I am learning basics of control systems from following link


It says that automatic washing machine & traffic signal system are example of open loop system. It does not gives detail why??

Despite the fact that "automatic" appears before washing machine

Also please kindly find attached photo of a common door closer. What type of control system is being used by it?open loop or close loop? enter image description here

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    \$\begingroup\$ Go back to the start of the page and read again. The second sentence says: "If in a physical system there is no automatic correction of the variation in its output, it is called an open loop control system." Think about how a washing machine would know how clothes are clean, or how traffic lights would know how to best get the most cars through. If they just turn on stuff for a fixed time, they're open loop. \$\endgroup\$
    – Graham
    Sep 3, 2019 at 19:00

3 Answers 3


Both, the automatic washing machine and the traffic signal system are open-loop for the same reason the website mentions for the home heating system.

An open-loop system usually has a timer which instructs the system to switch on the furnace for some time and then switch it off.

The Washing machine uses a timer to turn on and turn off washing and drying without measuring how washed or dried the clothes are.

The traffic light uses timers to switch lights without measuring how many cars are actually on the road.

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    \$\begingroup\$ Must be an old site if its author thinks that's how traffic lights work. Almost every one round here (UK) has sensors to detect waiting cars and whether any pedestrians who pressed the cross button have walked away. \$\endgroup\$ Sep 3, 2019 at 22:41
  • \$\begingroup\$ @PeteKirkham I would be surprised if washers these days didn't use sensors to inform the control system of over-full loads, under-full loads (from asymmetric loading causing wobble of the drum), etc. \$\endgroup\$
    – Alexander
    Sep 4, 2019 at 2:45
  • \$\begingroup\$ @Alexander I think of those are low level close loop systems. If the actual wash cycle is a fixed length it's probably "open loop" even if say the drum speed is maintained by a closed loop controller. But if the wash cycle runs until somehow the washer determines they're clean it's closed loop (our dryer does this, using I guess moisture content) \$\endgroup\$ Sep 4, 2019 at 3:26
  • \$\begingroup\$ @Alexander true, and there will also be a closed loop in a subsystem of any washer that allows setting a water temperature, but they aren't measuring how clean the clothes are, whereas the traffic lights can measure how long the traffic waits and adjust timing to minimise that, closing the loop on the system goal. \$\endgroup\$ Sep 4, 2019 at 9:20
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    \$\begingroup\$ @PeteKirkham "but they aren't measuring how clean the clothes are" true! They can be closed loop over different metrics, but not that primary goal metric \$\endgroup\$
    – Alexander
    Sep 4, 2019 at 14:28

Because it has no feedback. Washing machine does just a programmed sequence of operations, this is so called automatic. The manual operation would be for example that you determine the step of the sequence and duration. Also the traffic light control is just an automatic sequence.

Closed loop is for example a temperature control, where you measure the temperature and take action in base of that.

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    \$\begingroup\$ And the washing machine does of course both. Most washing machines today even measure the soakedness of the laundry. \$\endgroup\$
    – Janka
    Sep 3, 2019 at 9:17
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    \$\begingroup\$ @Janka You do realise that this is just a beginners example, don't you? \$\endgroup\$ Sep 3, 2019 at 9:30
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    \$\begingroup\$ It's a bad example, hence the confusion. \$\endgroup\$
    – Janka
    Sep 3, 2019 at 9:46

It's worth first explaining what open loop control system is. Open loop

This is a control system where you essentially blindly set an value. This may work well, based on experience. You may for instance know that a sausage is warm after 10 minutes in warm water bath, without actually checking the internal temperature of the sausage. For some processes, this is OK, because it's either cheap to overrun a little bit, or cheap to underrun.

An example of this would be my home ventilation; I don't check humidity, temperature or any other factors; it gets turned on at 09:00, and off at 22:00, no matter what. The cost of running it is tiny, and the cost of sensors and programming high.

This is also used in most washing machines; they run based on a time, not any measurement of the process value, which is how dirty the clothes are.

In short; a open loop system does not look at the system to determine where the process is. It blindly sets the set point.

Imagine a sled, on a rail, which accepts commands to go left or right for some distance, e.g. go 10 cm left. If you are at the leftmost position, and send this command, it will lead to a crash. In such a situation, open loop system will not suffice.

A closed loop on the other hand, has a feedback.

Closed loop The Process Value (PV) is fed back into the controller, modifying the set point. Different types of controllers exists; for analogue values, a PID controller is commonly used.

But let's continue with the previous example of the sled on rails. Now the controller knows where the sled is, so the set point to the controller can be changed from go left by 10 cm to go to five centimeters right of the left end stop, and the controller will know which direction it should drive the sled, and for how far.

Another example is the simple thermostat on an oven. The oven (actuator) heats up the room, and the process value is sensed by the thermostat, which turns off the actuator when the setpoint matches the process value.

Control engineering is a big topic, and this is a very brief introduction.


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