What is a supervisory system?

Future of Supervisory Systems in Process Industries: Lessons for Discrete Manufacturing

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By Ian Nimmo, President, User Centered Design Services.

What is a supervisory system?

Basic definition

The early 1970’s were the commencement period for computer control. Memory and disk space was very limited, and the interface to the computer was very basic. The initial control was what we describe today as Supervisory control. The computer had limited output capability, but could influence instrumentation systems by changing the setpoint to PID electronic controllers.

Examples

During the startup phase of a continuous polymer process the characteristics of the polymer change. Based on plant throughput different process control setpoints require adjustment. In the old days the operator used to tweak the controller setpoint but would often over compensate and introduce new disturbances in the process. The early computers only changed the setpoints to these controllers but within a couple of years the mathematical model of the PID controller was developed within the computer program. This allowed not only automatic changes to the setpoint but changes to the tuning constants of the controllers also. This was very useful for non-linear processes.

However, the User Interface was still very crude and plant supervisors used to interface to the computer by identifying the address a variable and entering it through a set of 16 piano style keys, then entering new data points using the same keys. The process was very dangerous, especially when we consider that the data was not entered in normal base 10 numbering but base 8 octal or binary formats.

Soon more powerful monolithic control computers were introduced but were limited by small amounts of memory and often no disk storage capability. The Supervisory Control Computer System held the main program and instructions for the “target” control systems. The Supervisory system downloaded the latest software version to the control system and the supervisor initiated a booting or startup sequence. During the plant control the target system fed information about the process back to the supervisory computer system, including batch records, historical plant data and any abnormal condition reports.

The control system user interface improved slightly allowing operators to change the control computer setpoints and alarm values directly at the control computer system. This, however, required regular updates back to the Supervisory system to allow easy alignment should the computer shutdown and require reloading of the basic program. This meant that the Supervisory Control Computers required several versions of the plant software, the basic or “virgin image”, a “sucked back copy” which held the latest setpoints and other data parameters, and often development versions of the software which was either under test or being developed by a programmer.

The Supervisory system became a very powerful system especially on batch processes and as Digital Corporation’s VAX computers and commercial VMS operating system became available and cost effective, the use of off-the-shelf database products improved the historical data capture and replay capabilities.

As the Supervisory systems continued to evolve, Honeywell introduced a new more powerful Distributed Process Control Computer System (DCS) that could work independently of the Supervisory systems. They had good human interface, historical data capture and replay capabilities and powerful alarm management systems. The Supervisory Systems became a place to store large amounts of historical data and for the optimization or mathematical modeling environment.

Today, the DCS works independent of any supervisory system but has relinquished some responsibility for advanced control to more powerful supervisory systems, however, even this is changing due to the power of the new open system technologies being used for control. It is becoming difficult to tell were the control system starts and ends and if a separate supervisory system as we know it is required anymore.

The ICI Robot story

The development of more capable computers allowed mechanical systems, such as robots, to be used for many highly labor intensive activities that only needed muscle and little in the way of human skills. However, there are some lessons for us as we consider the introduction of this new technology. The robot was identified for repetitive tasks. Also people and robots never should work together. The robots were put into large safety cages and isolated from all contact with people.

I remember my first robotics application within Imperial Chemical Industries (ICI) in the UK. The company were keen to identify and use time saving and cost effective automation. A group of engineers with ideas for applications of this technology was formed and several trial applications were investigated. It became obvious that there were many reasons why people should not trust this technology. The British Government had identified high risks to people from the robots as they were very robust and could seriously hurt a person or even kill them should people get in the way of their movements. The answer was simple. Isolate the robots from people by placing them in high security prisons. Should a person need to enter the room where the robot was working, elaborate isolation equipment was employed to secure the robots. This made them expensive and no longer cost effective.
 
My first robot was no exception to this rule, I had the great idea to use my robot as a shrink wrapping system for 300Kg bales of staple fiber. The existing bales of  what looked like very large cotton pillows were wrapped in polythene which needed to be shrunk by heating the polythene at very high temperatures. The existing system consisted of a large open electric oven with a conveyor through its middle. The conveyor carried the bale from the wrapper to the oven, to labeling and the warehouse for transportation to customer sites. The electric oven was very inefficient and was costing millions of dollars in energy usage, the wrapping was poor due to the large side ears that resulted because of the wrapping and shrinking process.

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