Bumpless Transfer

What’s all the fuss about Bumpless Transfer?

Methods for implementing Bumpless Transfer are widely used in process automation systems.  However the concepts and the rationale behind the methods are many times a mystery to process operators and automation engineers alike.  Let’s explore this subject in greater detail to unveil the underlying issues and gain better system awareness and insights.

What is Bumpless Transfer?

What is Bumpless Transfer (BT) and why do controllers need it?  Our definition is as follows:  Bumpless Transfer is the method by which a controller can be transitioned from Manual mode to Automatic mode  without disrupting the process.

Why do plant operators place control loops in Manual mode in the first place?  There are many reasons why plant operators place control loops in Manual mode: following procedures for plant start-up, troubleshooting valve problems and improper tuning of controllers are the most common.  When the plant operator decides to transfer a control loop from Manual mode to Automatic mode they would like this transfer to occur without disturbing the process.  Without bumpless transfer this transition could create major process upsets due to a large “bump”, or movement, of the valve.  This bump could also be responsible for setting off multiple alarms and in general erodes the confidence that the operator has in the control system.  The reason for these bumps when transferring from Manual to Automatic mode is illustrated in the following sections.

What is the source of the bumps?

As most folks are aware a PID controller takes the difference between the setpoint (SP) and the process variable (PV), referred to as the Error, and calculates a valve position change to drive the PV toward the SP.  The amount and the speed at which the controller moves the control device (valve, damper) is dependent upon the tuning parameters of the controller.  When a control loop is placed in Manual the PID controller is no longer determining the position of the control device.  That decision is now being made by the plant operator.  If the PID controller is not informed that the operator has taken direct control over the device the PID controller will still think that it is in control of the device.  While the loop is in Manual there is only a very small chance that the SP and the PV will have the same value.  If there is an Error which exists during the time that the loop is in Manual, and if the PID controller is not informed that the loop is in Manual, the integral component of the PID controller will continue calculate control moves to try and drive the PV toward the SP.  This integral action will wind up to either the completely open or closed position of the control device.  If this is allowed to occur there will be a major disturbance when the transfer from Manual back to Automatic mode is made.

Let’s look at an example of this:

At 10:00 AM the operator places a temperature control loop in Manual.  The SP = 280°F at the time of this transition.  The operator decides to “manually” decrease the control valve position from 40% to 30%.  The PV, after several minutes, settles out at 220°F.  At 11:00 AM the operator decides to switch the loop back in Automatic mode.  At that time the SP = 280°F and the PV = 220°F.  As soon as the operator “flips the switch” the control valve goes to 100% open (from 30%).  Alarms start going off, levels and pressures start swinging and, in general, everyone associated with operating the plant gets very upset.

Manual to Auto Transfer with Bump

Manual to auto transfer with bump due to CO wind-up.

The remedy here is quite straightforward.  The control logic should be designed to 1) Inform the PID controller when the loop has been placed in Manual mode, and 2) Have the PID controller track the value of the control valve position that is being established by the plant operator.  By having the PID controller track the actual valve position we will prevent it from winding up to either the completely Closed or Open position.  So we’re done right?  Well not quite.

Proportional Bump at the time of transfer from Manual to Auto

Assuming that we did our job and properly designed a PID tracking system that prevented the PID controller from winding up when the control loop was in Manual.  So let’s take a look at what could happen in our previous example:

At 10:00 a.m. the operator places a temperature control loop in Manual.  As before, the SP = 280°F at the time of this transition.  The operator decides to manually decrease the control valve position from 40% to 30%.  The PV, after several minutes, settles out at 220°F.  At 11:00 a.m. the operator decides to switch the loop back in Automatic mode.  At that time the SP = 280°F and the PV = 220°F.  As soon as the operator flips the switch the control valve goes to 50% open (from 30%).  Alarms may still trigger and disturbances to the process may still occur.  Maybe not quite as bad as without tracking but still very disruptive.  What happened?

Manual to auto transfer with bump due to P action

Manual to auto transfer with bump due to P-action.

