Spindle controller

RPM sensor

Measuring the speed of the spindle is done using an IR sensor. The hard part is finding a suitable location inside the router where the IR receiver can look at the spinning axle. In the Kress, there is a small black plastic "wheel" at the top end of the axle, just before the bearing. It has small indentations, so it is possible that it is was intended for measuring the speed, if it were painted.

The sensor/diode combination used is a Vishay TCRT5000. I tried a TCRT1000 as well, but its range was insufficient to obtain a reliable pulse signal. In the picture below, you can see the sensor mounted on a piece of protoboard and the white dot (corrector fluid) on the plastic wheel, just above the motor's armature.

To keep things simple, I'm using a single pulse per revolution. For a range between 2000 to 30.000 rpm, this translates to pulses with a frequency of 33 Hz to 500 Hz.

Triac driver

The application of triacs as a dimmer or motor controller is widespread; they can be found in appliances such as vacuum cleaners and light dimmers. If no specific control is necessary, a potmeter can be used to control the triac's phase angle, but we want to have a feedback loop through the microcontroller.

For the triac, I chose a BTA08-600, which can handle 8A at 600Vrms. Should be plenty.

It is possible to drive the triacs gate directly from a microcontroller if the triac is logic-level compatible. This means that the gate is more sensitive and can trigger faster (with a smaller current). But if this is not the case, there exist triac drivers that include both the driver circuit and an optocoupler to isolate the circuit parts, such as the MOC3022. Below is an application example from the datasheets, which fits perfectly for what we have in mind.

Microcontroller and peripherals

The STM8S is an 8 bit microcontroller from ST. Its major point is that it includes lots of onboard peripherals such as timers of various complexity. The timers can be configured extensively so that almost no support code is necessary during their operation.

In this case, we want to count the amount of time between pulses from the IR sensor to derive the router speed (an alternative is counting pulses in a certain interval but this is less accurate at low speeds). It involves using TIMER1 in Slave-reset mode, so that it resets on an interrupt on its input channel. The current counter value is then stored in its capture-compare register. I have already covered the code here.

RPM Timer

To cover the possible range of speeds with adequate accuracy, the timer is run at 2Mhz. At 30.000rpm this relates to 4000 pulses and at 2000 rpm 60.000 pulses:

TIM1_TimeBaseInit(7, TIM1_COUNTERMODE_UP, 65535, 0);

The measured intervals are stored in an array with moving index, and then averaged when the RPM value is calculated. To limit computation time with the division involved, everything is kept as an integer.

const u32 MultFactor = 120000000;

for (i=0;i<NBSAMPLES;i++){

    avg = avg + measuredValues[i];

}

avg = avg>>3; //Divide by 8

rpm = MultFactor/avg; //Convert to rpm as integer (accurate enough)

Triac phase angle timer

AC zero crossings are detected with a GPIO rising interrupt. At that point,TIMER2 is configured to count until the current phase angle setting, and started. As we only have to deal with intervals up to 25 Hz or 10ms, we choose the prescaler at 16. The phase angle value can therefore move in steps of 1us.

TIM2_TimeBaseInit(TIM2_PRESCALER_16, MAX_PHASE_ANGLE);

void EXTI_PORTA_IRQHandler(void) interrupt 3{

    TIM2->ARRH = (u8)(triacDelay >> 8);//Set the Repetition Counter value to the ignition wait-time of the triac

    TIM2->ARRL = (u8)(triacDelay);

    TIM2_SetCounter(0);//Ensure timer is zero

    TIM2_Cmd(ENABLE);//Start timer

}

At the TIMER2 update interrupt, the timer is stopped and an ignition pulse is created. Some nop()'s were inserted to ensure the pulse is long enough.

if (outputEnabled){

    GPIO_WriteHigh(GPIOG,GPIO_PIN_1);//Trigger triac

}

TIM2_Cmd(DISABLE);//Stop timer

TIM2_ClearITPendingBit(TIM2_IT_UPDATE);//Clear interrupt

nop(); nop(); nop();

GPIO_WriteLow(GPIOG,GPIO_PIN_1);//Stop trigger

Display and input

For the triac and its snubber, I used the following parts from Farnell:

1200784 CAPACITOR, 47NF, 630V for C4

1519308 CAPACITOR, 0.01UF, 630V, 10% for C2

1519302 CAPACITOR, 1000PF, 630V, 10% for C1

1357851 RESISTOR, 0.5W 5% 47R for R3

The resistor is on the small side, dissipation-wise, but it is only loaded during short-lived peaks.

Test video