Word for the day: CONTROL SYSTEM – OPEN LOOP / CLOSED LOOP?

 

 

On a bright beautiful day, as you are navigating your automobile thru a congested road, your hands, legs and eyes are working in perfect synchronization to control the automobile to avoid collision and reach office safely. All these are happening rather subconsciously and you are busy enjoying some soul touching music on your MP3 player. In another instance, the milk is boiling and overflowing from a hot vessel kept on a stove, while you are busy watching some exciting program on television.

 

We are definitely not interested in explaining about any television program or music in this article!

 

In first instance, your eyes and ears were observing all the happenings on road and were passing the information to brain. The highly sophisticated and complex central nervous system in your brain was passing necessary signals to hands and legs to control various parts of the automobile, like brake, clutch, accelerator, horn and indicator switches. All this was happening throughout your journey for one main purpose: to avoid collision and reach office in time. You cannot afford to close your eyes or ears even for a second in this situation.

 

In second instance, the stove was heating milk in vessel beyond its boiling point. No signals were passed from anywhere to control the stove's heat and milk started flowing out, dirtying the kitchen. If left unnoticed, entire milk will flow out, and still the stove will continue heating the vessel, wasting thermal energy. If the attention was given to milk, instead of television, your eyes would have seen milk reaching the boiling point and one of your hands would slowly turn down the heat regulator on stove. As the milk reaches boiling point, your hand would shut off stove and remove the hot vessel and store it in its place. Probably, you could have enjoyed a hot cup of coffee and continued to watch television, a little later!!

 

What is the difference between these instances? In first instance, the brain is continuously monitoring various happenings on road thru its "input" sensors (devices) namely, eyes and ears. Then, a series of "thinking" (internal calculations or processing) steps will occur in the central nervous system and brain will give required commands to "output" devices namely, hands and legs. We can represent this activity by a block diagram. Refer Figure-1.

 

As you can see from figure-1, there is a continuous and closed loop interaction between input, processing system and output devices. This type of controlling one or more parameters is generally called as CLOSED LOOP CONTROL SYSTEM. We can also call this as COMPLETELY REGULATED system. Once the required objective is defined, it will control all / necessary parameters to achieve the desired objective within acceptable tolerance limits.

 

Now let us represent the second instance by a similar block diagram. Refer Figure-2. As you can see from Figure-2, the heat supplied by stove to milk / vessel is not monitored. So, the milk will start boiling beyond its limit and will completely spill out. And stove will continue heating the vessel till either someone rushes and shuts off the stove or till the available energy is consumed and stove shuts down itself. Both the outcomes would have definitely caused some damage: Wastage of milk for sure and damage to the vessel / exhaustion of energy source. This type of a system, regulating one or more parameters is generally called as OPEN LOOP CONTROL SYSTEM. This can also be called as PARTIALLY REGULATED or UNREGULATED system. Even if the desired objective is known, a continuous or intermittent observation and control from time to time is required.

 

In the second instance, if you were standing close to stove, the system would become a closed loop or completely regulated type, monitoring the required parameters.

 

On the other hand, in first instance, if you close your eyes or ears, the system would become open loop or unregulated type. So, to a large extent, we can say that the system type can be inter-changed.

 

 

 

 

 

 

 

 

The type of control system required will be decided by application and end objective to be achieved. In any machine – independent of its complexity – some type of control system must exist and will be a combination of various electrical, electronic and mechanical components, which has to work in perfect synchronization to achieve objective.

 

Therefore, on a broader level, the control system can be divided into three elements: INPUT SENSORS, COMPARING OR PROCESSING UNIT and OUTPUT ACTUATORS.

 

Input Sensors: Any device that provides feedback about the desired objective can be included in this section. It can vary from human eyes watching boiling milk to a satellite that is monitoring global weather.

 

 

Comparing or Processing Unit: Any intelligent system that processes information received from input sensors to activate one or more output actuators. It can vary from human brain to weather forecasting station.

 

Output Actuators: Any device that acts in response to information received from comparing or processing unit to achieve the desired objective. It can vary from simple hand which regulates the stove knob to television network that beams information on prevailing or future weather conditions.

 

With sufficient background so far, let us now construct a small control system block diagram for a simple temperature control unit. The end objective is to achieve a stable temperature of 250°C (482°F) inside an oven.

 

List of items required for this will be: 1) A temperature controller 2) A temperature sensor 3) Electric Heater 4) Necessary wirings to complete the circuit. Refer to Figure-3 for details of this control system. This is a typical closed loop control system. Temperature controller closes the normally open contacts of the power contactor, which in turn supplies main power to the heaters. As the heat builds up, thermocouple senses the increase in temperature and gives a MILLIVOLT feedback to the controller. To avoid "overshooting" of temperature beyond 250°C (482°F), the controller opens contactor contacts at a present POSITIVE TOLERANCE value, which in this case is assumed to be at 251°C (483°F). Now depending on the load, temperature can drop over certain time.

