13 Working with PID/PID2 blocks In this chapter

Transcription

1 13 Working with PID/PID2 blocks In this chapter PID and PID2 blocks: an Overview 316 PID tuner/face plate display 318 PID Summary display 320 PID View Networks display 321

2 ProWORX NxT User s Guide PID and PID2 blocks: an Overview PID and PID2 blocks are software programming blocks that allow a process to be controlled with no changes or additions to hardware. PID stands for Proportional Integral Derivative. While the PID2 is a more advanced version of the PID, both operate in the same manner. Unless indicated, operations given in this chapter that can be performed on a PID can also be performed on a PID2. For more information on the differences between PID and PID2 blocks, see the ProWORX NxT online help. Process Input Set Point (SP) PID Calculation Output Value (OV) Process Variable (PV) Process Measurement Process Output The PID calculation compares a process variable (PV) with a desired control point called the set point (SP). The calculation uses the difference between the set point and the process variable to adjust the PID output value (OV), sometimes called the control variable. This output value is used to manipulate an input to the process so that, eventually, the measured process variable equals the desired set point. 316 PID and PID2 blocks: an Overview

3 Chapter 13 Working with PID/PID2 blocks PID blocks are highly complex pieces of logic. While they can be very useful, they can also be quite difficult to work with. ProWORX NxT includes a PID display with the following functions that make online programming of PID/ PID2 blocks easier: The PID Tuner/Face Plate display. The PID Summary display. The PID View Networks display. Quick access to the Data Watch Trend Window. Quick access to the Documentation and Register editors. Not all controllers support PID blocks, and even fewer support the more advanced PID2. If your controller doesn t support PIDs, you won t be able to add them into your logic database or access the PID display. To view the PID monitoring displays: From the online Network Editor, on the Data menu, click PID Summary. You can switch between the Summary, Tuner/Face Plate and View Networks displays by clicking the appropriate buttons along the top row. To return to the Network Editor, click Close. PID display shortcuts To open the Register Editor for a PID block, select the PID and click Tune. To open the Documentation Editor for a PID s process variable, select the PID and click Doc. PID and PID2 blocks: an Overview 317

4 ProWORX NxT User s Guide PID tuner/face plate display Sometimes it can be difficult to predict exactly how a PID s parameters will affect its behavior. Tuning a PID refers to the process of adjusting a PID s parameters until it operates the way you want. Often, the easiest way to determine the best PID settings is by observing changes in the PID s operation as they happen. The PID Tuner display provides PID faceplates for up to three PID/PID2 blocks at a time. This provides fast access to features necessary to monitor, control and tune each block. Each PID face plate display looks like this: The bar graphs indicate where the Process Variable and Set Point are between the low engineering range and the high engineering range. The higher the percentage, the closer it is to the high engineering range. The Process Variable is likely to change over time. You can change the Set Point using the Adjust button. From the PID face plate you can: This area lists the register address (left) and current value (right) of the PID s Process Variable, Set Point, and Output Variable. The PID s descriptor, as entered in the Documentation Editor, see Chapter 8 Documenting your PLC on page 223. Click Manual to toggle the PID s operating mode between Auto and Manual. Click Adjust to modify the PID s Set Point (in Auto mode) or Output Value (in Manual mode). Click Trend to open up a Trend window for this PID. Select Force Input Contact to force on the PID s input contact. Switching to Manual mode temporarily halts the self-correcting operation of the PID block. This lets you manually change the register settings to get a better idea of how your PID works. Switching back to Auto mode returns the PID to regular programming. To access the PID Tuner/Face Plate display: 1. If you aren t looking at the PID Summary or PID View Networks displays, on the Data menu, click PID Summary. The PID monitor dialog box appears. 318 PID tuner/face plate display

5 Chapter 13 Working with PID/PID2 blocks 2. The PID Tuner is one of three displays that can be shown in this window. If the PID Tuner display isn t the one that appears, click the Face Plate button. The PID Tuner display appears. 3. If your logic database has more than three PID/PID2 blocks, use the vertical scroll bar on the right to move to the one you want. Adjusting a PID s set point The Adjust Set Point dialog box (which appears when you click the Adjust button in the PID Tuner/Face Plate display) functions differently depending on whether the PID is in Auto and Manual mode. In Auto mode, it changes a PID s set point; in Manual mode, its output value. Mode Adjustment value Value restrictions Auto Set Point (SP) Between the High and Low Engineering Ranges Manual Output Value (OV) PID blocks: , PID2 blocks: between the high and low clamp values, normally set to 0 and 4095 respectively. Before adjusting the Set Point, check the High Engineering Range and the Low Engineering Range. The Set Point bar graph appearing in the PID s Face Plate is based on where the set point value falls within the Engineering Range. For example, if the Low Engineering Range is 100 and the High Engineering Range is 300 and you change the set point to 200, the Set Point bar graph would read 50%. To close the Adjust Set Point dialog box and confirm your changes, click Apply. To close the dialog box without making any changes, click Close. Tracking PID values in the Data Watch Window Clicking a PID Face Plate s Trend button from within the PID Tuner display brings up a Data Watch trend window for that PID. The trend window tracks the PID s register values including its process variable, set point, and output variable over time, plotting them on a graph. You can have Trend windows for multiple PIDs open at the same time. For more information on Trend windows, see page 308. Adjusting a PID s set point 319

6 ProWORX NxT User s Guide PID Summary display The PID Summary window displays a table with the following information on each PID and PID2 block in the logic database: The location of the PID in the network; The register address for the PID s process variable, set point and output variable; and A description of the PID, as entered from the documentation editor (see page 230). To view the PID Summary display: 1. If you aren t looking at the PID Tuner or PID View Networks displays, on the Data menu, click PID Summary. The PID monitor dialog box appears. 2. The PID Summary is one of three displays that can be shown in this window. If the PID Summary display isn t the one that appears, click Summary. The PID Summary display appears. 320 PID Summary display

7 PID View Networks display Chapter 13 Working with PID/PID2 blocks The PID View Networks display provides a quick look at where PID/PID2 blocks are located in your logic networks. You can t edit any logic from here, but it does let you jump from one PID to another very quickly. To view the PID View Networks display: 1. If you aren t looking at the PID Tuner or PID Summary displays, on the Data menu, click PID Summary. 2. The PID View Networks is one of three displays that can be shown in this window. If the PID View Networks display isn't the one that appears, click View Net. The PID View Networks display appears. To move the network view to a different PID: Click Previous or Next to cycle through the PID/PID2 instruction blocks located in your database. PID View Networks display 321

8