Note in the above example that at the time that the switch from Manual to Automatic occurred there was an Error present (SP-PV;  280°F -220°F = 60 °F).  So even though we had the PID Controller tracking the position of the Control Valve the first time it calculated a control valve position the Proportional Term of the PID controller would bump due to the presence of the Error.  The positive aspect is that the bump was less than that which occurred without PID Tracking; but there was a substantial bump nonetheless.  So now what should we do?  Well there are two options:

  1. Design the SP to track the PV when the control loop is placed in Manual
    Most modern control systems today will give you the option to have the SP track the PV when the control loop is placed in Manual.Pros:  Since the SP tracks the PV when in Manual the value of the Error at the time the operator “flips the switch” will be zero.  As such there will be no “proportional bump” at the time of the switch.Cons:  Operators will have to “remember” what the normal SP should be for the control loop when it is returned to Automatic mode.  By having the SP track the PV the operator no longer has the SP pegged at its normal value. There could still be a bump due to P action if large change in SP is made after transferring to Auto mode.

    Bumpless Transfer by SP tracking PV

    Option 1 – Bumpless Transfer by SP tracking PV

  1. Suppress the Proportional Action at the instant that the switch from Manual to Auto is made
    Some modern control systems provide the option of “suppressing” the Proportional action at the time the operator “flips” the switch from Manual to Automatic.Pros:  The SP remains at its nominal value during the period of time that the control loop is in Manual thus preventing the operator from having to remember where the SP should be during normal operations.  Eliminates the Proportional Bump.Cons:  If there is an Error present at the time the operator “flips the switch” it may take a considerable amount of time to “drive the PV to the SP” since the proportional action of the PID controller has been suppressed.

    Bumpless transfer by suppressed P-action

    Bumpless transfer by suppressed P-action

Optimal Solution

The best method of providing bumpless transfer is dependent on a variety of factors the most important of which is the process itself. The optimal solution is “to have your PID controllers tuned so well that the operators never feel the need to place the control loops in Manual due to improper tuning”. And what is the best way to ensure this: tune your loops with ControlSoft’s INTUNE tuning tools software.

Please contact us today to discuss how we can help you keep your loops in Automatic nearly 100% of the time thereby ensuring safe, reliable and consistent operation of your processes.

INTUNE and INTUNE+ v6.5.0.1 Available

We are pleased to announce that INTUNE and INTUNE+ v6.5.0.1 are now available. Highlights include:

  • Interface to PI System.
  • Performance Improvements.
  • Additional usability enhancements and bug fixes.

Connecting to a PI System

You can now use INTUNE+ CLPM Software to collect historical and real-time data from your OSIsoft® PI System®. Using the Auto Tune tool, you can interface to your PI System, query data and import queried data into INTUNE software to help optimize your plant performance.

INTUNE+ CLPM Software can analyze this data and make adjustments to process variables that will help your process run in a stable, predictable, and consistent manner, at or close to setpoint.

Performance Improvements

  • New Project Data Format – For improved performance when loading, executing, and saving projects, the INTUNE project format has been modified to a new data format. Projects saved in the new format can be reverted back to the original format.
  • Export/Import Loop Functionality – The Export/Import loop functionality has been updated for better performance.
  • Updates to Find Tags Dialog
  • Updates to Property Configuration Dialog
  • Report Metadata – Report Type properties containing Company information are now editable during runtime.
  • System I/O Reports – System IO reports have been discontinued.

For detailed information, please see the INTUNE Software v6.5.0.1 Release Notes, publication MN109A-EN, included with the software download.

INTUNE+ PI System OSIsoft

ControlSoft Joins OSIsoft Partner EcoSphere

ControlSoft is pleased to announce that we have been accepted into the OSIsoft Partner EcoSphere Program as an Application Partner.

The benefits of this partnership will be realized by customers who choose to interface the ControlSoft INTUNE®+ Control Loop Performance Monitoring (CLPM) with an existing OSIsoft PI System®. ControlSoft is developing software that allows INTUNE+ to connect like a client application at either the PI Archive or PI Asset Framework level. This enhanced method of data exchange between the INTUNE+ and the PI System improves usability and security beyond that available today.

OSIsoft’s PI System captures data from sensors, manufacturing equipment and other devices and turns it into rich, real-time insights for saving energy, improving productivity or developing new products. Over 1,000 leading utilities, 90 percent of the largest oil and gas companies and more than 65 percent of the Fortune 500 industrial companies rely on the PI System to get the most out of their businesses. Worldwide, the PI System manages over 1.6 billion data streams.

