Lab 5: Control and Feedback. Lab 5: Controls and feedback. Lab 5: Controls and Feedback

Transcription

1 Lab : Control and Feedback Lab : Controls and feedback K K You may need a resistor other than exactly K for better sensitivity This embedded system uses the Photo sensor to detect the light intensity of the environment and adjusts the light emitted by the LED to maintain a constant light intensity environment. PB0 SW0 SW SW SW SW To Computer Serial Port PB PB PB PB ATMEGA6L RXD TXD PC0 PC PC PC PC ADC0 (PA0) OC (PD7) LED0 LED LED LED LED 6 0 R 0 LED V_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. R 00K

2 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). 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. 6 Changes to the LED s intensity can occur as fast as the mcu computes and updates the PWM duty cycle register (OCR). For this application one updates every 00 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 00 milliseconds is fast but detectable. These timing considerations drive the requirement for only needing to use a proportional controller. Hint: A maximum change of % to % to the duty cycle every 0-00 ms implements a nice smooth transition of the LED s intensity 7 SW0 PB0 PB PB PB PB ATMEGA6L RXD To Computer Serial Port TXD PC0 PC PC PC 6 PC ADC0 0 (PA0) OC R (PD7) 0Ω LED SW SW SW SW LED0 LED LED LED LED V_Feedback R 00KΩ 8

3 SW0-SW are used to set the target level for the control loop. LED0-LED 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 Level SW0 (00%) Full LED intensity SW (7%) no_led +((full_led - no_led)*/) SW (0%) no_led +((full_led - no_led)/) SW (%) no_led +((full_led - no_led)*/) SW (0%) No LED intensity Timer(PWM) Timer is used to setup the Pulse Width Modulation (PWM) for the LED. A clock rate of 60KHz and a non-inverted PWM can be selected. The pulse width is determined by the 8-bit value in the OCR register. 0xFF is maximum pulse width and the initialization value of 0x80 has a pulse width of / cycle. You can choose any other values to suit your design. 9 0 Initialization code for Timer: // Timer/Counter initialization // Clock source: System Clock // Clock value: khz // Mode: Phase correct PWM top=ffh // OC output: Non-Inverted PWM TCCR=0x6; ASSR=0x00; TCNT=0x00; OCR=0x80; The output for Timer 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; Code to find the light with no LED and full LED intensity: // Find ambient light with no LED output. OCR = 0x00; delay_ms( 000 ); no_led = read_adc( 0x0 ); // Find ambient light with full LED output. OCR = 0xFF; delay_ms( 000 ); full_led = read_adc( 0x0 ); // Initialize variables for 0%. PORTC = 0xB; //0 turns on LED target_reading = no_led +((full_led - no_led)/);

4 #include <mega6.h> // Standard Input/Output functions #include <stdio.h> #include <math.h> bit update = 0; // Update the duty cycle. // Timer overflow interrupt service routine interrupt [TIM_OVF] void timer_ovf_isr(void) { // Reinitialize Timer value // Update every 00ms. TCNTH=0xE9; TCNTL=0x8A; update = ; } 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;????/ Delta V as a Function of R. Theoretical Max Delta V as a Function of R + V R Vout Delta V (V) Delta V (V) Ground R Value (Ohms) R Resistance (Ohms) 6

5 860 Response Time - Step Change from 0% LED Intensity to 80% LED Intensity V PWM_from_AVR V LED Intensity (ADC Integer Units) icom imeas 0% Detector_Output R R R Ohm LED R R6 R7 R _VDC_to_Aref Time (s) 7 8 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 +/- 0 New value of duty = old value of duty + error *.. Limit duty to range of 0 to Write duty to PWM register Output icom, imeas and duty on UART Wait 0 ms to give ~ 0 Hz control time If the switch is pushed, then change duty to new value as determined by switch number etc Software minimum requirements: () SW0-SW are used to set the target level for the control loop. according to the following table () LED0-LED 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 SW0 (00%) SW (7%) SW (0%) SW (%) SW (0%) Target Level Full LED intensity no_led +((full_led - no_led)*/) no_led +((full_led - no_led)/) no_led +((full_led - no_led)*/) No LED intensity 9 0