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Lab 5: Controls and feedback
Lab 5: Control and Feedback
This embedded system uses the Photo sensor to detect the light intensityof the environment and adjusts the light emitted by the LED to maintain aconstant light intensity environment.
You may need aresistor other thanexactly 2K forbetter sensitivity
22K
2K
ATMEGA16L
VTG
10K
SW
VTG
10K
SW
PB0 PB
PC
RXDTXD
To Computer
Serial Port
LED
VCC
VTG
10K
VTG
10K
SW
PC
PB
VTG
10K
SW
PB
VTG
10K
SW
PB
LED
VCC
VTG
10K
LED
VCC
VTG
10K
LED
VCC
VTG
10K
LED
VCC
VTG
10K
PC
PC
PC
OC
(PD7)
Sensor200K
LED
ADC0(PA0)
VTG
V_Feedback
R
R
Lab 5: Controls and Feedback
PID control
ā P: Proportional
ā¢ The error signal (error = desired ā current) multiplied by a
constant and fed out to the drive. (proportional = gain * error).
ā I: Integral
ā¢ The integral term is the sum of past errors, so adding the
past errors will eventually drive the output closer to thedesired output
ā D: Derivative
ā¢ Differentiator uses the derivative (rate of change) to predict
the future behavior.
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Lab 5: Controls and Feedback
In this lab, the plantās function is to generate a desired levelof ambient light. The sensor measures ambient light with aCadmium Sulfide (CdS) photocell. The sensor measuresthe plantās performance.
Lab 5: Controls and Feedback
The choice of a controller depends on theapplicationās requirements.
This lab builds an application that uses the dutycycle of the PWM signal to dynamically adjustambient light levels.
The controller increases the duty cycle of thePWM in order to apply more power to the LEDand thereby increase its intensity. The PWM isthe control, u(t), that is fed into the plant (that is,LED).
Lab 5: Controls and Feedback
Changes to the LEDās intensity can occur as fast as themcu computes and updates the PWM duty cycle register(OCR2). For this application one updates every 100milliseconds is more than sufficient. One milli-second isvery slow compared to how fast ambient light fills aroom.
At speeds of human perception 100 milliseconds is fastbut detectable. These timing considerations drive therequirement for only needing to use a proportionalcontroller. Hint: A maximum change of 2% to 4% to theduty cycle every 50 - 100 ms implements a nice smoothtransition of the LEDās intensity
Lab 5: Controls and Feedback
ATMEGA16L
VTG
10K
150 SW
VTG
10K
SW1 150
PB0 PB1 PC
RXDTXD
To Computer
Serial Port
1415
1 2 22
LED
VCC
VTG
10K
150
VTG
10K
150 SW
PC
23
PB
3
VTG
10K
150 SW
PB
4
VTG
10K
150 SW
PB
5
LED
VCC
VTG
10K
150
LED
VCC
VTG
10K
LED
VCC
VTG
10K
LED
VCC
VTG
10K
150
150
150
PC
24
PC
25
PC
26
OC (PD7)
21
130
Ī©
Sensor 200K
Ī©
LED
ADC0(PA0)
40
VTG
V_Feedback
R
R
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Lab 5: Controls and Feedback
#include <mega16.h>
// Standard Input/Output functions
#include <stdio.h>
#include <math.h>
bit update = 0; // Update the duty cycle.
// Timer 1 overflow interrupt service routine
interrupt [TIM1_OVF] void timer1_ovf_isr(void)
{
// Reinitialize Timer 1 value
// Update every 100ms.
TCNT1H=0xE9;
TCNT1L=0x8A;
update = 1;
}
Lab 5: Controls and Feedback
if( update )
{
// Read channel 0 adc.
adc_input = read_adc( 0x0 );
// Print the adc value.
//
printf( "Target Reading = %X\r", target_reading );
//
printf( "ADC Input = %X\r", adc_input );
// Find the difference between the adc input and
// the sensor reading.
error = adc_input - target_reading;
????/
Lab 5: Controls and Feedback
+5 V
Sensor
Ground
R
Vout
Delta V as a Function of R
R1 Value (Ohms)
Delta V (V)
Lab 5: Controls and Feedback
Theoretical Max Delta V as a Function of R
R1 Resistance (Ohms)
Delta V (V)
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Lab 5: Controls and Feedback
Response Time - Step Change from 30% LED Intensity to 80% LED Intensity
Time (s)
LED Intensity (ADC Integer Units)
iComiMeas
Lab 5: Controls and Feedback
R11kOhm R21kOhm
VCC
5V
Detector_Output
R
150 Ohm
LED
PWM_from_AVR
R41kOhm
R51kOhm
R61kOhm R71kOhm
VCC
5V
3_VDC_to_Aref
Lab 5: Controls and Feedback
If no switches are pushed, do the PID code.
Measure the LED intensity (iMeas)
Compare to commanded intensity (iCom)
Error = iCom ā iMeas
Limit error to +/- 10
New value of duty = old value of duty + error * 1.2.
Limit duty to range of 0 to 255
Write duty to PWM register
Output iCom, iMeas and duty on UART
Wait 50 ms to give ~ 10 Hz control time
If the switch is pushed, then change duty to new valueas determined by switch number
..... .. etc
Software minimum requirements:(1)
SW0-SW4 are used to set the target level for the control loop.according to the following table
(2)
LED0-LED4 are used to indicate to the user which switch has beenpressed.
(3)
The percentage of the LED intensity is sent to the computer terminalthrough the serial port.Switch
Target Level
SW0 (100%)
Full LED intensity
SW1 (75%)
no_LED +((full_LED - no_LED)*3/4)
SW2 (50%)
no_LED +((full_LED - no_LED)/2)
SW3 (25%)
no_LED +((full_LED - no_LED)*1/4)
SW4 (0%)
No LED intensity
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