1. Controlling the Heater
time
analog input
520
510
500
490
heat=off
heat=on
setpoint = 505
LCL= 500
UCL= 510

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3. Heater Control
You will choose a target temperature or a setpoint for your temperature control system.
What do you think should happen with your system when the temperature of the water is below the setpoint value?
Heater should turn on
What do you think should happen with your system when the temperature of the water is above the setpoint value?
Heater should turn off
How difficult will it be to keep the temperature at a single specific value?
Giving yourself some tolerance above and below the setpoint will help control the temperature in a more stable manner.
Tolerance above setpoint is called Upper Control Limit (UCL)
Tolerance below setpoint is called Lower Control Limit (LCL)

4. Finding UCL and LCL
Need to determine the random error of the analog input
Collect data points at room temperature over a span of time
Find the standard deviation, σ, of the temperature values collected
Why standard deviation? What is standard deviation?

5. Standard deviation, σ, is a value calculated to determine the amount of variation with in a data set
This curve represents data from a data set
μ represents the mean value of the data
Most data will be near the mean (μ)
As you move away from μ, there’s a decreasing probability of a measurement at that value
Moving one standard deviation (±1σ) away from μ will provide you with 68% of the data points
Moving ±2σ away from μ will provide you with 95% of the data points
Moving ±3σ away from μ will provide you with 99.7% of the data points

6. How the standard deviation relates to our exercise ?
The date we are measuring is the room temperature of a closed classroom over a period of a few minutes
Meaning, the data will practically not vary at all, or, σ should be very close to zero if the measurement is accurate
Any change in data can be attributed to the errors in the measurement system
Hence, for a system like ours, σ can be correlated to the errors ( fluctuation in data due to the measurement system )
Moving ±3σ away from μ will provide you with 99.7% of the data points AND remove 99.7% of the measurement errors

7. Finding UCL and LCL
Need to determine the random error of the analog input
Collect data points at room temperature over a span of time
Find the standard deviation, σ, of the temperature values collected
Why standard deviation? What is standard deviation?
Standard deviation, σ, is a value calculated to determine the amount of variation with in a data set
For your temperature control system, finding σ will allow you to account for the random error of the analog input from the thermistor circuit
Using ±3σ will account for 99.7% of random error
UCL = setpoint + 3σ
LCL = setpoint - 3σ
How do you find standard deviation?
𝜎= 𝑥−𝜇 2 𝑛−1
or use the STDEV.S function in Excel
Note: If σ is less than 1, then set 3σ=3.

8. Find UCL and LCL of your thermistor
Setpoint Value is 25℃ (Room Temperature)
How can you write this an analog value?
Determine random error and calculate your UCL and LCL
Allow system to reach steady state (stop changing) at room temperature
Collect 20 “analog input” readings, taking a reading every 5 seconds
Enter these readings into Excel and compute the standard deviation, σ
UCL = setpoint + 3σ
LCL = setpoint - 3σ

9. Controlling the Heater
time
analog input
520
510
500
490
heat=off
heat=on
setpoint = 505
LCL = setpoint - 3 = 500
UCL = setpoint + 3 = 510
deadband
Simulate this process with an LED acting as the heater
Write a sketch that turns on an LED when the analog values are below the LCL and turns off the LED when the analog values are above the UCL.

10. no no yes yes start declare variables
calculate analog value of setpoint, UCL & LCL
read analog value of temperature no no
value < LCL
value > UCL
yes
yes
LED on function
LED off function