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.