Friday, February 18, 2011

Easy pulse width modulation (PWM) with PICs to control brushless motor ESCs, servos, and other things

Today I present a brushless motor controlled by an electronic speed controller (ESC) and a PIC microcontroller. The PIC microcontroller performs the function of a R/C reciever by taking in a signal (here generated by a pot) and outputting the appropriate signal (here a PWM square wave) for the ESC to use to determine the rotation speed of the brushless motor.

Simple background: this brushless motor was bought at Hobbyking for less than $20 and can put out something like 600W. Although that is pretty high, it is not to be used for wheeled vehicles, which have highly dynamic loads and high current spikes. These brushless motors are intended for R/C planes' propellers, which have very steady and predictable loads. I have it connected to an electronic speed controller (ESC), which I bought, that dishes out the current at the right times to the brushless motor based on a simple pulse-width modulated signal it receives. The ESC is necessary because brushless motors operate in a totally different way from DC motors. Brushless motors are 3-phase, and as a result have three wires that need to be connected to a power source. Where you could just apply a steady voltage to DC motor and the commutator takes care of current alternation, brushless motors require more deliberate control. This control is not the subject of this post. I use a PIC to easily create and control a PWM signal using PIC's onboard timers for the ESC to interpret. So this post is basically about programming PICs specifically, to generate a PWM signal, of any flavor or kind if you like.

The signal generated here is a standard servo signal of 50 Hz (period 20 ms) with a square wave pulse that ranges in width from 1 to 2 ms.

What the software does: The software uses PIC's Timer1 and Timer0 and interrupts to make the signal, whose length varies (of the signal that can vary from 1 to 2 ms, the 50 Hz cycle is fixed) based on an analog input from a potentiometer. The PIC is constantly polling and converting the signal from a potentiometer to a value from 0 to 255, while in the background the Timers are counting down. Timer1 has a fixed time length (of course defined by me) of 20 ms, and Timer0 has a length from 1 ms to 2 ms, determined by the ADC. Timer1 dictates when the pulse, whose length is determined by Timer0, occurs. Notice that every cycle there is at least a period of 18 ms where the signal is low. The timers counting down and the potentiometer signal polling occurs simultaneously. When the Timers countdown, interrupts stop the PIC from whatever it was doing, executes a specified procedure, and then the PIC resumes whatever it was doing. In this case, the interrupts would be used to stop and start signals.

The c code:
//Using default Internal Clock of 4 Mhz
//To use TMR1 at 50 Hz and 1:1 prescaler, set offset to 45535.
//TMR0 can vary from 7 to 132, or 2 ms to 1 ms, at 1:1 prescaler.
//Use TMR0 to control pulse width, even though TMR1 has much higher resolution, because TMR1 offset is 16 bits and may require lots of time to compute proper TMR1 offset from ADC value.


unsigned int pulse = 0;

//Interrupt function
static void interrupt isr(void)
RC0 = 0;
TMR1IF = 0;
//Start 50 Hz cycle period.
TMR1L = 0b11011111;
TMR1H = 0b10110001;
//Turn on variable pulse.
RC0 = 1;
if(RC1 == 1) //safety switch (must be pressed).
TMR0 = pulse;
T0IF = 0;
else //off.
TMR0 = 132;
T0IF = 0;

void main(void)
//Setting ports.
TRISA = 0x00001001; // Set A0 (for ADC) and A3 (MCLR) to input, others output
PORTA = 0x00;
TRISC = 0b00000010; //Set C1 to input for safety on switch.
PORTC = 0x00;
//ADC configuration.
ADCON1 = 0x00;//So that ADC conversion clock is set to Fosc/2.
ADCON0 = 0b00000001;//ADC enabled.
ANSEL = 0x01; // Set A0 to analog, others digital

//Timer configuration
OPTION = 0b00000010; // Timer0 configuration, 1:8 prescaler.
TMR1L = 0b11011111;
TMR1H = 0b10110001;
T1CON = 0b00000001; // Timer1 configuration, including 1:1 Prescaler and TMR1ON = 1.

//Timer interrupt enabling.
T0IE = 1;
TMR1IE = 1; // Timer1 interrupt enable, needed with PEIE and GIE bits set.
PEIE = 1; // Peripheral interrupt enable, for Timer1.

//General interrupt enabling.
GIE = 1; // Global interrupt enable, for Timer1 and Timer0.

GODONE=1; // initiate conversion.
while(GODONE) continue; // Wait for conversion to finish.
pulse = 7+ADRESH*125/255;

The c code is turned into a hex file via MPLAB and I program the microcontroller with the PICKit 2. The microcontroller is a PIC16F690. Analog-to-digital conversion signal (from potentiometer) input is at A0 and ouput PWM signal is at C0. There is an input at C1 for a momentary pushbutton, which needs to be held in order for an input at C1 to be high. This is for safety, as I want it to be easy to turn the motor off by just letting go of the pushbutton. So no matter how high a speed the potentiometer is set to, nothing will happen unless the pushbutton is held.

Since the ADC gives values from 0 to 255, the resolution, or how many speed levels, you have is limited to 256. So it does not matter whether you use Timer0 or Timer1 for determining the signal length (Timer1 has a much higher temporal resolution). I use Timer0 because it probably takes less time for the PIC to do the arithmetic of converting the ADC value to the appropriate number. Timer0 uses 8 bits to define counter number (up to 255 in decimal), while Timer1 uses 16 bits (up to 65535 in decimal).

Remember, when using PICs as standalone portable devices, two things should be done: there should be a capacitor across the power supply pins (here, .1 microFarad, and yes this is absolutely necessary, else the PIC just keeps turning off and on) and a resistor tying MCLR pin to the power supply (here 10 kOhm). Also, I tied the input C1 pin to ground, so that electrical noise wont make it go high and low unintentionally.

Wednesday, February 16, 2011

Putting together a prototype/validation for CFD...

This is my second foray into building something mechanical, my previous venture utilizing only junkyard parts to build a testing platform for something similar to the Fanwing last summer (this is before I even touched CFD, and is incidentally why I picked up CFD). It ended up weighing 80 pounds, using a 5hp motor I had used in 8th grade to power a massive, hot roller mill for making rubber bands for a science fair project (10+ ingredient chemical recipes, fun times) and v-belt transmission for a cross-flow-fan test section of about a foot in diameter and less than 2 feet in span (picture of it here maybe if i remember). I guess it ended up not being very useful, but for the amount of metal-working and welding I had to do, I have at least a modicum of pride in it.

Anyways this time I used all of the internets to find non-junkyard parts for use in conjunction with a water jet cutter, which is awesome and I now have access to, to build a prototype for CFD validation. The design in Autodesk Inventor (which, unlike SolidEdge ST3, allows easy use and integration of variables for dimensions so you do not have to redo your whole design if you need changes in a few parts) looks very tight and I am excited to build it.

Anyways, I always post to share something that I think may be useful. For this post, it is some vendors in case you are planning to build something:
Main vendors:
- McMaster-Carr (Everything mechanical)
- Hobby King (Motors, batteries, etc.)
Places that had stuff that the main vendors did not have:
- SDP SI (Has a whole lot of mechanical stuff, but I used them for their extensive variety of timing belt pulleys)
- Fastenal (For 4-40 square nuts, and if you do not want to buy set screws or square nuts, etc. by the 100's or more)
- Digikey (Angle brackets; usually an electronics supplier; I bought PIC microcontrollers from these guys before; Use google to search their stuff)

Although this post feels pretty unsubstantial, it is but the calm before the storm of substance, if you will. I will be posting more about my mechatronic CFD validation, uhh... machine.