Prelab: Introduction and Greenhouse Construction

Transcription

1 Prelab: Introduction and Greenhouse Construction In this lab, you will create a PID control system that will regulate temperature and humidity of a greenhouse-like enclosure. You will learn the concepts of PID control, microcontroller coding and principles of op-amps. The following is a block diagram of the entire system; however, each component of the circuit will be discussed. Figure 1 Block diagram of the greenhouse control system. Figure 1 gives a description of how the system will be implemented. The microcontroller takes input voltages from both an analog temperature sensor and a digital humidity and temperature sensor. These voltage values will be used to generate an error signal that represents the difference between the actual and desired temperatures. The input from the digital sensor will be used as a comparative voltage for an on/off control for the humidifier via a relay. The microcontroller will then take these values, and output them to a LCD. In addition, the microcontroller, through PWM pins, will output a voltage to both a relay and PID control circuit. Figure 2 below shows the PID circuit the students will construct on breadboards to control the fan and light bulb to adjust the system s internal temperature.

2 Figure 2 PID circuit that the students will build to control the light bulb and fan of the greenhouse. The PID circuit is depicted in Figure 3 below. In the PID control circuit, the microcontroller will provide the actual temperature value from the sensors and a reference temperature voltage. This reference voltage will be chosen by the user and programmed into the microcontroller code. The PID will turn on a light bulb when the temperature of the system is lower than the desired reference temperature. Figure 3 Typical PID circuit and block diagram using op amps.

3 The PID will also turn on a fan when the actual temperature is higher than the reference temperature. The process of heating up the container and cooling it down, will continue until a relative equilibrium has been reached that is close to the reference temperature. The microcontroller will also output a voltage through a digital output pin set high (5V) to a relay. If the humidity is above the desired set point, this relay sends 15V to be summed with the output signal of the PID circuit by a summing op amp. The idea is, that 15 volts summed with whatever the PID is outputting is enough to turn on the fan, effectively by-passing PID control. Greenhouse Construction: Here, you will construct the greenhouse enclosure. To start, you will need a 128ml container. The one used in this lab, is a Hawaiian Punch container from a grocery store. 1. Using an empty container, trace and cut a hole for the fan near the bottom of the container. The bottom was chosen to minimize the amount of heat and humidity lost through the openings between fan blades. Figure 4 Greenhouse container with a focus on the hole for the fan. 2. Next, you will mount the fan making sure it will blow air out of the container, rather than into it.

4 Figure 5 Greenhouse container with the fan mounted.

5 3. Next you will hang the light bulb, analog temperature sensor and digital sensor from the top of the container and seal it with tape. Figure 6 Greenhouse showing the LM34 temp sensor, 12V light bulb, and digital T/H sensor hanging inside.

6 4. Later on, a humidifier will be placed into the greenhouse, so a small slit should be made at the bottom of the container to make room for the humidifier wire. Figure 7 Greenhouse with the humidifier placed inside at the bottom.

7 Table 1 Parts list needed to complete the greenhouse labs. Product Part Number 2 N-MOS FQP30N06L Op-Amp Temp Sensor Temp/Hum Sensor Fan Portable Ultrasonic Humidifier Hawaiian Punch 128 ml container Sylvania tail light UA741CPE4 LM35DZ/NOPB DHT11 AD0405MB-G70- LF B006N0JJVM AA P27/7W 2 darlington transistors 2N relays 26Y2W8 4 Potentiometer D1M 1130 Trimmer LCD 306E LCM-S01602DSF /A Arduino Uno R3 Functional Necessities (tape, solder, etc)

8 Lab 1: Programming the Microcontroller In this lab, you will write a microcontroller program that will be used to perform three operations. For the first operation, you will write a code for the digital temperature/humidity sensor and LCD display. This code should take temperature and humidity levels, and output them to an LCD. The code should also output the data to a CSV file. You won t have to worry about the serial output until a later lab, but you will need to get the ambient room temperature and humidity as these will aid in selecting your reference voltages for the humidity and temperature inputs. For the second operation, you will complete the skeletal code that will output reference voltages. For the third operation, you will complete the skeletal code for the relay circuit. In order to program an Arduino you must first download the Arduino IDE. Instructions for doing this can be found at: As you may or may not know, there will be libraries that will be needed to interface with various components such as the DHT sensor. These libraries were provided and an inorder to use certain functions such as dht.readhumidity(); you will have to include them by using #include DHT.h as illustrated in the sample code and skeleton. These libraries (contained in the greenhouselab.zip) will have to be placed in your local Arduino libraries folder, usually Documents/Arduino/Libraries before the IDE can use them. When writing this code make sure to keep a count of the pins that you are using on the Arduino Uno board. Certain pins marked pwm are capable of outputting a varied voltage level and should be used when providing the references to the PID control circuit. Additionally, pin 0 and 1 should not be used as a digital input pin for the DHT sensor.

