Chapter 5
Control Using Wireless
Transmitters
Measurement and Control Data Sampling Rate

To achieve the best
control response, the rule
of thumb is that feedback
control should be
executed four to 10 times
faster than the process
response time.

Most multi-loop controllers
used in the process
industry are designed to
oversample the
measurement by a factor
of 2 to 10 to minimize
delay being introduced by
IO access.
Impact of Update Rate on Battery Life

When a wireless
measurement transmitter
is used in a control
application, it is not
practical to provide the
same oversampling as a
multi-loop controller with a
wired transmitter because
it quickly depletes the
battery in the wireless
transmitter.

A wireless transmitter that
communicates a new
measurement value every
8, 16, or 32 seconds
typically has a battery life
in the range of 3–7 years.

There is also an
underlying assumption in
traditional PID control
that a new measurement
is available each time
control is executed and
that control is executed
at least four times faster
than the process
response time.

Depending on the
process response time it
may not be possible to
provide measurement
updates this frequently
and still achieve a 3–7
year battery life.
PID Control – Wireless Measurement Update Four
Times Faster Than the Process Response Time

The impact of wireless
measurement update
rate on control
performance can be
illustrated by considering
a control application

Lambda controller tuning
rules are applied to
traditional PID control for
a Lambda factor = 1.
–
Process Gain = 1
–
Process Deadtime
= 2 seconds
–
Process Time
Constant
= 6 seconds
Wireless Measurement Update Rate Two Times
Faster Than the Process Response Time
PID Control – Wireless Measurement Update
Rate Set Equal to the Process Response Time
Wireless Measurement Update Rate Two Times
Slower Than the Process Response Time

Wireless update time
exceeds the process
response time, the
control response to
setpoint changes and
disturbances becomes
oscillatory.

Only for applications
such as temperature
control and level control
that are characterized by
slow process dynamics is
it possible to use
wireless transmitter
update rates that are four
times faster than the
process response time
and still achieve 3–7 year
battery life.

Many of the control techniques and guidelines established during the
development of single loop digital controllers in the mid-‘70s are
based on providing a capability that mimics an electronic analog
controller.

With the introduction of battery powered wireless transmitters, such
update rates are impractical. Thus it is necessary to re-examine how
control should be structured for use with wireless measurements.
Implementations of PID Controller Reset

Manufacturers of DCS
have approached PID
implementation in a variety
of ways.

Many commercial products
create the reset component
using a positive feedback
network.

In a positive feedback
network the time constant
of the filter in the network
defines the reset time in
seconds per repeat.
Example – Process Response Exactly Matches
Reset Network Filter Response

When the PID reset is
implemented using a
positive-feedback network, it
is easy to see that the time
constant in the filter
contained in this network is
a direct reflection of the
process dynamic response.

Take, for example, a pure
lag process where the PI
controller is tuned for a
Lambda factor of 1. On a
change in setpoint, the PI
controller output changes
only once because the
dynamic response of the
filter exactly cancels the
dynamic response of the
process.
PIDPlus for Wireless Control

To provide the best
control when a
measurement is not
updated on a periodic
basis, the PID may be
restructured to reflect
the reset contribution
for the expected
process response since
the last measurement
update.

This PID
implementation is
known as PIDPlus.
PIDPlus with Continuously Updated Filter

To further enhance the
response for continuous
changes in setpoint, the
implementation of the
PIDPlus algorithm can be
modified as shown in this
figure.

PIDPlus tuning is based on
the process dynamics (for
example, RESET = process
time constant plus
deadtime).

PIDPlus reset automatically
compensates for variations
in the measurement update
rate and slow measurement
update rates.
PIDPlus Implementation

For those processes that
require derivative action,
the contribution to the PID
output should be
recomputed and updated
only when a new
measurement is received.

The derivative calculation
should use the elapsed
time since the last new
measurement to account
for the fact that a new
measurement value is not
available for each
execution of the PID.
Control for Wireless Measurement

When the PIDPlus
algorithm is used with
a wireless transmitter
in a control
application, the
performance will be
comparable to that
achieved using a
wired transmitter.

Example: PIDPlus
using wireless
transmitter compared
to a standard PI
controller where the
wired measurement
value.
Response for Measurement Loss
during a Setpoint Change

The reliability of
WirelessHART device
communication has
been well established.
Even so, in the event
of communication loss,
the expected control
behavior is of interest.

The example
compares loss of
communication with a
PIDPlus against a PID
with a wired transmitter
where the wired
measurement is frozen
for a period of time.
Response for Measurement Loss after a
Process Disturbance

The response
observed when the
measurement was lost
after a process
disturbance is shown

As illustrated by these
tests, the PIDPlus
provides superior
dynamic response
under these lost
measurement
conditions.

