The HART Communications Foundation has published another WirelessHART white paper that describes various scenarios for low-latency PID loop control. The paper, Peer-to-Peer Communication with WirelessHART describes this transmission of information between network participants on a WirelessHART network.
This standard is designed to support applications like monitoring, diagnostics, alarm and event detection, and more complex data forms required for vibration monitoring, valve signature testing and process control.
A simple, cooling jacket temperature loop is the basis for five scenarios ranging from everything communicating wirelessly to everything wired in the field, but communicating wirelessly back to the WirelessHART gateway. The example loop includes a temperature transmitter, a control valve with a digital valve controller and a controller running in the WirelessHART gateway or process automation system.
The scenarios vary where the PID control resides and what devices in this control loop are hardwired.
For example, the first scenario has the temperature transmitter hardwired via a 4-20mA HART signal to the digital valve controller. Both these devices are wired to a WirelessHART transmitter that communicates back to the gateway. The PID control runs in the digital valve controller. Monitoring and setpoint changes are done via wireless communications from the process automation system.
Another scenario has the control running back in a process automation system controller or in the gateway with the transmitter and digital valve controller connected wirelessly to the gateway. The measurement value, valve target position and valve actual position communicate via the WirelessHART protocol.
There are power considerations if control is resident in one of the field devices. On way to address this is to run power as mentioned in the first scenario. The second way is to use the publish-subscriber communications built into the WirelessHART standard. From the whitepaper:
In WirelessHART, wireless devices are designed to support publisher-subscriber communications using multiple burst mode commands. Burst mode commands sent by the publisher (e.g. transmitter) are received by the gateway, cached, and then redistributed to the subscribing clients (e.g. valve) that are registered for notifications.
This burst mode communications happens on a periodic schedule and includes a timestamp, so that the other devices and applications subscribing to this information know its freshness. A WirelessHART Network Manager handles the security/encryption/validation of this communications along with the routing and scheduling.
The white paper concludes:
The WirelessHART protocol allows for secure, highly reliable, low latency control with almost no impact on the bandwidth and absolutely no impact on process performance. All of this is automatically built into the WirelessHART standard with little or no input from users. WirelessHART is simple, reliable, and secure.
Consider giving this white paper a read to see how you might apply WirelessHART to some of your low-latency control applications.
Update: I inadvertently picked up the wrong hyperlink for the whitepaper. Both links have now been updated. Sorry about that!