Pulse Width Modulation Applications

Transcription

1 Pulse Width Modulation Applications Lecture 21 EE 383 Microcomputers

2 Learning Objectives What is DTMF? How to use PWM to generate DTMF? How to use PWM to control a servo motor? How to use PWM to control a DC motor?

3 PWM Applications Pulse-Width Modulation (PWM) can be used with any application that requires generation of a periodic rectangular waveform. With additional filter circuitry, rectangular waveforms can be used to generate triangular, sinusoidal, and other waveform types. Design of these filters is outside the scope of this class. PWM applications include: Servo control (standard or continuous-rotation) DC motor speed control Generation of audio tones Many others

4 Generating Audio Tones A pure audio tone is a sinusoid of a particular frequency. Sinusoids can be roughly approximated by 50% duty-cycle rectangular waves. The approximation can be improved by filtering the rectangular wave. The range of tone frequencies for human hearing falls roughly in the range 20Hz 20 khz. The extremes of this range are quite variable by individual. Aging tends to reduce the range, especially at higher frequencies. The Dragon12 PWM can easily generate waveforms in the 20Hz 20 khz range and, with a proper transducer (e.g. a speaker), can be used to produce audio output. The quality of this square-wave audio may be questionable.

5 Touch-Tone Telephone */E 0 #/F A B C D * 0 # Original Touch-Tone Keypad Modern Touch-Tone Keypad

6 DTMF Touch-Tone Standard Dual-Tone Multiple-Frequency (DTMF) encoding is used to transmit the state of the keypad when a button is pressed. Each keypress is indicated by a unique combination of two sinusoidal frequency components. One sinusoid indicates the keypad row and the other indicates the keypad column. Each tone must be held for a minimum of 40 ms.

7 DTMF Frequencies 1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz A 770 Hz B 852 Hz C 941 Hz */E 0 #/F D

8 Approximating DTMF with PWM DTMF requires the sum of two sinusoids. PWM produces rectangular waveforms. Two PWM channels (row and column) with appropriate low-pass filters may be used to create the DTMF sinusoids.

9 Dragon12plus - First Approximation of DTMF For a simple approximation of DTMF on the Dragon12plus, we can take advantage of the fact that we can set a separate frequency for each PWM channel. In addition, the Dragon12plus laboratory kits include two small piezoelectric speakers equipped to plug directly into the servo connectors on the Dragon12plus. Driving these two speakers with two PWM square-wave (50% duty cycle) signals is an acceptable first approximation for DTMF.

10 Dragon12 PWM Speakers

11 Selecting DTMF frequencies Frequency (Hz) Period (µs) 24 MHz cycles 697 Hz µs cycles 770 Hz µs cycles 852 Hz µs cycles 941 Hz µs cycles 1209 Hz µs cycles 1336 Hz µs cycles 1477 Hz µs cycles 1633 Hz µs cycles Because of the tight constraint on period accuracy (1.5%) we should consider using the concatenated PWM channels with 16-bit period counters for greater accuracy.

12 Selecting PWM parameters for DTMF Frequency (Hz) Period (µs) 24 MHz cycles (PWMPER/PWMDTY) 697 Hz µs cycles (34434/17217) 770 Hz µs cycles (31168/15584) 852 Hz µs cycles (28170/14085) 941 Hz µs cycles (25504/12752) 1209 Hz µs cycles (19852/9926) 1336 Hz µs cycles (17964/8982) 1477 Hz µs cycles (16250/8125) 1633 Hz µs cycles (14696/7348) The period counts here are the closest even integer so that the duty cycle counts provide for exactly 50% duty cycle. This will produce a sinusoidal signal with minimum distortion after filtering.

13 Setting up PWM for DTMF Let us choose to use concatenated PWM channels to give us the needed accuracy. Use PWM channels 4 and 5 for the DTMF row. Output is on pin 5. Use clock A or SA. Use PWM channels 6 and 7 for the DTMF column. Output is on pin 7. Use clock B or SB.

14 PWM Setup Code For DTMF BSET PWMCTL, #$C0 BSET PWMPOL, #$F0 BCLR PWMCLK, #$F0 BCLR PWMPRCLK, #$FF ;set CON67 and CON45 ;4,5,6,7 active high ;50% duty cycle so polarity doesn t matter ;4,5,6,7 use clock A/B ;clock scale = 2^0, clock is 24 MHz ;clock period now (1/24) microseconds MOVW #34433, PWMPER4 ;697 Hz, signal generated at CH5 MOVW #17217, PWMDTY4 ;50% duty cycle MOVW #14696, PWMPER6 ;1633 Hz, signal generated at CH7 MOVW #7348, PWMDTY6 ;50% duty cycle BSET PWME, #$F0 JSR DELAY_1s BCLR PWME, #$F0 ;4,5,6,7 enabled ;TURN OFF

15 Motor Control Applications Motors convert electrical energy into mechanical energy. Motor control is normally concerned with one or more of the following: Motor shaft position Motor shaft rotational speed Available rotational power (torque) Control is usually optimized for one of these factors and it is generally not possible to control all three quantities simultaneously. PWM is easily adapted to two simple motor types, DC motors and Servo motors.