“Our customers are demanding that the exchange of information between applications related to their process control system be done in a reliable, fault-tolerant and secure environment” said Joe Patella, General Manager, ControlSoft. Inc. “We believe that the infrastructure provided by the OSIsoft PI System will help ControlSoft meets these demands. For over 30 years our customers have placed their trust in us to deliver process control solutions which optimize their plant’s performance while stabilizing and improving the safety of their operations. We will diligently work to earn that trust and believe that joining the OSIsoft Partner Ecosphere Program is evidence of our continual commitment to Customer Satisfaction.”

OSIsoft, the OSIsoft logotype, and PI System are registered trademarks of OSIsoft, LLC. INTUNE is a registered trademark of ControlSoft Inc.

ControlSoft - Inc. 5000 List 2017

Continued Growth Earns ControlSoft a Spot on the Inc. 5000 2017 List

For the second year in a row, ControlSoft has been named to Inc. 5000’s List of America’s Fastest-Growing Private Companies.

While making it on the list one time is an extraordinary achievement, making it on the list a second time is something only 1/3 of honorees have been able to do. This list represents a unique look at the most successful companies within the American economy’s most dynamic segment — its independent small businesses – and is the most competitive crop in Inc.’s list history.

Variable Frequency Drives

Save Money with Good VFD Process Control

Realize Energy and Cost Savings

The use of variable frequency drives (VFDs) in flow control applications is gaining in popularity and eroding the hegemony of traditional control valves. Energy efficiency is the main reason for this change producing multiple economic payouts, although there may be benefits to process dynamics as well. VFDs are used to control the speed of motors which are frequently used to drive pumps, fans, and compressors. Typically, a VFD operates by receiving a speed setpoint, usually from a DCS (Distributed Control System) or PLC (programmable logic controller), and compares the setpoint to the actual motor speed. When there is a discrepancy, the VFD adjusts the electrical power to the motor in order to get the actual speed to match the setpoint. As with any control application, proper tuning of a VFD is critical.

Achieve the Best Results from Your VFD Installation

In considering a VFD in place of a control valve, a VFD will operate with faster dynamics while avoiding common valve problems such as hysteresis and stiction. When talking about VFD tuning, it may be helpful to think of cascade control. Process control usually controls variables such as flow rate, temperature, pressure, level, and product composition. Motor speed (which is controlled by the VFD) can be thought of as the inner loop in a cascade control scheme with the outer loop controlling one of the aforementioned variables. Control of the outer loop is most commonly handled by a standard PID controller. Figure 1 illustrates a common VFD controller implementation. The relationship between the process’s controlled variable and the motor speed should be determined by plant tests. Once this relationship has been established, the optimal tuning parameters for the outer loop can be calculated in order to achieve the best results from your VFD installation. ControlSoft’s INTUNE software is frequently used for such tests and tuning.

Figure 1: VFD control viewed as its own inner loop in cascade control.

VFD control

It should be noted that some applications that employ VFDs may also require the installation of a control valve to provide safe and reliable operation across the entire range of a controlled variable. To this point, VFDs are not immune to dead heading and may require a recirculation valve, as well as an on-off and/or check valve. A system curve can have high static head but little friction head, while a pump curve can be low sloped or even drooping, depending on both geometry and fluid properties. This could lead to an unstable operating region and is one reason to consider keeping a control valve after a VFD. Such an arrangement allows both the pump and system curve to be under control. As shown in figure 2 below, this allows for a wider operating range for both pressure and flow. In this example, a VFD can reduce the flow from 76% to 23%, but begins to risk unstable operation in the 23% range as the proximity of the curves becomes dangerous. If flow in this range is desired, it would be safer to keep the pump curve higher and use a control valve to reach low flows such as 8%. A back pressure valve is shown here to demonstrate the flexibility in pump curve ranges, but a recirculation valve may be used to similar effect.

Figure 2: Split range control of flow by VFD speed and throttling.

