As mentioned my previous block, the communication capability of WirelessHART devices and a modification in the way the PID enable control using wireless devices. The standard communication techniques that are supported by a WirelessHART device are defined in HART 7 specification that has been adopted as an international standard, IEC 62591Ed. 1.0. The device may be configured to communication new measurement values using one of five defined burst message triggered modes. For control applications, the two communication techniques that best fit control applicatons are:
- Continuous – The device wakes up at a configured update period, senses the measurement and then communicates the value
- Window – The device wakes up at a configured update period, senses the measurement and then communicates the measurement if the specified trigger value is exceeded.
Window communication is the preferred method of communications for control applications since for the same update period Window communications will always require less power that Continuous communications. When window communication is selected, and then a new value will be communicated only if:
- the magnitude of the difference between the new measurement value and the last communicated measurement value is greater that a specified trigger value
- or if the time since the last communication exceeds a maximum update period.
Thus, the measurement is communicated only as often as required to allow control action to correct for unmeasured disturbances or response to setpoint changes. For Windowed mode you must specify an update period, a maximum update period, and a trigger value. The update period and the maximum update period are separate configured values. For best control performance process disturbances, the update period configured for Window communications should be ¼ the process response. If disturbance response is not critical then a slower update period may be used.
To provide best control, the PID must be restructured to correctly handle continuous measurement updates that are communicated much slower than 4X the process response time. Also, the PID must be able to work with the non-periodic measurement updates that are provided when Window trigger mode is configured. The PIDPlus capability included in DeltaV v11 is specifically designed to address control using wireless measurements. The implementation of the PIDPlus in DeltaV v11 is shown below.
The filter output used in the positive feedback network is calculated in the following manner when a new measurement is received.
Control execution is set much faster than measurement update. This permits immediate action on setpoint change and update in faceplate. PIDPlus tuning is based on the process dynamic (e.g. RESET= process time constant plus deadtime); PIDPlus reset automatically compensates for variations in the measurement update rate. No change in PID tuning is required for varying update rate. In Solutions Part 2 we will examine other PID enhancement for control using wireless devices.