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Chapter 10

Chapter 10. Applying Wireless in Legacy Systems. PID Support for External Reset.

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Chapter 10

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  1. Chapter 10 Applying Wireless in Legacy Systems

  2. PID Support for External Reset When the PID supports external reset capability, a parameter is provided to select whether the PID output or an input to the PID is used as the input to the positive feedback network to calculate the reset component of the PID.

  3. Use of External Reset in Override Control Support for external reset becomes necessary for implementing override control. In this case external reset is enabled and the output of the control selector is the external reset input to the PID block. used in the override control.

  4. Construction of PIDPlus Capability In a legacy control system it is possible to construct PIDPlus capability if the PID supports external reset. This is done by setting the Reset of the PID to a value that eliminates the impact of the filter in the positive feedback network and then constructing the external reset input to duplicate that used in the PIDPlus

  5. Example – External Reset in Foundation Fieldbus PID PID function blocks based on the Fieldbus Foundation specification may use the BKCAL_IN parameter for the external reset input. For example, in one manufacturer’s implementation, the use of BKCAL_IN as the external reset is enabled by selecting Dynamic Reset in a PID parameter, FRSIPID_OPTS. When external reset is enabled the BKCAL_IN value is used in the positive feedback network rather that the PID output

  6. Composite for Calculation Outside the PID As a starting point in constructing the required calculation outside of the PID, a composite block EX_RESET was created that contains parameters to support connection with the PID block and a calculation block

  7. Composite Created to Detect New Measurement A composite block may be created to set a flag when a new measurement is detected based on a change in value. An example of the composite NEW_VAL that was created to detect a new measurement

  8. Enabling External Reset in the PID When control uses a wireless measurement then the EX_RESET and NEW_VAL composites can be added to the control module and external reset can be enabled in the PID as illustrated.

  9. Exercise: Adding PIDPlus to Legacy Control Systems This workshop provides several exercises that are used to explore how PIDPlus functionality may be added to a control module in a legacy control system and then tested in a simulation environment. In this workshop it is assumed that the PID supports external reset. • Step 1: Open a module that contains a standard PID function block and examine the parameters of the PID block that are used to enable external reset capability and to connect the external reset input. Enable external reset in the PID. • Step 2: Identify the two composites that were added to the module that enable the PID to provide functionality that is equivalent to PIDPlus (for PI control). Examine the construction of these composite blocks. • Step 3: Examine the parameters and composite block that are used to simulate a first -order- plus- deadtime process and wireless communication.. • Step 4: Enable wireless control in the composite, switch the PID to Auto and on a trend observe the response to changes in setpoint and process disturbances. Disable wireless control and on a trend observe the response for a change in setpoint and process disturbance .

  10. Process: Adding PIDPlus to Legacy Control Systems This workshop is designed to illustrate how PIDPlus capability may be added to a legacy control system. The resulting module is shown below.

  11. Gateway Connection with the Controller The gateway supports both Modbus remote terminal unit RTU over the RS485 serial port and Modbus Transmission Control Protocol TCP over Ethernet. It functions as a sub-device on the Modbus network and must be polled by a Modbus master or client (host system).

  12. Gateway Terminal Block Diagram The gateway is wired to an RS485 bus which typically leads to a serial I/O card or a Modbus I/O card. The interface allows a maximum of 31 gateways to be connected to a single I/O card in this manner. The Modbus connectors in the gateway terminal block are shown below

  13. .Modbus Communications Page It is important that the Modbus communication settings in the gateway match the settings in the controller I/O card. The Modbus communication settings can be found by navigating to Setup>Modbus>Communications in the gateway’s webpage as shown

  14. Modbus Register Mapping Interface After configuring communications, the next step is to configure register mappings. Register mapping is the process of assigning parameters from wireless field devices to Modbus registers. These registers can then be read by a Modbus master or client. The Modbus register mapping interface can be found by navigating to Setup>Modbus>Mapping in the gateway’s webpage as shown

  15. Typical System Architecture Using Security Setup Utility To connect the gateway with a workstation, a Security Setup Utility must be installed in the workstation that is linked to the gateway. The Security Setup Utility enables secure communications between the gateway and the host system, asset management software, data historians or other applications. The Security Setup Utility can support multiple gateways at once and each proxy can support multiple client application connections. A typical system architecture using the Security Setup Utility is shown.

  16. OPC Browse Tree A list of device tags may be accessed using the OPC Browse Tree. This speeds up the work in the site integration phase. The tag count is the basis for estimating the cost of integrating the asset health information and alarms into the DCS.

  17. OPC Mirror Connecting Two OPC Servers OPC Mirror connects OPC servers on multiple control systems and enables bi-directional data traffic from one system to another . OPC defines two kinds of applications: servers and clients. Servers provide data services to control systems, industrial buses or proprietary devices. The OPC proxy for the gateway is also a server. Clients access servers to make use of their data.

  18. Map Parameters in OPC Mirror OPC Mirror's standard Windows interface makes it easy to learn and requires no custom code to transfer data between all standard OPC systems. The user interface (UI) mapping parameters from both servers is shown.

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