1. Load Cells and Pressure Transducers – Overview of Operating Principles
|A load cell is a transducer that converts mechanical force into electrical signals. There are many different types of load cells that operate in different ways, but the most commonly used load cell today is the strain gage (or strain gauge) load cell. As their name implies, strain gage load cells use an array of strain gages to measure the deformation of a structural member and convert it into an electrical signal.|
Pressure transducers operate under the same principle. Strain gages, mounted on a diaphragm where the pressure is applied, measure the deformation of the diaphragm that is proportional to the pressure. The following sections describe the principle of operation of strain gage load cells and how to make a measurement from them, although the same applies for strain gage pressure transducers.
To understand how a load cell works, you need to first understand the basic theory behind the operating principles. As stated before, strain gages measure deformation, or strain, to determine the force (load) applied. Strain is defined as the fractional change in length. More specifically, strain is the change in length, dL, divided by the original length, L, and it varies directly proportional with the applied load. Figure 1 illustrates this concept. By sensing the strain and knowing the physical characteristics of the structural member to which the load is applied, you can accurately calculate the force.
Figure 1. Strain
While there are several methods of measuring strain, the most common is with a strain gage, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gage is the bonded metallic strain gage as shown in Figure 2.
Figure 2. Bonded Metallic Strain Gage
Because the changes in strain, and therefore resistance, are extremely small, you have to use additional circuitry to amplify the changes in resistance. The most common circuit configuration in a load cell is called a Wheatstone bridge. The general Wheatstone bridge, illustrated in Figure 3, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.
Figure 3. Wheatstone Bridge
The output voltage of the bridge, VO, is equal to:
Load cells typically use four strain gages in a Wheatstone bridge configuration, meaning that each resistive leg of the circuit is active. This configuration is called full-bridge. Using a full-bridge configuration greatly increases the sensitivity of the circuit to changes in strain, providing more accurate measurements. Although there is more in-depth theory about Wheatstone bridges, you do not need to know it because load cells are usually a “black box” with two wires for excitation (0 V and Vex) and two wires for the output signal (AI+ and AI-). Load cell manufacturers provide a calibration curve for every load cell, which correlates the output voltage to a specific amount of force.
2. Signal Conditioning Required to Make a Load or Pressure Measurement
The following section describes the necessary data acquisition and signal conditioning equipment to make an effective load cell or pressure transducer measurement. The basic requirements to make a load cell or pressure transducer measurement are excitation, signal amplification, and bridge balancing.
Load cell signal conditioners typically provide a constant voltage source to power the bridge. While there is no standard voltage level that is recognized industry wide, excitation voltage levels around 3 to 10 V are common. While a higher excitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors due to self-heating. It is very important that the excitation voltage be very accurate and stable.
The output of load cells and bridges is relatively small. In practice, most load cells and load-based transducers output less than 10 mV/V (10 mV of output per volt of excitation voltage). With a 10 V excitation voltage, the output signal is 100 mV. Therefore, load cell signal conditioners usually include amplifiers to boost the signal level to increase measurement resolution and improve signal-to-noise ratios.
Bridge Balancing and Offset Nulling
When a bridge is installed, it is very unlikely that the bridge outputs exactly 0 V when no strain is applied. Rather, slight variations in resistance among the bridge arms and lead resistance generate some nonzero initial offset voltage. There are a few different ways that a system can handle this initial offset voltage.
- Software compensation – The first method compensates for the initial voltage in software. With this method, you take an initial measurement before you apply the strain input. This is also referred to as auto-zero. This method is simple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is that the offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, therefore limiting the dynamic range of the measurement.
- Offset-nulling circuit – The second balancing method uses an adjustable resistance, or potentiometer, to physically adjust the output of the bridge to 0 V. By varying the position of the potentiometer, you can control the level of the bridge output – set the output to 0 V initially.
- Buffered offset nulling – The third method, like the software method, does not affect the bridge directly. With buffered nulling, a nulling circuit adds an adjustable DC voltage to the output of the instrumentation amplifier.
3. NI Solutions for Measuring Load Cells and Pressure Transducers
National Instruments offers many solutions for measuring load cells and pressure transducers depending on sampling rate, form factor, and channel count.
Figure 4. Examples of NI CompactDAQ, PXI, and SCXI Systems
NI CompactDAQ is ideal for low- to medium-channel count systems and includes a full-bridge analog input module that provide excitation and signal conditioning to measure load cells and pressure transducers.
The SC Express family for the PXI Platform combines data acquisition and signal conditioning into a single board for high-performance, reliable sensor measurements. The SC Express module utilizes a hot-swappable terminal block, and a wider variety of excitation voltages are also available. Multidevice triggering and synchronization via PXI Express make SC Express ideal for medium- to high-channel counts. SC Express can also provide higher sample rates than SXCI or CompactDAQ.
4. Connecting an Load Cell or Pressure Transducer to an Instrument
Similar procedures apply for connecting an amplified sensor to different instruments. For this section, consider an example using the NI cDAQ-9174 chassis and the NI 9237 C Series bridge module (see Figure 5).
Figure 5. NI CompactDAQ System
You will need the following equipment:
- NI cDAQ-9174 4-slot USB chassis for NI CompactDAQ
- NI 9237 four-channel, ±25 mV/V, 24-bit simultaneous bridge module
- Full-bridge load cell or pressure sensor
The NI 9237 has four RJ-50 receptacles or a 37-pin DSUB connector that provides connections for four half or full bridges, and an external voltage source. Figure 6 shows the correlation between the pin numbers of the RJ-50 10-position/10-conductor modular plug and the NI 9237 receptacle. If your sensor has an RJ-50 connector, you can plug it in to any of the 4 channels.
Figure 6. NI 9237 with RJ-50 Pin Assignments
If your sensor does not have an RJ-50 connector, you can refer to Figure 7 for the DSUB connector pinouts. You can connect your sensors to the appropriate channels as shown below.
Figure 7. NI 9237 with DSUB Pin Assignments
5. Seeing Your Measurement in LabVIEW
Now that you have connected your load cell to the measurement device, you can use LabVIEW graphical programming software to transfer the data into the computer for visualization and analysis. Figure 7 shows an example of displaying measured load data on a chart indicator inside the LabVIEW programming environment.
Figure 7. LabVIEW Front Panel Showing Load Data
6. Measuring Load Cell and Pressure Transducer Measurements
NI offers several platforms and solutions for measuring the variety of sensor in the market today. Combined with LabVIEW graphical programming, you can easily build a scalable load or pressure measurement system that meets your application requirements.