| Intermediate Technology Education | Control Technology | Pre-Design | Topic 3 | Activity 3 |

Control Systems

The purpose of this activity is to understand systems as a means of understanding control system
You will
  • Define systems
  • Describe the parts of open and closed loop systems
  • Identify examples of control systems

System

A system is made up of parts or components which, taken together, can do something that the individual parts cannot do. 

The function of a system may be described by a simple diagram.


Figure System Diagram

As the diagram shows, a system has three parts to its operation. 

  • Input.  Input is what you tell the system to do.  When you are driving a car, one input is to press the accelerator.
  • Process. This is what the system does after it receives input. In the car example, more fuel is fed to the engine.
  • Output.  This is what the system produces as a result of processing the input.  Again in the car example, as a result of the input and processing of the input, the engine operates at a higher speed and produces more power, and the vehicle moves faster.

This type of system is not self monitoring.  In the example of the car, as long as you press on the accelerator, the car will continue to speed up.  If you lay a brick on it and jump out it will continue to speed up.  By itself, the system has no way of knowing when it is going too fast.  Of course the driver does and can change the input any time to modify the car's speed.  This type of system is called an open-loop system because it is open ended and has no way of knowing the effect of its output.

System with Feedback

Some systems have an additional function, called feedback.  This diagram shows how that function fits in the system.


Figure System with Feedback

The diagram adds one part to the operation of the system

  • Feedback. Feedback is when the system compares the output (what happened) to the input (what you wanted to happen) and then makes adjustments so that they match.

This system is a self regulating system, and we call it a closed loop system.  It knows the results of its output and can modify the input in order to control the output. 

Does this look familiar?  If you think back to the previous activity on control, the definition of control was

Control is the process of comparing what actually happened to what you intended to happen, figuring out the differences between the two, and then making changes to get the desired results.

If you are paying attention, you are way ahead of me.  A system with feedback is nothing more than a control system.  A control system is a self regulating system  If we look at the car example again, we can convert it into a closed-loop system by installing a cruise control device.  Here is the system model for a car with cruise control


Figure Car with Speed Control (Control System)

Control System

Any system can be a control system. If it is an open loop system, it has no feedback loop and requires external guidance by a person. If it is a closed loop system it has a feedback loop to regulate the operation of the system, and is capable of autonomous operation.

When you are ready, go to Your Turn