lesson 1.pdf

# Overview of a DAQ System This lesson introduces the basics of data acquisition (DAQ). ## Topics * DAQ System Overview * Sensors * Signals * DAQ Hardware * Signal Conditioning * DAQ Software ## DAQ System Overview Data acquisition is the automatic collection of data from sensors, instruments, a...

# Overview of a DAQ System This lesson introduces the basics of data acquisition (DAQ). ## Topics * DAQ System Overview * Sensors * Signals * DAQ Hardware * Signal Conditioning * DAQ Software ## DAQ System Overview Data acquisition is the automatic collection of data from sensors, instruments, and devices: in a factory, laboratory, or in the field. The purpose of a DAQ system is to measure a physical phenomenon, such as light, temperature, pressure, or sound. A DAQ system includes the following building blocks: * Sensor/Signal * DAQ Hardware * Signal Conditioning * DAQ Software With these building blocks, you can bring the physical phenomenon you want to measure into the computer for analysis and presentation. ## Sensors Signal acquisition is the process of converting physical phenomena into data the computer can use. A measurement starts with using a sensor, also called a transducer, to convert a physical phenomenon into an electrical signal that a DAQ system measures. Sensors can generate electrical signals to measure parameters, such as temperature, force, position, sound, or light. Table 1-1 lists some common sensors. | Phenomena | Sensors | |---|---| | Temperature | Thermocouples <br> Resistive temperature detectors (RTDs) <br> Thermistors <br> Integrated circuit sensors | | Light | Vacuum tube photosensors <br> Photoconductive cells | | Sound | Microphones | | Force and pressure | Strain gage <br> Piezoelectric transducers <br> Load cells | | Position (displacement) | Potentiometers <br> Linear voltage differential transformers (LVDTs) <br> Optical encoders | | Fluid flow | Head meters <br> Rotational flowmeters <br> Ultrasonic flowmeters | | pH | pH electrodes | ## Types of Sensors Sensors are used for a variety of needs, such as measuring temperature, pressure, and fluid flow. Refer to ni.com/sensors for more information about sensors and where to obtain them. Different sensors have different requirements for converting a physical phenomenon into a measurable signal. For example, a resistance temperature detector (RTD) needs an excitation current to measure the temperature. A thermocouple does not need excitation current, but it does need cold-junction compensation. Strain gages use a configuration of resistors called a Wheatstone bridge to measure strain. Before you set up the system, you should know if the sensor has any special requirements. Contact the vendor of the sensor for more information about how to properly use the sensor. ## Signals A sensor converts a physical phenomenon into a signal that you can acquire. However, there are also times that you want to acquire voltage signals that are not produced by a sensor. Not all signals are measured in the same manner. You first need to categorize the signal as digital or analog. After you categorize the signal, decide which type of information you want from that signal. The possible types of information you can obtain from a signal are state, rate, level, shape, and frequency, as shown in Figure 1-2. * **Analog** * 5.0 Volts *Level* * Rise Time = 1msec *Shape* * 0.2 Hz *Frequency* * **Digital** * ON / OFF *State* * 0-3 *Rate* **Note:** This discussion of signals assumes that you are acquiring the signal. However, most of the points also apply to generating a signal. The difference between acquiring and generating a signal is that you do not need analysis when you generate a signal with a specific frequency. ## Analog Signals Unlike digital signals, an analog signal can be at any voltage level with respect to time. Because an analog signal can be at any state at any time, the physical aspects you want to measure differ from those of a digital signal. ### Analog Signal Information You can measure the level, shape, and frequency of an analog signal, as shown in Figure 1-3. * **Level** - Measuring the level of an analog signal is similar to measuring the state of a digital signal. The difference is that an analog signal can be at any voltage level, but a digital signal is either a low or high voltage signal. * **Shape** - Measuring the shape of the signal is often important because analog signals can be at any state with respect to time. For example, a sine wave has a different shape than a sawtooth wave. Measuring the shape of a signal can lead to analysis of other aspects of the signal, such as peak values, slope, or integration. * **Frequency** - Measuring the frequency of an analog signal is similar to measuring the rate of a digital signal. However, you cannot directly measure the frequency of an analog signal. You need to perform software analysis on the signal to extract