Peer-to-Peer Communication with WirelessHART

Peer-to-Peer Communication with WirelessHART
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Peer-to-Peer Communication
with WirelessHART
HCF_LIT-129, Revision 1.0
Release Date: September 5, 2008
Abstract:
The WirelessHART standard is the first open wireless
communication standard for measurement and control in
the process industries. It uses wireless mesh networking
between field devices, as well as other innovations, to
provide secure, reliable digital communications that can
meet the stringent requirements of industrial applications.
This is one of a series of papers helping users recognize
the benefits of WirelessHART, as well as addressing
specific questions about WirelessHART.
HART Communication Foundation Document Number: HCF_LIT-129
Document Title: Peer-to-Peer Communication with WirelessHART
Date of Publication: September 5, 2008
Document Distribution / Maintenance Control / Document Approval
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approval please contact the HART Communication Foundation (HCF) at the address shown below.
Copyright © 2008 HART Communication Foundation
This document contains copyrighted material and may not be reproduced in any fashion without the
written permission of the HART Communication Foundation.
Trademark Information
HART® is a registered trademark of the HART Communication Foundation, Austin, Texas, USA.
Any use of the term HART hereafter in this document, or in any document referenced by this
document, implies the registered trademark. WirelessHART™ is a trademark of the HART
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trademarks of their respective companies. For more information contact the HCF Staff at the
address below.
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HART Communication Foundation Document Number: HCF_LIT-129
Document Title: Peer-to-Peer Communication with WirelessHART
Peer-to-Peer Communication with WirelessHART
WirelessHART has been designed from the ground up to support
a wide range of applications.
These applications include
monitoring, diagnostics, alarm and event detection, block transfer
to support applications such a vibration monitoring and valve
signature, and control. To support these applications several
forms of communications are required. Although these forms of
communications are largely transparent to the user, it is useful to
understand that they exist.
This paper describes these
communication
patterns
as
forms
of
peer-to-peer
communications.
Peer-to-peer communication is defined as the transmission of
information between participants in a network. Participants can
be physical devices such as a transmitter and a valve, or they
can be applications such as monitoring, control, diagnostics, and
event detection. They can also be more complex applications
performing calibration, a vibration analysis or a valve signature.
These applications can reside in workstations, controllers,
gateways, measuring devices, and final control elements such as
valves.
The WirelessHART standard is
the
first
open
wireless
communication
standard
for
measurement and control in the
process industries. It uses
wireless
mesh
networking
between field devices, as well as
other innovations, to provide
secure,
reliable
digital
communications that can meet
the stringent requirements of
industrial applications.
This is one of a series of papers
helping users recognize the
benefits of WirelessHART, as well
as addressing specific questions
about WirelessHART.
This paper describes the various forms of peer-to-peer communication. A typical control application
is used to illustrate these forms of communication.
Communication patterns
When a control algorithm runs in a dedicated controller such as a Distributed Control System or a
PLC, the communication is between the controller and field devices. In other cases control may be
distributed between controllers and a gateway-type device in a wireless network, or between
controllers, gateways, and the field devices themselves. Running control in the field devices – often
called "control in the field" – can be implemented in multiple ways. One way is to wire the 4-20mA
signal from the measurement device directly to the controlling device, such as a valve. Another is
to distribute and run software function blocks in the devices, and communicate process variables
and control signals in digital form over the communications network.
In all of these scenarios parameter values are being communicated. These communications are
built on top of a communication infrastructure. The role of the communication infrastructure is to
transport process variables, parameters, alarms and other values wherever they are needed. The
scenarios that follow illustrate how WirelessHART supports these many forms of communication.
Base scenario
We'll use the following control loop as the starting point for these scenarios.
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HART Communication Foundation Document Number: HCF_LIT-129
Document Title: Peer-to-Peer Communication with WirelessHART
In this example loop, a raw temperature measurement from a reactor cooling jacket is converted to
engineering units, filtered, and checked for violation of alarm limits using an analog input (AI)
function block.∗
Next, a PID function block is used to maintain the temperature about the operating target -- in this
case, the reactor temperature setpoint.
An analog output (AO) function block then drives the final control element. In this example, it
adjusts a valve to manipulate the flow rate through the cooling jacket.
Each of the connecting lines in the drawing represents the transfer of a parameter value and a
status indicating the quality of the value. If the source and destination of the line are physically
separated – for example, because they are in different devices -- then the parameter value and
status must be communicated by either wired or wireless connections.
Scenario 1: Control in valve, signals communicated through current loop
Controller
WirelessHART
Gateway
Communicate
1- Adapter
2- Transmitter
3- Valve
WirelessHART
Adapter
External
Power
In the first scenario, the measurement and control functions are
contained within the transmitter and valve, respectively. The
transmitter has a physically wired 4-20mA connection to the
valve, and the processed measurement value and status are
communicated directly to the valve over this connection. Since
the control is in the valve, the actual and target position are
communicated directly between the control function and the
valve.
