Transducers 7

With DAQ, the signals you are measuring or controlling will ultimately be translated into voltages compatible with your DAQ device. Even a current signal is converted to a voltage at some point; with current-reading DAQ devices, this conversion will occur internally. The devices that convert any real-world parameter to or from an electrical signal, often a voltage, are called transducers. NI has application notes devoted to many of these transducer types—this chapter is simply meant to give you a quick overview of the most common transducers. For an entire book on LabVIEW and transducers, read Sensors, Transducers, and LabVIEW: An Application Approach To Virtual Instrumentation, by Barry E. Paton, a physics professor. For a list of transducer manufacturers, go to www.LCtechnology.com/daq-hardware.htm and see the Transducers section.

For a few very common types of transducers, LabVIEW has customized hardware and software. These will be discussed on a per-transducer basis throughout this chapter. When you see SCXI, this is a type of NI hardware. Notice that you cannot spell “excessively expensive” without SCXI. SCXI DAQ devices are very rugged and are housed in beautiful aluminum chassis. SCXI devices are designed for industrial applications—but there are often less expensive alternatives from NI as well. An example of an SCXI system is shown in Figure P-4 of the preface.

Following is a categorization of the most common types of transducers:

    Temperature

        Thermocouples

        RTDs

        Thermistors

        Other Devices

    Force and Pressure

        Strain Gauges

        Other Devices

    Flow Rate

    Position

    Other Transducers

7.1 TEMPERATURE TRANSDUCERS

This section presents a wide range of temperature transducers, including thermocouples, RTDs, thermistors, and specialty devices.

7.1.1 Thermocouples

A thermocouple is perhaps the simplest type of transducer in existence; it converts temperature into a voltage. Simply twisting together any two pieces of wire, each made of a different type of metal, produces a thermocouple! Iron and constantan (a copper-nickel alloy) are two metals that will produce a J Type thermocouple. Other types of thermocouples are E, K, R, S, and T, each with their own two types of metals, and each useful over a different range of temperatures. Thermocouples can withstand higher temperatures than can most other temperature transducers, but they do have a couple of unique downsides: (1) they produce a very small voltage, and (2) they require a second temperature transducer called a CJC (cold-junction compensation sensor). The CJC and its physical connection to the thermocouple usually limit the thermocouple’s measurement accuracy. With NI’s present hardware, this accuracy ranges from +/- 0.9° C to +/- 1.4° C.

Figure 7-1 shows a simple thermocouple, which is simply two wires of different metals connected.

Figure 7-1
A simple thermocouple.

There are many packages for thermocouples involving connectors, shielding, and various other bells and whistles, but the essence is always a pair of metal wires inside.

The following table lists some common thermocouple types, as well as their temperature ranges.

Some of NI’s DAQ devices have their own CJCs built in. For example, a few of the SCXI terminal blocks have a CJC, like the SCXI-1328, and many of the SCXI modules, like the SCXI-1120, are recommended for thermocouples. If you are a good electrical engineer, you can supply your own CJC, such as the National Semiconductor LM-35CAZ. You can use any generic DAQ device with a high gain, with differential inputs, and with a +5 volt power supply (for the CJC) to measure thermocouples over a wide temperature range. You’ll need an extra analog input for the CJC. Of course, you’ll also need to read the spec sheet on your CJC to hook it up correctly, and you’ll want to thoroughly understand the application note mentioned at the end of this section.

With thermocouples, the relationship between voltage and temperature is highly non-linear, and an equation is required. LabVIEW provides thermocouple equations in the Functions»Data Acquisition»Signal Conditioning palette’s Convert Thermocouple Reading .vi: For more detailed information on thermocouples, read NI’s Application Note 043, Measuring Temperature with Thermocouples—A Tutorial.

7.1.2 RTDs

An RTD, or resistance-temperature detector, is a device whose resistance increases with temperature. In practice, an RTD is typically either a wire coil or a deposited film of pure metal. The most common type of RTD is platinum, which has a nominal resistance of 100Ω at 0° C. RTDs are very accurate, some of them having an accuracy as high as 0.026° C at 0° C. RTDs come with two-, three-, or four-wire connections, depending on their purpose, but no matter the number of wires, each RTD is just one resistor. Figure 7-2 shows some typical RTDs.

Figure 7-2
RTDs for temperature measurement.

Many of NI’s DAQ devices are appropriate for RTDs. As with any resistive device, an excitation voltage is required, and certain types of DAQ hardware provide this—for example, the SCXI-1121.

With RTDs, the relationship between voltage and temperature is fairly linear, but not quite, so an equation is required. LabVIEW provides RTD equations in the Functions»Data Acquisition»Signal Conditioning palette’s Convert RTD Reading.vi: . For more detailed information on RTDs, read NI’s Application Note 046, Measuring Temperature with RTDs—A Tutorial.

7.1.3 Thermistors

Like RTDs, thermistors are thermally very sensitive resistors, but they are semiconductors made from metal oxides. The two types of thermistors are negative temperature coefficient (NTC), in which the resistance decreases with temperature, and positive temperature coefficient (PTC), in which the resistance increases with temperature. Figure 7-3 shows the basic element of a thermistor.

Figure 7-3
A thermistor.