the frequency information, usually with a Fourier Transform. ## Digital Signals A digital signal has only two possible states—ON (logic high) or OFF (logic low). A common type of digital signal is a Transistor-to-Transistor Logic (TTL) signal. The specifications for a TTL signal define a voltage level between 0 and 0.8 V as logic low and a voltage level between 2 and 5 V as logic high. Most digital devices accept a TTL-compatible signal. ### Digital Signal Information You can only measure two aspects of a digital signal: state and rate. * **State** - A digital signal has only two possible states: ON or OFF. One of the aspects of a digital signal that you can measure whether the state is ON or OFF. * **Rate** - A digital signal changes state with respect to time. You can measure the rate, or how the digital signal changes states over time. Consider an example of measuring the state of a digital signal, as shown in Figure 1-7. Assume you have a switch that you want to monitor. This switch turns a light on or off. In the example in Figure 1-7, when the switch is open, you measure 0 V (OFF). When the switch is closed, you measure 5 V (ON). By measuring the state of the digital signal, you can determine if the light is on or off. See Figure 1-8 for an example for measuring the rate of a digital signal. Assume you have a motor, and you want to determine how fast the shaft of the motor is spinning. An encoder is a transducer that can convert the rotary motion of the motor shaft into a digital signal. When an encoder rotates, it produces two digital signals. Each digital signal is a series of alternating on or off states, called a pulse train. For each increment of rotation, you get a pulse. The increment of rotation depends on the encoder. For example, the BNC-2120 terminal block you use in this course has an encoder that gives 96 pulses per revolution. You can measure the rate of one of the pulse trains to determine how fast the shaft rotates. You can measure both pulse trains to determine not only how fast the shaft rotates, but also the direction in which it rotates. ## DAQ Hardware The purpose of DAQ hardware is to transfer the data between your software and your sensor or signal. DAQ hardware can both acquire and generate analog and digital signals. The DAQ hardware transfers signals to and from the DAQ software through a bus. A few examples of available buses include PCI, PCI Express, PXI, PXI Express, and USB. ## Signal Conditioning You cannot always connect the signal directly to a DAQ device. You might need to alter the signal to make it suitable for a DAQ device to measure. Signal conditioning is the process of measuring and manipulating signals to improve accuracy, isolation, filtering, and so on. Signal conditioning is not always required. The purpose of signal conditioning is to take a signal that is difficult for your DAQ device to measure and alter the signal to make it easier to measure. Many sensors need some sort of external hardware to perform their job. For example, resistance temperature detectors need an excitation current, and strain gages need a configuration of resistors called a Wheatstone bridge. Also, to measure signals from sensors, you must convert them into a form which a DAQ device can accept. For example, the output voltage of most thermocouples is very small and susceptible to noise. Therefore, you might need to amplify the thermocouple output before you digitize it. This amplification is a form of signal conditioning. Sometimes, signal conditioning can occur in the sensor itself. For example, some microphones and accelerometers contain a built-in amplifier to output a more readable signal. Signal conditioning can also occur along the path between the sensor or signal and DAQ hardware. For example, you can place external amplifiers and filters along the path to help reduce noise. Signal conditioning can also occur in the DAQ hardware. There are many National Instruments DAQ hardware products that have built-in signal conditioning, such as amplification, Wheatstone bridge completion for strain gages, cold-junction compensation for thermocouples, lowpass filtering, and more. Refer to ni.com/signalconditioning for more information about National Instruments signal conditioning hardware. ## DAQ Software After you acquire your data from the DAQ hardware, you often still need to do something with the data. You can use DAQ software to generate a report, interact with the data, manipulate signals, analyze signals, log the data to file or a database, and much more. ## Summary A DAQ system consists of the following components: * Sensors that convert a physical phenomenon into a measurable signal * Signals, either digital or analog. Depending on the signal, you can measure the state, rate, level, shape, or frequency. * Signal conditioning, which makes signals easier to measure with a DAQ device * DAQ hardware * DAQ software