9 A) 1. The first step will be to write a code for the digital temperature and humidity (DHT) sensor. The DHT will be used to control the humidity in the system, and the analog temperature sensor will be used to control the temperature, as it can be read from constantly, while the DHT requires a waiting period. A skeleton is provided, but the implementation is generally open to how you want to do it. You must however use the DHT library provided, or another library used for interacting with the DHT sensor. The one provided can be found at: For how to implement this, take a look at the sample code provided in the library labeled 'DHTtester.ino (%localpath%/libaries/dht/dhttester/dhttester.ino). This will show how the functions provided in the library interact with the sensor. You will have to make a loop that will activate a relay when the humidity is too low, or another relay for the fan when the humidity is too high. This can be done by using the digitalwrite(pin, HIGH); or digitalwrite(pin, LOW);. Make sure that you designate the pin to be a output pin by using pinmode(pin, OUTPUT); You will have to make sure that you take in the input from the sensor, using the provided sample code, and then compare it to a reference. The point at which the fan should turn on is up to you. This means that you can set a humidity buffer for how far the humidifier can overshoot the target humidity. It can be interesting to observe the effects that this can cause. 2. The second step will be to write a code that will be based on setting up the LCD display to display temperature and humidity. A fantastic tutorial to using an LCD with an Arduino is provided at the link directly below, and goes much more in depth than is possible in this lab writeup. You will see that you must set the position of the cursor and the use lcd.print();

10 3. The third step will be recording the reference values that were generated from the digital sensor (temperature and humidity) V Temp = V Humdity = These references can be found either from a serial output to the computer or from the LCD. A serial output can be set up using the tutorial below. To view the serial output, make sure that the Arduino is connected to the computer and go to tools>serial monitor B) 1. In this part of the lab, you will be finishing the skeletal code that will output the reference voltages from the microcontroller and send them to the PID controller. The code should also take in an analog signal from the analog temperature sensor (LM34), convert it to a digital signal and then back to a analog signal via one of the PWM output pins. The analog temperature sensor is powered off of 5V, and its input must go into one of the analog in pins of the arduino. It can be read using analogread(pin). To convert it to degrees fahrenheit it must be multiplied by the scaling factor, (5.0V/ levels ); Be sure to include a calibration, as your temperature sensor may constantly be off by a consistent amount of degrees. In order to output the reference using a digital PWM pin the temperature reading in fahrenheit must be converted. there is 255 different voltage levels that the pin is capable of outputting and the output voltage is 5 volts. So to output the temperature like 75 degrees as 0.75 the output must be multiplied by the factor of temp*(255 levels / 5V).

11 Lab 2: Building the System In this laboratory, you will build a PID control circuit to control the light bulb and fan based on the readings from the DHT11 digital humidity/temperature sensor and the LM35 analog temperature sensor. Next, you will implement two relays that will be used to control the humidifier and fan. A) Create PID Control Circuit: The PID control circuit will control the light bulb and the fan to increase or decrease greenhouse temperature, as well as allow for a signal from a relay to control the fan. 1. The following schematic of the PID controller, shows how the PID will be implemented using op-amps, resistors, capacitors, transistors and potentiometers. Create the circuit shown in Figure 8 on a breadboard. It will be helpful for debugging if you use a color-coded wiring scheme, such as red for +Vcc, blue for -Vcc, green for GND, yellow for wires feedback loops and to connect to the next stage. A sample photo of what this color scheme would look like is provided in Figure ####. Figure 8 PID circuit to control temperature of the greenhouse. 2. The first three op amps from left to right of Figure 8 above shows how the error term is generated using an inverting, a summing, and unity gain buffer op amp configurations. Show that the output of the inverting summer is: Verror = -(Vref -Vactual) Note: Vref is the desired temperature. 3. Figure 3 above shows a general way to implement a PID control circuit using op amps. Note that it differs from the implementation shown in Figure 8, in that it is lacking a low pass filter in series with the differentiator component of the PID. Why do you think in practice it would be a good idea to have this low pass filter for the differentiator? From Figure 3, show that the transfer function is: VV cccccccccccccc VV eeeeeeeeee = RR pp2 RR pp1 + 1 RR ii CC 1 ss + RR ddcc 2 ss = KK pp + KK ii 1 ss + KK ddss

