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page 521

30. CONTINUOUS CONTROL

A controlled system includes,

-a basic system that does not achieve objectives by normal means - for our purposes a mathematical model a such a system is very important.

-some objective for the operation of the system.

-a controller that is matched to system, and tuned to meet the control objective.

Objectives for control systems might include,

-fast response

-stability

-energy efficiency

-etc.

The systems below are often directed by control systems.

valve

q1

valve

q2

 

 

h

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

motor

 

speed, acceleration

controller

Vin

Some Notes:

-Neither of these systems have controls.

-They both have inputs, and outputs.

-The output is not always something exiting the system. eg. the water tank height is the system state.

-We can treat these systems with ‘black boxes’ with lumped parameters.

All of the inputs and outputs are expressed as physical quantities (as will be seen later).

page 522

30.1 CONTROLLING CONTINUOUS SYSTEMS

• We can use some type of feedback from the system to change the system input (this is called the control variable).

e.g.

We have the tank of water from before, and we want to control the height of the water in the tank by adjusting the flow in. To keep the system linear, we will assume the output flow rate is fixed.

For control add,

feedback1. Some means of measuring the water height (system state) setpoint 2. Some input for desired control height

system error 3. Some error compensation

4. An actuator to change the system input

q1

Main water supply

q2

x

1. Feedback of hydrostatic pressure through a rubber tube.

2. This input slider adjusts the position of the bellows (can be adjusted with a screwdriver).

3. Bellows expand/contract as pressure increases/decreases, and move the rod that closes/opens the valve

4. The valve changes the flow into the tank, thus changing the water height.

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