1 Lab 5: Controls and feedback. 2 Lab 5: Control and Feedback This embedded system uses the Photo...

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Lab 5: Controls and feedback

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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 a resistor other than exactly 2K for better sensitivity

22K2K

3

ATMEGA16L

VTG

10K150

SW0

VTG

10K150

SW1

PB0

PB1

PC0

RXD

TXD

To Computer Serial Port

14

15

1

2

22LED0

VCC

VTG

10K

150

VTG

10K150

SW2

PC123

PB23

VTG

10K150

SW3

PB34

VTG

10K150

SW4

PB45

LED1

VCC

VTG

10K

150

LED2

VCC

VTG

10K

LED3

VCC

VTG

10K

LED4

VCC

VTG

10K

150 150 150

PC224

PC325

PC426

OC2(PD7)

21

130Sensor

200K

LED

ADC0(PA0)

40

VTG

V_Feedback

R2

R1

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Lab 5: Controls and Feedback

• PID control– P: Proportional

• The error signal (error = desired – current) multiplied by a constant and fed out to the drive. (proportional = gain * error).

– I: Integral• The integral term is the sum of past errors, so adding the

past errors will eventually drive the output closer to the desired output

– D: Derivative• Differentiator uses the derivative (rate of change) to predict

the future behavior.

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In this lab, the plant’s function is to generate a desired level of ambient light. The sensor measures ambient light with a Cadmium Sulfide (CdS) photocell. The sensor measures the plant’s performance.

controllerPWM

r(t) u(t) y(t)plant h(t) LED

1

+-e(t)

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• The choice of a controller depends on the application’s requirements.

• This lab builds an application that uses the duty cycle of the PWM signal to dynamically adjust ambient light levels.

• The controller increases the duty cycle of the PWM in order to apply more power to the LED and thereby increase its intensity. The PWM is the control, u(t), that is fed into the plant (that is, LED).

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• Changes to the LED’s intensity can occur as fast as the mcu computes and updates the PWM duty cycle register (OCR2). For this application one updates every 100 milliseconds is more than sufficient. One milli-second is very slow compared to how fast ambient light fills a room.

• At speeds of human perception 100 milliseconds is fast but detectable. These timing considerations drive the requirement for only needing to use a proportional controller. Hint: A maximum change of 2% to 4% to the duty cycle every 50 - 100 ms implements a nice smooth transition of the LED’s intensity

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ATMEGA16L

VTG

10K150

SW0

VTG

10K150

SW1

PB0

PB1

PC0

RXD

TXD

To Computer Serial Port

14

15

1

2

22LED0

VCC

VTG

10K

150

VTG

10K150

SW2

PC123

PB23

VTG

10K150

SW3

PB34

VTG

10K150

SW4

PB45

LED1

VCC

VTG

10K

150

LED2

VCC

VTG

10K

LED3

VCC

VTG

10K

LED4

VCC

VTG

10K

150 150 150

PC224

PC325

PC426

OC2(PD7)

21

130Sensor

200K

LED

ADC0(PA0)

40

VTG

V_Feedback

R2

R1

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• SW0-SW4 are used to set the target level for the control loop.

• LED0-LED4 are used to indicate to the user which switch has been pressed.

• The percentage of the LED intensity is sent to the computer terminal through the serial port.

• Switch Target LevelSW0 (100%) Full LED intensitySW1 (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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• Timer2(PWM)• Timer2 is used to setup the Pulse Width

Modulation (PWM) for the LED. A clock rate of 460KHz and a non-inverted PWM can be selected.

• The pulse width is determined by the 8-bit value in the OCR2 register. 0xFF is maximum pulse width and the initialization value of 0x80 has a pulse width of 1/2 cycle.

• You can choose any other values to suit your design.

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Lab 5: Controls and Feedback• Initialization code for Timer2:• // Timer/Counter 2 initialization• // Clock source: System Clock• // Clock value: 460.000 kHz• // Mode: Phase correct PWM top=FFh• // OC2 output: Non-Inverted PWM• TCCR2=0x62;• ASSR=0x00;• TCNT2=0x00;• OCR2=0x80;• The output for Timer2 PWM mode is PD7. Bit 7 of Port D is set up to be an output.• Initialization code for Port D:• // Port D initialization• // Set up PD7 as the output driver to the external LED• PORTD=0x00;• DDRD=0x80;

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Lab 5: Controls and Feedback• Code to find the light with no LED and full LED intensity:• // Find ambient light with no LED output.• OCR2 = 0x00;• delay_ms( 2000 );• no_LED = read_adc( 0x0 );•• // Find ambient light with full LED output.• OCR2 = 0xFF;• delay_ms( 2000 );• full_LED = read_adc( 0x0 );• // Initialize variables for 50%.• PORTC = 0x1B; //11011 turns on LED2• target_reading = no_LED +((full_LED - no_LED)/2);•

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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;• }

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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;• • ????/

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+5 V

Sensor

Ground

R1

Vout

Delta V as a Function of R1

0

0.2

0.4

0.6

0.8

1

1.2

0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

R1 Value (Ohms)

Del

ta V

(V

)

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Theoretical Max Delta V as a Function of R1

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500

R1 Resistance (Ohms)

Del

ta V

(V

)

17


Response Time - Step Change from 30% LED Intensity to 80% LED Intensity

720

740

760

780

800

820

840

860

0 5 10 15 20 25

Time (s)

LE

D In

ten

sit

y (

AD

C In

teg

er

Un

its

)

iCom

iMeas

18


R11kOhm

R21kOhm

50%

VCC

5V

Detector_Output

R3150 Ohm

LED1

PWM_from_AVR

R41kOhm

R51kOhm

R61kOhm

R71kOhm

VCC

5V

3_VDC_to_Aref

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• If no switches are pushed, do the PID code.– Measure the LED intensity (iMeas)– Compare to commanded intensity (iCom)– Error = iCom – 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 value as determined by switch number .. . . . .. etc

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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 been pressed.

(3) The percentage of the LED intensity is sent to the computer terminal through the serial port.

Switch Target LevelSW0 (100%) Full LED intensitySW1 (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

1 Lab 5: Controls and feedback. 2 Lab 5: Control and Feedback This embedded system uses the Photo...

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