Guidelines for Setting Filtering and Module Execution Rate

Guidelines for Setting Filtering and Module Execution Rate
Terry Blevins Principal Technologist

Presenters Terry Blevins, Principal Technologist Kent Burr, Gary Law, Joe Nelson – DeltaV Product Engineering

Introduction Filtering and module execution period can directly impact control performance. In this workshop we will be addressing: Protection against 50-60hz pickup provided by analog input card and Charm analog input. Filtering of process measurements –configuration guideline to void aliasing and to minimize impact of process noise. Control execution – configuration guideline for setting execution period based on process dynamics, impact on control performance. Guidelines for setting filtering and execution period are presented and examples used to illustrate their impact.

Protection against 50-60 Hz pickup The DeltaV analog input card uses a two pole hardware (RC) filter to provide -3 dB at 2.7 Hz and > -40dB attenuation at 50-60 Hz. The CHARM analog input uses the A/D software ( FIR ) and configurable 2nd order software filter after the A/D. By default will provide -3 dB at 2.7 Hz and approx – 70 dB attenuation at 50-60Hz. 1st Order Configurable Software Filter A/D Converter Hardware Filter DeltaV Analog Input Card A/D Converter 3rd Order Sigma Delta Converter FIR Digital Filter 2nd Order Software Filter* CHARM Analog Input *DeltaV v11.3.1

A/D FIR Filter – 50-60 Hz Attenuation

Filtering of process measurements The impact of aliasing for noise containing frequencies higher than ½ the module execution frequency (Nyquist frequency) is illustrated in this examples. Filtering to prevent aliasing can not be added at the module level since at this point the data is already aliased. Field Input of 4.5 Hz (green), AI output (blue) of Module executing at 5 Hz (200 msec) - Scaled inTime

Example – Process Noise

Configuring Anti-aliasing Filter Rule 1: If a measurement is characterized by process noise then anti-aliasing filtering should be applied at the IO channel. Note: Help is providing in setting this filter based on module execution period.

Filtering Within a Module Rule 2: To remove process noise the filter time constant of an analog input in a module should be no more than 10% of the process response time. Example: For a process response time of 5 seconds the input filter time constant should be no more than 0.5 seconds.

Response Time – Self-regulating Process O2 – O1 Note: Output and Input in % of scale Most processes in industry may be approximated as first order plus deadtime processes. A first order plus deadtime process exhibits the combined characteristics of the lag and delay process. Gain = I2 – I1 The process dynamic of a self-regulating process may be approximated as first order plus deadtime and the response time assumed to be the process deadtime plus the process time constant. T2 – T1 Dead Time = T3 – T2 Time Constant = O2 63.2% (O2 - O1) O1 Value Output I2 I1 Input Time T1 T2 T3

Response Time – Integrating Process For integrating processes, the response time may be assumed to be the deadtime plus the time required for a significant response to a change in the process input. Integrating Gain = When a process output changes without bound when the process input is changed by a step, the process is know as a non-self- regulating process. The rate of change (slope) of the process output is proportional to the change in the process input and is known as the integrating gain. O2 – O1 Dead Time = T2 - T1 (I2 - I1 ) * (T3 – T2) Note: Output and Input in % of scale, Time is in seconds O2 O1 Output Value I2 I1 Input Time T1 T2 T3

Example: Impact of Filtering (Cont)

Example: Impact of Filtering Process Gain=1, TC=4 sec, DT=1 sec * Time to return within 2% of setpoint.

Control Execution Period To minimize delay introduced by IO processing, analog inputs are oversampled at a rate sufficient to support the fastest module execution rate. To reduce controller load, the module execution rates is adjustable. The default execution rate is 1/sec. Process Output 63% of Change O Time Constant ( ) Deadtime (TD ) Process Input I Control Execution New Measurement Available

Control Execution Rule 3: Control loop execution period should be ¼ the process response time or less to achieve best control performance. Rule 4: The module execution period should be 2X the Process Deadtime or less. Note: Executing control faster than the guideline provides little improvement in setpoint and load disturbance response. Quality of control will be degraded if execution is set significantly slower than the Guideline.

Example: Control Execution - Rule 3 Module Execution Impact - Process Gain=1, TC=3 sec, DT=1 sec

Example: Control Execution - Rule 3 (Cont)

Example: Control Execution - Rule 4 Module Execution Impact - Process Gain=1, TC=2 sec, DT=2 sec

Example: Control Execution - Rule 4 (Cont)

Examples – Applying Execution Rules Rule 4 applies Note: Maximum was limited to 60 sec. Faster update may be needed for operator visibility, calculations or alarming

Business Results Achieved Control variability caused by process noise and unmeasured load disturbances can be minimize through tuning and by following the guidelines for module execution period and input filtering. When plant throughput is limited by an operating constraint or variation from target operating conditions impacts operating efficiency or product quality, then a reduction in process variation provides direct economic benefit in plant operation. Maximum $ Lost Maximum $/HR Profit $ Lost $/HR Profit Time Time Maximum $ Lost Maximum $/HR Profit $ Lost $/HR Profit Time Time “Better” Control “Better” Control

Summary Easy to follow filtering and execution guidelines are proposed as a means of improving control performance and reducing process variability. These guidelines are based on the process response time to changes in setpoint and disturbance inputs. A reduction in process variation can provide direct economic benefit in plant operation when throughput is limited or variations impact operating efficiency or product quality.

Where To Get More Information DeltaV Product Data Sheet, DeltaV S-Series Traditional I/O DeltaV Product Data Sheet, S-series Electronic Marshalling W.L. Bialkowski and Alan D. Weldon, The digital future of process control; possibilities, limitations, and ramifications. Vol No. 10, Tappi Journal, October, 1994. Jeffrey Li, A PID Tuning Method Using MINLP with Nonparametric Process and Disturbance Models, AIChE 2010 Spring National Meeting, San Antonoio, TX.

Guidelines for Setting Filtering and Module Execution Rate

Guidelines for Setting Filtering and Module Execution Rate

Presentation Transcript

Guidelines for Setting Filtering and Module Execution Rate