PID response is
significantly worse and
may not be acceptable
in many process
applications.
Example – Enabling PIDPlus in a Control Module

In addition, in many common
applications such as flow or
pressure control of a liquid or
gas stream, an update rate
that is four times faster than
the process response time
cannot be achieved if there is
a requirement for a 3–7 year
battery life.

In such cases the PIDPlus
should be used to implement
control using a slower update
rate such as 8 or 16 seconds.

When PIDPlus is available as
a standard feature of the
distributed control system, the
PIDPlus capability is selected
through an option parameter of
the PID.
Disabling Filtering in the Control Path

A timestamp accompanies
new measurement values
that are communicated by
a transmitter to the
WirelessHART gateway.

However, some distributed
control systems detect the
communication of a new
measurement when the
value changes. If the
PIDPlus uses this
mechanism to identify a
new communication it is
critical that filtering is not
applied in the module
processing
Setting Module Execution Rate Example

When control using a WirelessHART
measurement is implemented, the
module execution rate should be set
much faster that the communication
update period.

For example, the module may be set
to execute every 0.5 seconds even
though the communication update
rate is set to 8 seconds.

Scheduling the module execution in
this manner can minimize any delay
in a new measurement value being
used in control. This is necessary
since the module execution within the
DCS is not synchronized with the
measurement communication.
Single Use Bioreactor (SUB) with
Wireless Instrumentation

The benefits of using
WirelessHART transmitters with a
single use bioreactor have been
demonstrated by Broadley
James, a major manufacturer of
bioreactors for product
development and production.

A skid was instrumented with a
100L SUB (Single Use
Bioreactor) with WirelessHART
pH, temperature and pressure
transmitters

The bioreactor pH and
temperature were controlled over
a series of batch runs using
WirelessHART measurements.
Bioreactor Process

A wireless pressure
transmitter was used to
monitor pressure within
the bioreactor.

The pH measurement
was communicated on a 1
second window
communications.

The temperature was
reported on a 2 second
using continuous
(periodic)
communications.
Wireless Temperature Control
Response in SUB Unit

A mammalian cell culture
was used for each batch
run.

For the purpose of
comparison, wired pH
and temperature
measurements were also
available during each
batch run.

This screen capture
shows the setpoint
response of temperature
control based on the
WirelessHART input.
Wireless pH Control Response in SUB Unit

Similarly good
performance was
seen for pH
control using the
WirelessHART
input.

The response to
0.05 changes in
pH setpoint is
shown in this
screen capture.
Stripper Column at UT

During the development of the
PIDPlus, the performance was
also verified in several field trials
where the PIDPlus was used for
control with WirelessHART
transmitters.

The control of the Stripper
Column shown on the left portion
of the picture was addressed in
a field trial conducted at the J.J.
Pickle Research Campus,
University of Texas
Stripper Pressure and Steam Flow Control

Standard WirelessHART
pressure and flow transmitters
were installed to demonstrate
and test control using the
PIDPlus.

The control system was
configured to allow the operator
to switch between control using
WirelessHART and PIDPlus
and the wired transmitters and
PID.
The stripper column pressure
control is shown in Figure 5-26 for
two periods of operation:

1. PID control of steam flow
and column pressure using
wired measurement
transmitters.

2. PIDPlus control of steam
flow and column pressure
using WirelessHART
measurement transmitters.
The same dynamic control
response was observed, as
illustrated in these screen
captures. For these tests, the
same tuning was used for both
wired and wireless control.
Field Evaluation of Wireless Control

The control performance is shown
for column pressure and steam
flow control for PIDPlus control
using WirelessHART
measurement transmitters (Test
2) vs PID control using wired
measurement transmitters (Test
1).

Comparable control performance
was achieved using
WirelessHART measurements
and PIDPlus vs control with wired
measurements and PID.
However, the number of
measurement samples with a
WirelessHART transmitter vs a
wired transmitter was reduced by
a factor of 10 for flow control and
a factor of six for pressure control.
Exercise: Control Using Wireless Transmitters
This workshop provides several exercises that can be used to further explore the control
using a wireless measurement.

Step 1: Open the module that will be used in this workshop and observe the control and
simulated processes.

Step 2: Initialize the Performance Index (IAE) and then change the SP parameter of
both control loops by 10%. Observe the control response using a plot of the setpoint,
control measurements and output.

Step 3: Note the IAE and the number of communications for the wireless and wired
control. A significant difference should be seen in the number of communications for
wired vs wireless control that were required to respond to the setpoint change.

Step 4: Initialize the Performance Index and change the Disturbance input from zero to
10. Observe the response of the PID and PIDPlus to this unmeasured process
disturbance.

Step 5: Note the IAE and the number of communications for the wireless and wired
control. A significant difference should be observed in the number of communications
for wired vs wireless control that were required to respond to the unmeasured process
disturbance.
Process: Control Using Wireless Transmitters
A simulation of two identical heater processes is used to compare the control performance of
PIDPlus using a wireless transmitter and PID using a wired transmitter.