DAQ Hardware

DAQ(Data Acquisition) Hardware

A data acquisition (DAQ) system uses a data acquisition device to pass a conditioned electrical signal to a computer for software analysis and data logging. You can choose a data acquisition device that uses a PCI bus, a PCI Express bus, a PXI bus, or the computer USB or IEEE 1394 port. This section explains the hardware used in a data acquisition system and how to configure the devices.

A typical DAQ system has three basic types of hardware—a terminal block, a cable, and a DAQ device, as shown in Fig-1.

After you have converted a physical phenomenon into a measurable signal with or without signal conditioning, you need to acquire that signal. To acquire a signal, you need a terminal block, a cable, a DAQ device, and a computer. This hardware combination can transform a standard computer into a measurement and automation system. Terminal Block and Cable A terminal block provides a place to connect signals. It consists of screw or spring terminals for connecting signals and a connector for attaching a cable to connect the terminal block to a DAQ device. Terminal blocks have 100, 68, or 50 terminals. The type of terminal block you should choose depends on two factors—the device and the number of signals you are measuring. A terminal block with 68 terminals offers more ground terminals to connect a signal to than a terminal block with 50 terminals. Having more ground terminals prevents the need to overlap wires to reach a ground terminal, which can cause interference between the signals. A cable transports the signal from the terminal block to the DAQ device. Cables come in 100-, 68-, and 50-pin configurations. Choose a configuration depending on the terminal block and the DAQ device you are using. Cables, like terminal blocks, are shielded or non-shielded.

Fig.1 - DAQ Hadware.

DAQ Signal Accessory Fig-2 shows the terminal block you are using for this course, the DAQ Signal Accessory. The DAQ Signal Accessory is a customized terminal block designed for learning purposes. It has three different cable connectors to accommodate many different DAQ devices and spring terminals to connect signals. You can access three analog input channels, one of which is connected to the temperature sensor, and two analog output channels. The DAQ Signal Accessory includes a function generator with a switch to select the frequency range of the signal, and a frequency knob. The function generator can produce a sine wave or a square wave. A connection to ground is located between the sine wave and square wave terminal

Fig.2 - DAQ Signal Accessory—terminal block..

A digital trigger button produces a TTL pulse for triggering analog input or output. When you press the trigger button, the signal goes from +5 V to 0V and returns to +5V when you release the button. Four LEDs connect to the first four digital lines on the DAQ device. The LEDs use reverse logic, so when the digital line is high, the LED is off and vice versa.

The DAQ Signal Accessory has a quadrature encoder that produces two pulse trains when you turn the encoder knob. Terminals are provided for the input and output signals of two counters on the DAQ device. The DAQ Signal Accessory also has a relay, a thermocouple input, and a microphone jack.

DAQ Devices

Most DAQ devices have four standard elements—analog input, analog output, digital I/O, and counters.

You can transfer the signal you measure with the DAQ device to the computer through a variety of different bus structures. For example, you can use a DAQ device that plugs into the PCI or PCI Express bus of a computer, a DAQ device connected to the PCMCIA socket of a laptop, or a DAQ device connected to the USB port of a computer. You also can use PXI/CompactPCI to create a portable, versatile, and rugged measurement system.

If you do not have a DAQ device, you can simulate one in Measurement & Automation Explorer (MAX) to complete your software testing. You learn to simulate a device in the Simulating a DAQ Device section of this lesson.

Analog Input

Fig.3 - DAQ Hardware Analog Input

Analog input is the process of measuring an analog signal and transferring the measurement to a computer for analysis, display or storage. An analog signal is a signal that varies continuously. Analog input is most commonly used to measure voltage or current. You can use many types of devices to perform analog input, such as multifunction DAQ (MIO) devices, high-speed digitizers, digital multimeters (DMMs) and Dynamic Signal Acquisition (DSA) devices.

Analog Output

Analog output is the process of generating electrical signals from your computer. Analog output is generated by performing digital-to-analog (D/A) conversions. The available analog output types for a task are voltage and current. To perform a voltage or current task, a compatible device must be installed that can generate that form of signal.

Digital I/O

Digital signals are electrical signals that transfer digital data over a wire. These signals typically have only two states—on and off, also known as high and low, or 1 and 0. When sending a digital signal across a wire, the sender applies a voltage to the wire and the receiver uses the voltage level to determine the value being sent. The voltage ranges for each digital value depend on the voltage level standard being used. Digital signals have many uses; the simplest application of a digital signal is controlling or measuring digital or finite state devices such as switches and LEDs. Digital signals also can transfer data; you can use them to program devices or communicate between devices. In addition, you can use digital signals as clocks or triggers to control or synchronize other measurements.

Counters

A counter is a digital timing device. You typically use counters for event counting, frequency measurement, period measurement, position measurement, and pulse generation.

When you configure a counter for simple event counting, the counter increments when an active edge is received on the source. In order for the counter to increment on an active edge, the counter must be armed or started. A counter has a fixed number it can count to as determined by the resolution of the counter. For example, a 24-bit counter can count to:

2(Counter Resolution) – 1 = 224 – 1 = 16,777,215

When a 24-bit counter reaches the value of 16,777, 215, it has reached the terminal count. The next active edge forces the counter to roll over and start at 0.