Profi Lab-Expert: Application for PC Data Acquisition Cards （2）

The data acquisition hardware can either be connected to one of the PC's ports or can take the form of an inter­nal expansion card. Before we can start on a project it is of course necessary to install the acquisition hardware (whether it is a PCI card or a USB device) along with its software, as well as ProfiLab-Expert.

In this example we are going to measure the temperature of a process and display how it changes over time. We want to be able to calibrate the measurements in 1 % steps to ±1 °C. We also want a direct indication of when the temperature goes above or below a threshold of 10 =C.

We will use a PtlOO platinum resistance thermometer with a signal conditioner connected to the analogue input of the data acquisition card (Figure 5).

Start ProfiLab-Expert and create a new project. Select the acquisition card from the component library and drag it to the work area. In the 'Properties' dialogue (reached via the context menu) enter the port number and A/D range.

Next the circuit must be entered into ProfiLab-Expert as shown in the example circuit diagram (Figure 6). The input signal digitized by the data acquisition card is first taken to an averaging unit (MW1) to smooth the value. Then it is scaled by a fixed value (FW1) by a multiplier (MUL1). The scaled signal is now taken to an adder to allow for calibration: using potentiometer PT1 a positive or a negative offset can be added to the scaled value to correct the measured value. At the same time the offset value is displayed on the application's front panel using ND2. The parameters for the offset adjustment and for the offset value display can be configured using the 'Properties' dialogues. The signal at the output of the adder (which is the measured temperature plus or minus the offset) is displayed using digital display ND1; its changes over time are recorded using the Y(t) plotter. The measured signal is also taken to a comparator. This switches two lights according to whether the temperature is above or below preset value FW2. FW2 thus repre­sents the desired temperature threshold.

The circuit can be tested in simulation. The settings of var­ious parameters (such as offset voltages and fixed values) can be altered while the simulation is running.

If the application is to run in stand-alone mode on another computer the project must be compiled. Adjust­ment of port settings may be needed when running on another machine.

Conclusion