Real World Feedback and Control

Motion Activated Hand Sanitizer Dispenser:

Motion sensors and the things they control are a common feedback and control system. For hand sanitizer dispensers, the sensing mechanism is a motion detector located at the bottom of the device. When the motion detector senses something moving, ideally someone’s hand, it begins to dispense hand sanitizer and stops after a set amount of time. This action is often accompanied by a blinking light, which all together make up the actuation mechanisms. The control computation is done by a preset code within the device.

As long as the motion sensor is in a place that will not encounter unintentional movement underneath it, this feedback and control system works well. It allows people to get hand sanitizer without having to touch any surfaces, preventing them from both picking up and leaving behind germs. If the hand sanitizer dispenser is placed on the wall in a spot where bags or jackets may be close to it when people walk by, the device may dispense sanitizer on these objects, which is not ideal. There is also a delay between when the motion is detected and the sanitizer is dispensed. This delay may result in a person pulling their hand away too soon and the sanitizer being wasted.

Refrigerator:

Similar to how a thermostat controls the temperature in a building, a temperature sensor is used to maintain a constant temperature in a refrigerator. This feedback and control system uses a closed-loop feedback system. A temperature gauge, the sensing mechanism in this system, checks the temperature of the fridge and sends this information to the computing mechanism, a preprogrammed device within the fridge. This device then compares the measured temperature to the desired temperature that is set by the user. If the actual temperature is above the desired temperature, the computing mechanism sends a signal which turns the cooling device on, the actuation mechanism. While this is happening, the thermostat continues to measure the temperature which is compared to the desired temperature at a regular interval. When the actual temperature is equal to the desired temperature, or if the temperature is lower than desired, the cooling mechanism is turned off.

When working correctly, refrigerators do a good job at keeping the temperature constant therefore maximizing the amount of time food remains fresh. The doors on a fridge get opened often and each time this happens, the temperature can drastically change. The feedback and control system allows the fridge to keep the temperature fairly constant without the requiring anyone to have to make any manual adjustments. If the sensor is in a location where it is getting blocked by something in the fridge, this feedback system could run into problems. This could cause the sensor to detect a temperature higher or lower than the average within the fridge, which would hinder its ability to reach the desired temperature. The system could also produce undesired outcomes if the sensor is too close to the door, which would cause drastic temperature fluctuations and could cause the fridge to overcompensate by lowering the temperature too much.

One Cards:

When trying to get into many buildings on campus, it is necessary to use a One Card to unlock the door. This process is an example of an open-loop feedback and control system. When a card is swiped, the box acts as the sensing mechanism and begins the feedback and control system’s actions. The box then uses computer technology to read the magnetic stripe in the card. If the data in the magnetic stripe matches what the box is programmed to look for, then the light on the box flashes green and the door is unlocked. If the box does not recognize the data on the magnetic stripe, then it flashes a red light and the door remains locked. Both of these actions are part of the actuation mechanism.

For the most part One Cards work well, but if the card is swiped too quickly, then the box may not have enough time to read it and the door will not unlock. Also, if the card is not in contact and parallel with the back of the slot which it is being swiped in, the box will not be able to read it accurately and the door will remain locked. Although it does not work perfectly every time, this feedback and control system is one that is used by everyone on campus and it makes controlling who has access to different places on campus much easier than using traditional keys. It is often the case that technology does not work as well as people want it to, but it is still better than the alternative.

Electric Kettle:

An electric kettle uses two feedback and control systems in order to heat water. On the kettle there is a small screen and buttons that allow the desired temperature to be chosen. After the temperature is set, pressing the power button, the sensing mechanism, turns the heating element on and the power button begins to flash. These actions are the actuation mechanisms and because they happen only once and do not depend on further feedback, this process is a closed-looped system.

The second feedback and control system starts directly after the first one ends. There is an electric thermostat in the kettle, the sensing mechanism, that measures the temperature of the water. This temperature is then compared to the desired temperature that was set. The difference between these values is the “feedback error”. If the feedback error is zero (or close to zero if the delay between the time the device is shut off and the water stops heating is being taken into account), then the heating element turns off and the kettle beeps, which is the actuation mechanism. If the feedback error is not zero, the kettle stays on and the process is repeated until the desired temperature is reached. This control computation is done using a negative feedback loop.

Depending on the amount of water in the kettle and what the desired temperature is, the amount of time between when the heating element is turned off and the amount of time it takes for the temperature of the water to stop rising will change. The variation in this time makes it difficult for the kettle to determine when to turn off in order to achieve the desired temperature, which means the actual final temperature will usually vary from the desired temperature. Because it is usually not necessary to have water heated to an exact temperature, this feedback and control system is well suited to perform its task.