired operating level irrespective of the voltage drop across the cables. To summarize, the six-wire system integrates a simple voltage controller design with the Wheatstone bridge.
This shows that while a four-wire strain gauge load cell is easily affected by the varying resistance of the supply wires, the six-wire type is not affected because the amplifier will make the needed gain adjustments to the power supply. Therefore, with the latter design, it is possible to adjust the cable lengths without introducing error.
Finally, an additional wire may be present in both the four- and six-wire bridge types. This wire is called the shield wire. The shield wire is not connected to the strain gauge; rather it is connected to the body of the transducer. It protects the internal circuitry from electromagnetic interference.
Important Electrical Characteristics
The various electrical characteristics of a strain gauge transducer can be found in the product’s datasheet. The following are some of these specifications:
- Recommended Excitation Voltage: This is the electrical voltage that should be applied to the transducer. It usually has a range of about 10V to 15V. As explained above, a six wire load cell will maintain this level of voltage regardless of cable voltage drops.
- Full-Scale Output: This is the ratio of the output voltage for a certain excitation voltage and applied rated load. It can also be called the sensitivity of the transducer. The unit is mV/V and a value of 2mV/V is typical at rated capacity.
- The Zero Balance: This is the ratio of transducer output voltage to excitation voltage when no force is applied. The unit is also mV/V.
- Input Resistance: This is the resistance value obtained at the supply terminals of the force transducer under standard test conditions, zero loads, zero excitation, and with open circuit output terminals.
- Output Resistance: This is the resistance value obtained at the signal output terminals under standard test conditions, zero load, zero excitation, and open circuit input terminals.
- Insulation Resistance: This is the resistance measured between the body of the transducer and the interconnecting node of all the device’s wires. A good transducer should have an insulation resistance that is greater or equal to 2 Gigaohms.
- Temperature Effects: The temperature effect on the sensitivity and zero balance is a measure of how the output voltage changes with respect to temperature variations when the rated load and no-load are applied respectively.
- Temperature Range: This shows the range of temperatures within which measurements must be taken to ensure accuracy.
This article presents the various issues relating to the electrical connections of a strain gauge transducer. Although specifically strain gauge load cells were described, the same conceptual understanding can be applied to other transducers like the pressure and piezoelectric transducers. For example, the compensating techniques for the influence of power supply cables can be applied to these other transducers.
Design and implementation considerations beyond the scope of this article exist. A force transducer always needs output signal amplification because the output electrical signal is small – of the order of mV for strain gauge load cells and pC for piezoelectric transducers. Instrumentation amplifiers are used for strain gauge transducers while charge amplifiers are used for piezoelectric transducers. While boosting the output signal, other signal conditioning processes such as noise filtering and isolation are implemented to ensure accuracy.
Finally, calibration of the measuring chain to mitigate systematic errors further guarantees the quality of output measurements. These and other concepts related to load cells are discussed in more detail in the articles below.
- Load Cell FAQ
- An Overview of Load Cells
- How to Test for Faults in a Load Cell
- Load Cell Minimum Weight
- Calibrating the Force Measuring System