1. Overview of Amplified Bridge-Based Pressure Sensors
Wheatstone bridge-based sensors require external excitation and output a low-voltage signal that is measured by a DAQ device. Because most analog-to-digital converters (ADCs) have an input range of 10 V, amplification is required to take advantage of the ADCs full range and resolution. Traditionally, amplifiers have been separate from the sensors and part of the actual measurement device.
In more recent years, bridge-based pressure sensors have evolved to include onboard amplifiers, in which case the sensor has a high-level output (that is, 0-10 V or 0-20 mA). These types of sensors require an external power supply (9-28 V) and are referred to as internally amplified bridge-based sensors. More general terms include amplified sensors, powered sensors, and high-level output sensors. Load and displacement sensors also have variations in the amplified sensor class.
Figure 1. Internally Amplified Pressure Transducer
2. Pros and Cons of Using Amplified Bridge-Based Pressure Sensors
Internally amplified sensors move amplification and other signal conditioning components closer to the sensor, simplifying the requirements of the DAQ hardware. Sensor-specific conditioning is no longer required on the hardware so a variety of sensor types can be mixed and easily measured by the same analog input devices.
Internal amplifiers are housed in the same unit as the sensor, so the system is less vulnerable to electrical noise and creates a higher signal-to-noise ratio. Internal amplifiers do not add much error to the measurements and the larger output gives ADCs the ability to create higher-resolution output.
Internal amplifiers may not be feasible under certain conditions. Specifically, the circuitry in the amplifier cannot be subjected to extreme temperatures. If a sensor is placed in a location inaccessible to users (for example, a hazardous environment, small space, or long distance), zero and span adjustments may not be able to be tweaked when needed. Internal amplifiers increase the overall size of the unit, which may be concern in some applications.
Many amplified sensors have built-in temperature compensation to remove errors from self-heating and changes in the environment. This is an advantage over traditional Wheatstone bridge-based pressure sensors that can be sensitive to temperature-induced error.
3. NI Solutions for Measuring Amplified Bridge-Based Pressure Sensors
National Instruments offers many solutions for measuring internally amplified pressure sensors depending on sampling rate, form factor, and channel count. NI CompactDAQ is ideal for low- to medium-channel-count systems and includes a variety of analog input and output modules that can be combined to the supply excitation and measure amplified sensors. NI CompactDAQ includes options for simultaneous measurements and channel-to-channel isolation.
Figure 2. Examples of NI CompactDAQ, PXI, and SCXI Systems
The PXI platform offers many modules that can supply excitation and measure amplified sensors as well. PXI can provide high-voltage excitation and includes options for simultaneous sampling and channel-to-channel isolation. The PXI platform is ideal for systems with the most stringent requirements or high-channel counts. Lastly, the SCXI family has a module designed to supply excitation and measure the high-level pressure output for low-speed, high-channel-count systems.
4. Connecting an Amplified Pressure Sensor to an Instrument
Similar procedures apply for connecting an amplified sensor to different instruments. For this section, consider an example using the NI cDAQ-9178 chassis, the NI 9205 analog input module, and the NI 9264 analog output module (see Figure 3).
Figure 3. NI CompactDAQ System
You will need the following equipment:
- cDAQ-9178, an 8-slot USB chassis for NI CompactDAQ
- NI 9205, a 32-channel, ±200 mV to ±10 V, 16-bit, 250 kS/s analog input module
- NI 9264, a 16-channel, ±10 V, 16-bit, 400 kS/s analog output module
- Amplified sensor
The NI 9205 is a 32-channel single-ended or 16-channel differential analog input module that can measure output from the amplified sensor. Figure 4 lists the signal names of the terminals and the correlated pin numbers. There is a diagram for the screw terminal and the D-sub connectivity options. Figure 5 shows the input circuitry on one analog channel on the NI 9205.
Figure 4. NI 9205 Terminal and Pin Assignments
Figure 5. NI 9205 Input Circuitry
You can use a differential measurement configuration to attain more accurate measurements and less noise. A differential measurement configuration requires two inputs for each measurement, thus reducing the number of available channels on the NI 9205 to 16. Figure 6 shows the signal pairs that are valid for differential connection configurations with the NI 9205.
Figure 6. NI 9205 Differential Pairs
The NI 9264 is a 16-channel analog output module that can supply excitation (up to 10 V) to the amplified sensor. Figure 7 lists the signal names of the terminals and the correlated pin numbers. There is a diagram for the screw terminal and the D-sub connectivity options. If you require greater than 10 V excitation, you can use the NI 9269 for up to 40 V output.
Figure 7. NI 9264 Terminal and Pin Assignments
5. Seeing Your Measurement in NI LabVIEW
Now that you have connected your amplified sensor to the measurement device, you can use LabVIEW graphical programming software to transfer the data into the computer for visualization and analysis. Figure 8 shows an example of displaying measured strain data on a chart indicator inside the LabVIEW programming environment.
Figure 8. LabVIEW Front Panel Showing Pressure Data
NI Offers Many Solutions for Measuring Amplified Sensors
NI offers several platforms and solutions for sensor measurements from basic thermocouple measurements to the variety of pressure sensors in the market today. Amplified sensors can simplify DAQ software and hardware because of the high-level outputs and relaxed signal conditioning requirements.