In this scenario communication between the control system and
the devices is routed through the adapter.
Wireless
communications are used only to monitor the control loop,
adjust the setpoint in the valve, and monitor and adjust other
parameter values.
∗ Despite their name, Analog Input (AI) and Analog Output (AO) function blocks work with the digital information used in
WirelessHART communications.
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HART Communication Foundation Document Number: HCF_LIT-129
Document Title: Peer-to-Peer Communication with WirelessHART
Scenario 2: Control in gateway or controller, connection to valve wired
When using self-powered wireless devices, it's important to minimize
power usage in order to extend the time a device can run without
needing battery replacement.
One approach is to run the
measurement and control functions in centralized, line-powered
controllers. Putting those functions in the gateway can also provide
faster response times that are required in some applications.
In this case the measurement value is communicated wireless. The
valve target and actual valve position are communicated through the
wired connection between the valve and the gateway.
Scenario 3: Control in gateway or controller
When using self-powered wireless devices, it's important to minimize
power usage in order to extend the time a device can run without
needing battery replacement. The approach shown here is to run the
measurement and control functions in centralized, line-powered
controllers. Putting those functions in the gateway can also provide
faster response times that are required in some applications.
In either case, the measurement value, the valve target, and the actual
valve position are communicated wirelessly between the devices and
the gateway.
Scenario 4: Control in valve, publishing through gateway
In this case it is desirable to keep the control in the field devices and
create peer-to-peer communications between them. The way to do
this is to publish parameter values from devices to the gateway, then
republish those same values from the gateway to devices that are
registered as subscribers to those parameters.
In the example shown below, the transmitter publishes a measurement
value and status to the gateway, which republishes the data to the
valve – which then uses it in the control algorithm.
In WirelessHART, wireless devices are designed to support publishersubscriber 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.
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HART Communication Foundation Document Number: HCF_LIT-129
Document Title: Peer-to-Peer Communication with WirelessHART
One of those clients could be another WirelessHART device.
Devices receiving messages from the gateway must be preconfigured to subscribe to the
measurement value and status. The subscribing field device is scheduled to periodically listen for
published information and to utilize the information communicated in this manner.
When burst mode reporting is used, the data is sent on a predetermined, periodic schedule.
Parameters can be sent as scheduled, or only if the value has changed by a significant amount or
has not been updated within a default reporting time (report by exception).
All burst mode messages are communicated with a timestamp. The subscribing device can be
designed to utilize the timestamp, such as to determine when a new value has been received.
The way the WirelessHART gateway redistributes messages has many technical advantages over
other approaches. The communicated values can, in addition to being re-published back out to
other devices, published to multiple client applications. Also, the gateway-to-device session keys,
routing information, and network scheduling is managed automatically by the WirelessHART
Network Manager.
Scenario 5: Control in valve, publishing between devices
Controller
WirelessHART
Gateway
To address some special applications, it may be necessary for field
devices to form peer-to-peer sessions among themselves. In this
case a device must publish directly to another device rather than
through the gateway.
Such communications may be established by publishing using a
burst mode operation in one device and subscribing using a catch
mode operation in the other device. In this case the Network
Manager can automatically allocate routes and communication
resources to provide the additional session between each pair of
peer-to-peer devices.
The WirelessHART specifications define the network management
services required to form these types of communications.
Manufacturers may use these services to allow automatic
establishment of publisher-subscriber communications between
devices.
Let's look more closely at what has to happen to set up this peer-to-peer communication. A good
engineering or configuration system will do most of the work automatically, without the user having
to worry about each of the steps outlined below.
As a starting point, consider the basic steps to set up a control strategy involving a wired field bus.
The engineering tools must perform the following tasks:
•
•
•
•
•
Configure parameters in the device
Schedule function block execution with the device relative to the macrocycle
Configure Virtual Communication Relationships in source and destination devices that have an
inter-device function block link
Configure Link Objects in source and destination devices that have an inter-device function
block link
Update Link Active Scheduler schedules in the primary Link Master device
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Document Title: Peer-to-Peer Communication with WirelessHART
•
Update Link Active Scheduler schedules in the secondary Link Master device.
WirelessHART uses a somewhat different approach. The configuration system sorts out the control
strategy, what needs to be communicated (i.e. the connections on the drawing), the communication
rates (i.e. module execution rate), and the WirelessHART Network Manager takes care of the rest.
All communications in WirelessHART are reliable and secure. The WirelessHART network also
allocates redundant paths for communications and automatically manages sessions for
communication between devices – all communications within WirelessHART are encrypted and
validated.
Conclusion
The bottom line on any technology is how well it meets users' needs. Users generally think in terms
of applications such as control rather than capabilities such as peer-to-peer communications. With
WirelessHART, however, those capabilities – including direct device-to-device communication -- are
readily available to support the applications users care about. 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.
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