12 B) Relay Implementation: The relays will be used to control the humidity in the system. The relay chosen for this lab is a 5 pin mechanical relay that runs off of 5V (you can power it off of the microcontroller). Two pins are the source and ground of the object that is being switched on and off, and one pin is the output. The other two pins are the ones that run off of 5v and are used to actuate the relay. When there is current running between these two pins the relay will go to the on position (active on). You will hear a small mechanical click when this happens. Look at the diagram below: Figure 9 Schematic of the relay(s) used to control the fan and the humidifier. This is one implementation of the relay circuit. This circuit uses a NPN transistor to stop the flow of current until the it is triggered from the Arduino out. Be sure to include a resistor between the Arduino and the NPN transistor and to have similar grounds for the transistor and the switch circuit. If a different relay is used, make sure to read the documentation on that relay and match the correct pins. Once the relay circuit is hooked up, the humidifiers wire from the power supply to the humidifier can be cut and the wires attached to the relay, so that the relay essentially stops to current flow to the humidifier and when it is activated, turns on the humidifier.

13 The fan is similar in when the relay is turned on it forces the PID control circuit to output the -15V required to fully actuate the fan. So place the output of the relay into the summing op amp on the right side of the circuit of Figure 8, where it says From relay.

14 Lab 3: Testing and Debugging The Circuits In this laboratory, you will finish complete the construction of the PID and relay circuits. Once they are built, you will start testing and debugging both of the circuits. You will test the op amps to make sure that their output voltages agree with their theoretical values. A) Testing the op-amp: 1. Use this code to generate a reference voltage and simulate an actual temperature: 2. At the output PWM pins selected for the previous step, check to see if your reference and simulated actual voltages are what you expect. Record them below. V Ref = V actual = 3. Run through the circuit up to and including the proportional term of the PID and check if your voltages make sense. 4. From left to right of figure 8, after the first inverting op-amp, what is the voltage? Does this make sense? Why or why not? V inverting input = V inverting output =

15 5. After the first inverting summer op-amp what is the voltage? Does this make sense? Why or why not? V Summer input1 = V Summer input2 = V Summer output = 6. After the proportional term op-amp, what is the voltage? What resistors did you use? What gain should you have gotten and what gain did you get? Do these values make sense? Why or why not? V Proportional input = V Proportional output = R Feedback = R Input = Gain Theoretical = Gain Actual = 7. Now remove the buffer that comes after the proporional op-amp. Measure the output voltage. Is your output voltage the same as before? Why or why not? V Output with buffer = V Output with out buffer = B) Testing the relay: 1. The simplest way to test the relay to ensure that it is working properly is to add LEDs between the output and ground pins on the relay, so that when the relay is turned on the LEDs will light up. To do this stick the short end of the LED (cathode) into the ground pin s rail on the breadboard, and then add a 5-15K resistor between the anode of the LED and the output pin of the relay. Once you are sure that the relays work properly, ensure that the fan is actuating when setting the references on the code to the correct levels. This can be done by adjusting the target levels of the humidity in your code.

16 Lab 4: Overall System Testing and Tuning the Circuit In this lab, you are going to test and tune the circuit. Furthermore, you will run the microcontroller that is connected to both the relay and the PID controller and you will also write microcontroller code to produce a serial output. Using the serial output, you will create a CSV file, create and analyze a response graph, and tune the circuit appropriately. A) 1. Connect the relay to the humidifier and the PID controller. 2. Connect the PID controller and analog and digital sensor to the microcontroller. 3. Compile and upload your microcontroller code using ambient temperature and humidity. Get the ambient temperature and humidity as found in lab 1 and run serial output. 4. From here, take your serial output and export it to a CSV file. Next, create and analyze a graph of the system response using this CSV file. 5. Now, run and export your serial output for the case of desired temperature and humidity are greater than actual. You will do this by changing your reference voltage values. Suggested temperature and humidity conditions: Case Temperature Humidity 1 V Temp +10 V Humidity +20% 2 V Temp +2 V Humidity +5%

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