Real World Feedback and Control

Feedback and control systems are more familiar than their fancy name makes them sound. A large range of objects, from automatic doors and faucets to drones and spaceships rely on feedback and control systems. 

These systems have sensors, computers, and actuators that change the behavior of the machine based on observed light, movement, sound, heat, interference, etc. The control system, for example, can use if/else statements to make the machine behave one way when its sensor observes a specific property in its surrounding and to act differently otherwise. 

The diagram above shows the general mechanism by which feedback and control systems work.

In class, we have seen two types of feedback and control systems -- open and closed loop. We have also used light sensors, ultrasonic sesnsors, encoders, and switches in combination with Arduino sketches to change and control the behavior of SciBorgs. 

Below, I present four common feedback and control systems I encounter daily: automatic doors, automatic water dispensers and faucets, automatic phone operators and smoke detector alarms. 

1) Automatic doors 

Such doors use light sensors, touch switches, or other sensors to open doors for a given amount of time in response to an observed change in the surrounding. 

PIR Sensor

Some automatic doors use PIR (Passive Infrared Sensor) to detect the presence of humans. The near-black-body irradiation from our body due to our temperature is mostly in the form of infrared waves (~9500 nm). And the intensity (or the number of photons reaching the sensor) increases with shorter distance between the source and the detector. So the intensity of IR photons detected by the sensor on an automatic door increases when a human is near the sensor. The PIR has a crystal whose temperature increases upon irradiation (by the IR waves reaching it). This temperature creates an electric signal/charge that communicates with the micro-controller. The micro-controller orders the motors and other actuators in the system to move such that the door opens when there is a high IR signal (or change thereof) and close after a certain delay, and remain closed when the IR signal is low in terms of intensity: the person is far; or in terms of temperature: the radiation reaching the sensor is only that of the surrounding inanimate materials. 

Note: This sensor is called "passive" because unlike ultrasonic sensors, for example, it doesn't send out waves and measure the time delay before they return to observe something about the surrounding. It simply detects input from the environment. 

This has the following uses: 

•  If a person cannot open the door due to a disability, because their hands are full, or they are too weak to do so (for example, a child), it is convenient to have an automatic door that doesn't need force to open.
• Since the amount of time the door stays open is controlled, it avoids the risk of leaving the door open for too long and wasting either heater or AC energy.

Some drawbacks:

• Since the specific sensor I discussed only detects IR reaching it an not the identity of the emitter, it can let either animals or children in our out the of the building without adult supervision. 
• It adds to the load of power use by the building. We would spend more energy on doors than we would if we manually opened them. But often this drawback is overcome by the many advantages of such systems. 

2) Automatic faucets

Automatic faucets can also use PIR but since one might want to wash items with temperatures lower than that of humans, such faucets also use ultrasonic sensors or infrared light next to a an IR detector to "measure the presence" of an item within a predefined distance. The object interrupts a light beam from the ultrasonic or the IR source and which reflect back to the sensors. If the reflection time is in the range of the calibrated value for a hand under the faucet (AKA not the sink or other items in the surrounding), an electric signal is sent to a solenoid valve (an electromagnet) to turn on the faucet. Otherwise, the solenoid valve isn't activated and the faucet remains closed. 

Advantages: 

• It prevents water waste because if a person forgets to close the faucet, the IR or ultrasonic interruption is no longer present and the faucet turns off instantaneously
• It is also sanitary especially in public bathrooms where manually opening faucets may introduce us to several germs. 

Drawbacks: 

• The specific systems described above don't allow the user to choose the water temperature. 
• While we save a lot of water, we have to sacrifice some electrical power. 

3) Ionization Smoke Detectors/Alarms

Smoke detectors contain a very tiny amount (on the order or 10^-6 g) of Americium which decays radioactively and emits alpha particles. These alpha particles collide with the nitrogen and oxygen molecules in the air and trigger a current flow in the opposite direction to an original current that would activate an alarm. This means the net current flow when there is ample N2 and O2 in the air is zero; the alarm can't go off. However, when smoke blocks off the clean air from occupying the space near the detector, not enough charges are created to counter the current that triggers the alarm. And depending on the occasion, either a life is saved or the user rushes to wave a towel under the detector (that is how my roommate and I handle completely random alarms). The following video from YouTube offers a good explanation for how this feedback and control system works: 

Advantages:

• The detector is very sensitive and gives a very early warning to the start of fire before too much damage happens. Also because smoke detectors sit on top of the room, hot air from the fire reaches them quickly through convection. 
• Needless to say, they save lives.
• While the radioactive material sounds dangerous it poses little threat due to its small amount and proper packaging.

Drawbacks:

• Disposal of smoke detectors containing radioactive material can be a health and environmental hazard if not done properly. 
• There is a chance the alarm would go off even though there is no risk of fire. It is so sensitive, steam or dust can set it off sometimes. This is because it detects the absence of ions rather than the specific material that is present in their stead. 

4) Fume Hoods

Fume hoods are "cupboards" that enclose a working area to isolate and remove harmful gases from the room in which a chemist (mostly) works. They have feedback and control systems that monitor air flow and window height (opening), as well as allow the user to turn exhaust fan on and off. 

Fume hoods have an air flow monitor that detects the face velocity (velocity of air at the fan opening). When the detected air flow is too low either due to excess exhaust, the sash being opened too high, or fan malfunction, an alarm goes off.

Advantages: 

• Not only does it do the primary job of pumping out unwanted gas and replacing it with fresh air, it assesses the product: how high the air flow is. 
• It prevents health and environmental hazards while allowing us to work with a variety of chemicals

Disadvantages: 

• It runs for several hours and pumps out air from the room that has been thermally conditioned. So it ends up consuming too much energy.