16 Servo Motors Servo motors employ internal feedback control electronics to allow the motor to be controlled by a simple pulse sequence. The servo motor control signal has a fixed frequency of 50 Hz and a duty cycle that varies from (nominal) 5% to 10%. Varying the duty cycles causes the servo motor to respond by changing some aspect of its operation. For a standard servo, the control signal is used to determine the position of the motor shaft. This is often constrained to less than one full revolution. Rotational speed is typically low. Torque is typically high. For a continuous-turn servo the control signal is used to determine rotational speed. Shaft position is uncontrolled. Torque is typically high.

17 Servo 3-pin connector GND, 5V, signal

18 Port P Servo Headers on Dragon12 3-pin connectors GND, 5V, signal (PWM)

19 Servo Connections The typical electrical connection for a servo motor is a 3-wire interface: Ground Power (typically 5-6V DC) Control input (50Hz, nominal 5%-10% duty cycle) On the servo motors purchased from the ground connection is a black wire, the 5V connection is a red wire, and the control input is a white or yellow wire. The Dragon12plus provides 4 connectors that provide servocompatible control. The signal lines for these connectors are Port P pins 4-7.

20 Driving Servos from Dragon12plus The Dragon12 provides 4 connectors that provide servo-compatible control. The signal lines for these connectors are Port P pins 4-7. Servo motors can draw considerable power, especially when loaded. Care must be taken when driving servos directly from the Dragon12plus to prevent overloading the onboard voltage regulator. If you plug in a servo to the Dragon12plus board and it draws too much power, it is likely that the Dragon12plus will reset when the output of the voltage regulator is pulled too low. When you are asked to use servos in the laboratory, a separate power board for the servos will be provided.

21 Controlling a Standard Servo Motor The position of the servo motor shaft is controlled by a single pulse-width modulated control signal. A 50 Hz pulse train with a period of 20ms with an active high pulse of 1.5ms (7.5% duty cycle) will cause the servo to move to the center of its range of motion then hold the shaft at that point. Longer duty cycles (> 1.5ms/20ms) will cause the servo to rotate counterclockwise. Shorter duty cycles(< 1.5ms/20ms) will cause the servo to rotate clockwise. The official valid range of duty cycles is from 5% (1ms/20ms) to 10% (2ms/20ms). The relative position of the servo is proportional to the difference from the center point to the extremes.

22 Controlling a Continuous-Rotation Servo The speed and direction of the servo are controlled by a single pulsewidth modulated control signal. A 50 Hz pulse train with a period of 20ms with an active high pulse of 1.5ms (7.5% duty cycle) will cause the servo to hold at a constant fixed point. Longer duty cycles (> 1.5ms/20ms) will cause the servo to rotate counterclockwise. Shorter duty cycles(< 1.5ms/20ms) will cause the servo to rotate clockwise. The official valid range of duty cycles is from 5% (1ms/20ms) to 10% (2ms/20ms). The relative speed of the servo is proportional to the difference from the center point to the extremes.

23 Example Code MOVB #$30, PWMPRCLK ;PWM CLOCK B SCALE = 8 MOVW #60000, PWMPER6 ;PWM CHANNEL 6&7 PERIOD = 20ms MOVW #4500, PWMDTY6 ;PWM CHANNEL 6&7 DUTY CYCLE 7.5% BSET PWMCTL, #% ;CONCATENATE 6&7 BSET PWMPOL, #% ;CHANNEL 6&7 ACTIVE HIGH BCLR PWMCLK, #% ;CHANNEL 6&7 CLOCK B BSET PWME, #% ;ENABLE PWM CHANNEL 6&7

24 DC Motor Control Generally, a DC motor is designed so that when a constant voltage is applied to the motor, the shaft will turn at a constant speed. A DC motor will be rated to have a maximum input voltage. A DC motor may be operated at a reduced voltage to produce a reduced speed. However, this is very inefficient and below a certain level will not work at all as the applied voltage will not produce sufficient torque to turn the motor, especially when loaded. A better way to control the speed of a DC motor is to rapidly turn it fully off and fully on to produce a desired average speed while maintaining full torque while it is on. PWM is appropriate for this.

25 Controlling a DC Motor using PWM The duty cycle of a PWM control for a DC motor determines the fraction of the maximum supplied energy provided to the motor to make the shaft turn. If the duty cycle is chosen too short, the motor may not be able to develop enough torque to make the shaft turn before the pulse ends. If the period is chosen too long, the motor will stop between pulses rather than continuing to spin from inertia. The appropriate PWM period and duty cycle are very dependent on the physical properties of the motor. Larger motors have greater inertia so, in general, pulses must have longer duration than for small motors to overcome the inertia initially but need shorter pulses to maintain rotation.

26 Driving DC motors from Dragon12plus The PWM outputs on the Dragon12plus are very limited in the amount of current they can provide. Attempting to drive a DC motor directly from the Dragon12 is likely to damage the microprocessor I/O pins. It is important to buffer the PWM outputs so that the drive current for the DC motor does not come directly through the microprocessor. The DC motors you will utilize in the laboratory will be accompanied by a circuit using a ULN2803A buffer.