The main advantage of thermistors is their sensitivity. As should be obvious, the tradeoff is that their temperature range is smaller than that of RTDs or thermocouples. Figure 7-4 shows temperature-resistance plots of a typical NTC thermistor versus a typical RTD.

Many DAQ devices are appropriate for thermistors. As with any resistive device, an excitation voltage is required, and certain types of DAQ hardware provide this—for example, the SCXI-1123. Thermistors’ relationship between voltage and temperature is highly nonlinear, as shown in Figure 7-4, so an equation is required. LabVIEW provides thermistor equations in the Functions»Data Acquisition»Signal Conditioning palette’s Convert Thermistor Reading. vi: . For more detailed information on thermistors, read NI’s Application Note 065, Measuring Temperature with Thermistors—A Tutorial.

Figure 7-4
Thermistor versus RTD curves.

7.1.4 Other Devices

There are a few other types of temperature devices out there, some of which produce either a current (from Omega) or an RS-485 digital signal (National Semiconductor has some really nice chip-level devices). Either of these signal types is highly immune to noise, so consider them if they have the range and the accuracy you need. Concerning temperature range, you might wind up with some melted transducers if they can’t handle the heat!

7.2 Force and Pressure Transducers

Transducers that measure force and pressure will be briefly presented in this section.

7.2.1 Strain Gauges

Strain, in the context of DAQ, is the “deformation of a material body under the action of applied forces,” according to my dictionary. Since the physics behind strain gauges is fairly cumbersome to describe, I won’t attempt it here; instead, please refer to the application note mentioned at the end of this section. Although there are a number of devices to measure strain, the most common is the strain gauge, whose electrical resistance varies with the amount of strain in the device. In other words, the more you bend the strain gauge, the greater its electrical resistance. A strain gauge can be a wire, but more often, it’s a piece of plastic or similar material with metallic foil bonded to it in a grid pattern. Figure 7-5 shows an example of a common strain gauge layout, which is a metal foil bonded to a slightly flexible, electricity-insulating base, like plastic.

Figure 7-5
A strain gauge’s basic parts.

The grid maximizes the length of wire exposed to the strain, as compared to a single wire. This entire strain gauge is then glued with epoxy (or some similar substance) to the device being measured. For accuracy reasons, multiple strain gauges with multiple wires are often used to measure one point of strain. As with any resistive device, an excitation voltage is required, and certain types of DAQ hardware provide this—for example, the SCXI-1121 and SCXI-1122 are designed for strain gauges.

Strain gauges have fairly complex equations associated with them. LabVIEW provides strain gauge equations in the Functions»Data Acquisition»Signal Conditioning palette’s Convert Strain Gauge Reading.vi: For more detailed information on strain gauges, read NI’s Application Note 078, Strain Gauge Measurement—A Tutorial.

7.2.2 Other Force and Pressure Devices

Other devices for measuring force and/or pressure are load cells, accelerometers, torque cells, and pressure switches, all of which are often based upon strain gauges. Specialized transducers exist for low pressures (usually gasses, as in a vacuum) and high pressures (often for liquids or gasses).

7.3 FLOW RATE TRANSDUCERS

Some common names for flow rate (of gasses or liquids) transducers are flowmeters, mass flowmeters, or if they can control the flow, mass flow controllers. In many flow rate transducers, temperature sensors are used as the substance is flowing through a tube; in some cases, heat is applied to such a tube as well. Magnetic flowmeters have no moving parts (thus are more rugged), but work only with electrically conductive fluids by passing an electromagnetic field through said fluid flowing through a tube. If a fluid has a minimal amount of suspended particles or bubbles, ultrasonic flowmeters work by passing sound waves through the fluid as it flows through a tube. Figure 7-6 shows a mass flow controller (MFC) attached to some equipment I was recently testing.

Figure 7-6
A mass flow controller used to test semiconductor-making equipment.

Of course, output is a part of DAQ as well. There exist hordes of valves that can control the flow rate of almost any gas or liquid.

7.4 POSITION TRANSDUCERS

Figure 7-7 is a photo of a Daytronic LVDT (Linear Variable Differential Transformer).

Figure 7-7
An LVDT.

An LVDT is usually a piston that is mechanically attached to the specimen whose position is to be measured. It has magnetic coils inside that produce an AC signal. The LVDT’s output is usually an AC or DC voltage signal.

Another common position sensor is the ultrasonic transducer. This transmits and receives pulses of sound, and measures the time it takes for the sound to bounce back from the object being measured. You could think of it as a little radar. Bats come with these devices organically preinstalled, although their frequencies dip into the audible range.

Ultrasonic transducers aren’t much to look at, hence the bat illustration in Figure 7-8. However, bats are difficult to interface to LabVIEW programs.

Figure 7-8
An organic ultrasonic transducer.

7.5 OTHER TRANSDUCERS

Almost any physical parameter that can be measured has some form of transducer available. Light intensity, pH, humidity, conductivity, all frequencies of electromagnetic radiation (radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma radiation), and so on all have specialized transducers. When the physical parameter becomes very specialized, such as measuring small concentrations of particles in a high-purity gas, expect a specialized transducer. Particularly, expect to pay lots of money for this specialized transducer, and to get some sort of bus-based interface to the computer instead of a simple voltage or current.