Designing Control Applications with Data Acquisition Hardware and NI-DAQmx

With hundreds of hardware options to choose from, you have several factors to consider when selecting the right data acquisition (DAQ) device for a control application. First, you should understand the channel count, data type, filtering and signal conditioning needs for the sensors you would like to use in your data acquisition application. Next, you’ll need to know the performance you need from your DAQ device: the accuracy, resolution, determinism, and latency needed to acquire clear signals at appropriate sample rates. This will inform which DAQ platform you should choose.

Once your I/O is fully understood, you can select the right hardware platform for your application. All the systems below can be programmed using the NI-DAQmx API, to access data and perform control quickly and easily.

NI-DAQmx, the driver software used to communicate with NI DAQ measurement devices, is included with more than 200 National Instruments data acquisition devices. NI-DAQmx has support for multiple programming languages including NI LabVIEW, LabWindows™/CVI, ANSI C/C++, Visual Basic 6.0, and C#/Visual Basic .NET. For control applications, NI-DAQmx has specific deterministic features like hardware-timed single-point function calls and support for LabVIEW Real-Time operating systems.

This white paper will focus on using the NI-DAQmx driver with LabVIEW. LabVIEW graphical programming software offers an easy-to-use application development environment designed specifically for the needs of engineers and scientists. Without any prior programming experience, you can configure your hardware, take sensor measurements, compare the measured value to the desired setpoint, and update output signals. The NI-DAQmx driver provides graphical functions to read and write values and directly interact with hardware I/O. LabVIEW delivers the best integration between software and hardware, reducing the overall programming complexity and saving you development time.

The NI-DAQmx application programming interface (API) is completely scalable from simple ON/OFF control to PID and other advanced control algorithms. The following few sections offer examples of the graphical programming required to implement various control systems.

A feedback control system can be as simple as taking a measurement, comparing the measured value with the desired value, and then turning an actuator on or off to get closer to the desired value. This typically involves driving a relay or switch that controls the power flowing to an actuator. Everyday air conditioning systems, for example, work in this manner. A temperature reading device (usually your thermostat) monitors the current temperature and engages a relay or switch if the temperature exceeds the desired value. The relay turns the air conditioning unit on until the temperature returns to a comfortable level.

Figure 3 shows an example of a LabVIEW block diagram for creating a basic on/off control system. The DAQmx Read.vi function takes a temperature reading, and the temperature value is compared to the Setpoint value using the Greater? function. If the temperature is greater than the Setpoint value, a TRUE value is sent to the digital output channel controlling the relay with the DAQmx Write.vi function.

Many buildings have both an air conditioner and a heater to help regulate the temperature of a room. While cooling and heating are typically done independently, you can use a PC-based data acquisition system to operate both simultaneously.

Figure 4 shows an example of a LabVIEW block diagram for controlling both cooling and heating. In this block diagram, the measured temperature value is compared to two different setpoints, each connected to either the air conditioner or the heater.

With additional logic, you can customize the control system even further and add features such as setpoint scheduling, temperature averaging, and dead band configuration. You can also add data-logging and report generation capabilities to the same PC-based control system.

Simple on/off control may not provide enough granularity to accurately control a particular system with the required speed and precision. Proportional integral derivative (PID) control is the most common control algorithm used in industry. The popularity of PID controllers can be attributed to their robust performance in a wide range of operating conditions as well as their simple, straightforward manner of operation. LabVIEW features a PID Control Toolkit that you can use to install prebuilt graphical functions to easily implement a PID control system. The basic PID.vi function is shown in Figure 5.

As the name suggests, the PID algorithm consists of three basic coefficients, proportional, integral, and derivative, which you can adjust to get optimal response. The basic idea behind a PID controller is to read a sensor and compute the desired actuator output by calculating proportional, integral, and derivative responses. The controller takes the sum of these three components to compute the output. Figure 6 shows an example block diagram with a PID control loop using one analog input channel and one analog output channel.

Beyond PID control, there are several advanced control systems based on the state-space model of a plant and several custom control parameters based on the desired system response. Designing such a control system requires knowledge and experience in control systems theory. You can use the LabVIEW Control Design and Simulation Module, which is included with LabVIEW Professional, for applications that require this level of sophistication. This software can help you test and iterate on a newly designed control algorithm, which can save you considerable time and resources when trying to prototype and validate an advanced control system.

NI-DAQmx is a powerful API for data acquisition and control applications, but some applications call for more determinism, lower latency, and faster control loops than DAQmx can provide. A real-time operating system (RTOS) provides the maximum level of software determinism and reliability for control systems by dedicating all resources to a deployed application. CompactRIO and PXI controllers can make use of this real-time OS for advanced determinism with the LabVIEW Real-Time Module, an add-on for LabVIEW. When installed, this software compiles LabVIEW graphical code and optimizes it for the selected real-time target. Using the LabVIEW Real-Time Module, you can develop and deploy applications to all NI real-time hardware targets including standard desktop PCs and PXI systems. With the NI-DAQmx driver software, you can easily migrate PCI, PCI Express, and PXI platform devices from LabVIEW for Windows to LabVIEW Real-Time on CompactRIO or PXI and retain the same function calls and hardware configuration.

For applications that require even higher-performance control loops with hardware-timed speed and reliability, consider using the LabVIEW FPGA Module with CompactRIO or multifunction reconfigurable devices. FPGAs are reconfigurable silicon chips that you can program with the same LabVIEW graphical development environment. CompactRIO and reconfigurable devices feature user-defined onboard processing as well as completely flexible I/O timing and triggering. You can configure device functionality by creating LabVIEW block diagrams with the LabVIEW FPGA Module. Your block diagram executes in hardware, giving you direct, immediate control of all I/O signals without needing to communicate with a processor. With CompactRIO and reconfigurable devices, you can create multiple PID control loops that execute at speeds up to the MHz range.

The combination of LabVIEW Real-Time and LabVIEW FPGA offers unsurpassed advantages in any high performance embedded control application. For more information on FPGA-based data acquisition devices, learn more about CompactRIO and R Series Multifunction RIO.

• Read about how CERN used LabVIEW and reconfigurable hardware to control world’s largest particle accelerator
• Multifunction DAQ Hardware Pricing and Specifications product page
• Try LabVIEW Today for Free download page

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