RTFM

I made a start on my PWM control software this evening. The first thing that I discovered was that I had connected the motor drive to the wrong pin on the MSP430F2013. There are two pins that can be driven from the timer, TA0 and TA1, but only TA1 is capable of PWM. I should have studied the datasheet more carefully! Easily fixed but a bit of a time waster.

I wrote a small test program to try different PWM duty cycles and frequencies :-

#include "msp430x20x3.h"

#define LED           BIT0
#define MOTOR         BIT2
#define FLOATING      (BIT1 + BIT3)
#define CURRENT       BIT4
#define GND           BIT5
#define I2CPINS       (BIT6 + BIT7)

#define OUTPUTS       (I2CPINS + MOTOR + LED + FLOATING)

int main( void )
{
WDTCTL = WDTPW + WDTHOLD;             // Stop watchdog timer
//
// Set MCLK to 16 MHz, SMCLK = 2 MHz
//
BCSCTL2 = SELM_0 | DIVM_0 | DIVS_3;   // MCLK = DCO, SMCLK =  DCO / 8
BCSCTL1 = CALBC1_16MHZ | XT2OFF;
DCOCTL  = CALDCO_16MHZ;
//
// Set up I/0
//
P1OUT = MOTOR;                        // Motor off
P1DIR = OUTPUTS;                      // Define output pins
P1SEL = MOTOR;                        // Motor = TA2 output
//
// Set up timer
//
TACCR0 = 20000;                       // 2 MHz / 100
TACCR1 = 15000;                       // 25%
TACCTL1 = OUTMOD_7;                   // Reset / set
TAR = 0;                              // Start counting from zero
TACTL = TASSEL_2 + MC_1 + TACLR;      // SMCLK, upmode, clear
}

I found that 50-100 Hz seems to work well. 1KHz is too fast as the current doesn't have chance the build up to its full value due to the motor winding inductance.

Here is a graph of motor current versus duty cycle with no mechanical load :-


As the duty cycle gets smaller the motor slows down so it generates less back e.m.f. making the current increase.

I chose a current sense resistor of 0.27Ω which gives voltages up to 1.75V however the full scale voltage of the ADC is only 0.6V so I could do with it being smaller. I could just attenuate it with a potential divider but as it gets hot and wastes power it is better to reduce its value to 0.1Ω. Again, I should have paid more attention to the datasheet. I guess I will be taking a trip to Maplin tomorrow lunch time.

Less speed more torque

This weekend I built a speed controller for my milling spindle motor, AKA Minicraft drill, after attending and recovering from the Stockport Beer & Cider Festival . I used one of the MSP430F2013 micros I described in the previous post. I built it on Veroboard, but hopefully all my future boards will be milled PCBs.



I never plan my Veroboard layouts in advance, I just make it up as I go along. In fact this circuit was so simple I didn't even draw a schematic. I did come a cropper this time though. I started on a board that was too small!



I only realised this after I had put on a regulator, LED and connector, so I had to desolder them and start again. The LED did not survive the ordeal. In my experience, they are one of the most fragile electronic components. They certainly don't like stress on their legs while being soldered.

The circuit is very simple, I will publish a schematic and the software when I complete it, assuming it works, which I have no reason to think it won't.

You may think "how is that puny little micro controlling a 40W motor"? What you can't see in the picture above are two surface mount FETs on the underside.



The big one is a BTS134D "smart low side power switch" with over voltage, over current, thermal and ESD protection. It has an on resistance of only 50mΩ so I can get away without a heatsink. With devices like these I don't know why anybody uses Darlingtons for switching nowadays. Darlingtons have a fundamental flaw in that they cannot be saturated so the on voltage is over a volt leading to significant power dissipation, and hence a large heatsink, for these sorts of currents.

The tiny FET next to it simply boosts the gate drive from the 3V output of the micro up to 12V to ensure the big FET delivers the smallest possible on resistance.

The picture below shows the board being tested. It is connected to a 39W PSU, the drill motor and a scope.



I have verified that it can turn the motor on and off OK. It just needs some software now. The micro has a timer with a PWM facility so controlling the speed of the motor should be pretty simple. The large resistor at the bottom left is for sensing the motor current. The micro has an ADC so I should be able to measure the speed to allow some feedback and also shut it off if it stalls. The unpopulated connector is the I²C link which will go to HydraRaptor's main controller.

PIC your micro

The official RepRap project uses Microchip PIC micro controllers. I have done several projects at work with these because they have such a wide range of parts that they often have the best fit price wise. However, I have never liked them very much. They have a horrible instruction set which does not lend itself to running C efficiently. Also, the development kit is a bit primitive by today's standards. When doing a home project ease of use and development kit cost becomes more important than part price. I have been following the RepRap forums and people are having lots of problems with a buggy open source PIC compiler and programmer.

I recently did a project at work which needed a micro with a very low power consumption so we used a Texas Instruments MSP430 series micro. I was very impressed with the USB key development kit which only costs $20. For that you get an IAR C compiler, assembler, simulator and debugger. The instruction set of the chip is very small but has just the right instructions to run C efficiently and the C compiler does a good job of using them. The code size is limited to 4K but, as I am only using a 2K part, that doesn't matter. The USB key contains a programmer / "Spy Bi-Wire" in circuit debugger and a little detachable target board. The chip is surface mount but the target board brings all the pins out to a 0.1 inch header so you can incorporate it into a through-hole board and you can get additional target boards at $10 for three.





So for $30 I get a development kit and micros to control four of HydraRaptor's heads.

The smaller parts don't have UARTs but they do have I²C which is easier to make a multi-drop bus with.