Split Flow Control

Such an installation might employ split range control with a lower VFD limit on speed. Depending on system dynamics and constraints, a more elegant solution might be to employ a Coordinated Control (CC) algorithm like that found in ControlSoft’s MANTRA control system. With a CC implementation, the controller will act to 1) provide the tightest control possible in the short term while 2) minimizing energy consumption in the long term. Figure 3 illustrates that, for a certain flow setpoint, there is a continuum for valve position and motor speed. Cost is minimized by reducing power, which is reduced with motor speed. A CC will always drive the system toward the operating point that maximizes cost savings, so toward reduced speed in this case, while remaining within operating constraints and balancing other considerations.

Figure 3: A Coordinated Control (CC) system maximizes cost savings and minimizes power consumption.

VFD cost savings

Proper tuning of controllers is of paramount importance if the operator is going to reap the economic benefits from a VFD installation. ControlSoft offers a number of tools to address this situation. From our INTUNE PID tuning tools which assist in finding the best tuning parameters for your controllers to our Coordinated Control algorithm (which effectively deals with multiple manipulated variables controlling a single process variable) ControlSoft can help to ensure the proper operation of your VFD installation. Effective VFD process control requires a good understanding of all relationships and interactions between a VFD, control valves, and the controlled variable.

Optimize Your VFD Performance

The popularity of VFDs has increased dramatically in recent years as plant operators strive to achieve maximum efficiency from their operating equipment. The financial impact that VFD operation has on electrical power usage is often a KPI (Key Performance Indicator) that is tracked and reported by a plant’s Control Loop Performance Monitoring (CLPM) system. The potential for savings is very real, but a large capital investment in VFDs might be lost without good process control.

ControlSoft, Inc. is here to make sure you get the full value out of your installed equipment and capital projects. We offer a wide range of products and services to help you plan, implement, and optimize process control. Our INTUNE tuning tools is ideally suited for determining optimal tuning parameters for the most difficult applications, including VFDs and more. Our INTUNE+ control loop monitoring system can track and report detailed statistics regarding your control system’s performance, while also defining and reporting on KPIs. Our engineers have hundreds of man-years devoted to helping our clients improve all aspects of their process control systems. We would be pleased to discuss with you how we can help you achieve your own operating goals.

We would be pleased to discuss with you how we can help you achieve your operating goals. Contact us today!

INTUNE and INTUNE+ v6.3.0.0 Available

We are pleased to announce that INTUNE and INTUNE+ v6.3.0.0 are now available. Highlights include:

  • Enhanced INSIGHTTM Applications Framework (formerly WebOPC).1
    Enhancements focused on performance improvements and ease of deployment.
  • Improved OPC Client Protection and Performance.
    Improved connectivity and performance for multiple simultaneous OPC clients.
  • Additional usability enhancements and bug fixes.

Additionally, INTUNE+ software includes:

  • Enhanced INSIGHT Mobile App
    INSIGHT mobile app communicates diagnostics on process controllers and final elements for corrective action. Major enhancements are implemented in the Message Center functionality:

    • Improved message interface
    • Filtering and sorting messages by criticality, class, frequency, type and time.
    • Message center now allows I&C users to add comments to any message. Multiple comments can be added to a message, and users can track specific messages for maintenance purposes. Messages and comments can be exported for other reports.
  • Expanded Capability and Increased Performance of Diagnostic Scripts
    Larger and more complex diagnostic scripts are supported with faster and more efficient execution.

1 We plan to release INSIGHT software in the third quarter of 2017. A beta version of the software is currently available.

ControlSoft Awarded Weatherhead 2016

ControlSoft Honored with Seventh Weatherhead 100 Award for Growth

In December 2016, ControlSoft was once again named one of the fastest growing companies in Northeast Ohio. The Weatherhead 100 awards are the premier celebration of Northeast Ohio’s leading companies.

ControlSoft Wins Weatherhead 2016

ControlSoft PID Tuning Pocket Guide

Updated PID Loop Tuning Pocket Guide Now Available

We’ve updated our PID Loop Tuning Pocket Guide to help you better understand the types of PID loops, how to tune Closed, Open and Cascade loops, and we’ve included parameters for the more common PID controllers used in plants today. By investing in tuning your PID loops (temperature, level, pressure and flow), your control loops will operate efficiently, rather than just being functional.