 

As soon as the temperature once again reaches NEGATIVE TOLERANCE value, which in this case is assumed to be at 245°C (473°F), the contactor is switch on again and cycle continues. This is one of the most basic methods of controlling the desired parameters. It is called ON-OFF type control system. There are better and highly sophisticated methods available to monitor, control the parameters. A brief summary about some of them are given in subsequent paragraphs.

 

1. ON-OFF Type Control System: A very simple method where the desired objective is achieved by only turning the output actuator(s) ON or OFF. It is a very basic and simple method of controlling any parameter, but has an extremely high tendency to overshoot or not achieve the target value. The ability of a control system to achieve target within shortest possible duration is called Response Time. The illustration described above is a good example. Usually, duty cycle is the commonly used terminology with this type of system. Duty cycle is the ratio of ON duration to sum of ON & OFF duration (Duty Cycle = ON duration / (ON duration + OFF duration))
   

   
    

2. PROPORTIONAL Type Control System: In this method, the objective is achieved by continuously varying output actuator. Adjusting the fan speed using electronic regulator, controlling speed of an engine by electronic governor and motor speed by regulating input voltage are some of the examples of proportional type control system. The amount by which a system overshoots or does not achieve the target over certain duration is called Hysteresis.
   

   
    

   Response Time and Hysteresis are most commonly used terminologies in the control system domain.
   

   
    

   The proportional type of control system are further classified into PROPORTIONAL INTEGRAL (PI) & PROPORTIONAL INTEGRAL DERIVATIVE (PID) types of which PID is the most popularly used method. We will elaborate on PID type control system in subsequent newsletters since it requires few pages for explanation.
   

   
    

   It is important to bear in mind that any type of control system can be selected for a given application. But technical complexity, initial investment, running cost and the actual application should be critically studied before selecting. For instance, it is not justifiable to integrate a PID control for an ordinary fan. On the other hand, using a basic ON-OFF type control for controlling motor speed is not appropriate as well.
   

   
    

   Independent of the application that is under consideration, control system will be playing an important role in achieving the desired objective. The accuracy of a machine – for example a CNC machining center or an automatic welding robot – is totally dependent on various elements of a good control system.
   

    

   
    

   
    

   
    

   The picture in next page gives a lighter approach to how complex a control system can get in a typical steam generation plant. Take a good look at how complex the system is, and you can also laugh at it in your break time!!
   

   
    

   It also depicts how a control system has to work on "real-time" basis to keep the parameters under control and achieve the desired objective.
   

   As a refresher, following are the common terminologies one should be familiar with, when working or troubleshooting or designing or selecting a control system.
   

   
    

• Response Time – How fast a system reach the target value (Example: A racing motor bike definitely "responds" quicker to accelerator and reaches top speed FASTER than a typical Scooter. So the racing bike responds faster for the desired objective, which is reaching top speed within short time)
  

  
   

• Hysteresis – Does the system "hunt" too much before reaching the target value (Example: When a spring is compressed and instantly left free, it "hunts" or oscillates between higher and lower value for certain time before stabilizing at its free length position)
  

  
   

• Tolerance Limit (Bandwidth) – In many applications, it is quite impossible to achieve target value precisely. The closeness to which a control system can achieve desired target is the tolerance limit and is an important point to be considered while dealing with a control system. An incubator for nurturing new born infant has to have very "tight" tolerance limit compared to an oven baking your favorite bread.

  
   

• Control Type – ON-OFF / PI / PID / Just Proportional type has to be clearly decided based on the application requirement. An impeller pump filling water to overhead tank can have ON-OFF control type compared to a similar pump filling "fixed" quantity of beverage into a can which must have more precise type – say PID for example – of control system.
  

  
   

 

 

 

 

Control system is a vast and diverse field applied to every equipment that is intended to achieve some desired objective. It is a science by itself and can be studied in depth if you are interested. Contact us for providing you with any specific knowledge, information, update on this exciting subject.

 

A simple control system exists in the form of a thermostat of Iron Box to complex Autopilot system on an aero plane!

 

There are many more application specific parameters for selecting a control system. For example, if a drive needs to be selected for regulating motor speed, few specific parameters that can be considered are: Acceleration Ramp, Deceleration Ramp, Regenerative braking time, Bleed down time, Feedback device. Explaining those will be beyond the scope of this newsletter, whose intent was to provide an overview.