Producing Low frequency PWM signals for low step motor speeds

[SuperveS]

Hi guys,
I have a PIC18F4550 , with a 20 Mhz Osc.
In order tho get the rotational speeds i need to produce a square signal with
in the range of 0-40 Hz.
Need your help....
I tried using the following link:
PIC PWM Calculator & Code Generator

the lowest frequency available in , according to the data sheet \ Flexible Oscillator Structure is
"8 user-selectable frequencies, from 31 kHz
to 8 MHz"

How is it possible to produce those low frequencies?
Thanks Guys!

 

[ronsimpson]

I don't understand your motor! Why 0-40hz?
This is hard. 20mhz/X=0 Thats makes X really really big.
Send more on the motor and why?

 

[SuperveS]

I am upgrading an "Instron" machine which tests mechanical qualities of materials. i.e - Tenslie, Compression and fatigue tests.
Instron : Materials Testing Machines for Tensile, Fatigue, Impact & Hardness Testing
for that i need really slow speeds from the motor. for example - if the tesion is applied at a rate of 0.5 Cm/Min - and i use a 1.8 deg. step motor - i need the pulses frequency to be 0.2 pulses/sec.
I can disconnect the external oscillator (20 Mhz) and use the "8 user-selectable frequencies, from 31 kHz to 8 MHz" (from the data sheet)

this is the motor i use:
**broken link removed**

Thanks!

 

[ronsimpson]

There must be 100 ways to do this.
1)Use the Secondary Oscillator to run the counters. The first OSC is running at 20mhz and the second OSC is running from a watch crystal 32,768khz. That is about 1000 times slower.
2)Use a software PWM. Its source could be the 32khz internal to the PIC the output of any of the timers. In software you can have more bits in the counters.

 

[Mike - K8LH]

I am upgrading an "Instron" machine which tests mechanical qualities of materials. i.e - Tenslie, Compression and fatigue tests.
Instron : Materials Testing Machines for Tensile, Fatigue, Impact & Hardness Testing
for that i need really slow speeds from the motor. for example - if the tesion is applied at a rate of 0.5 Cm/Min - and i use a 1.8 deg. step motor - i need the pulses frequency to be 0.2 pulses/sec.
I can disconnect the external oscillator (20 Mhz) and use the "8 user-selectable frequencies, from 31 kHz to 8 MHz" (from the data sheet)

this is the motor i use:
**broken link removed**

Thanks!

That really seems slow. Are you saying you're using a stepper motor and that 0.2 pulses/second or 12 pulses/minute, a mere 21.6° of rotation, will move the mechanism 0.5 Cm? I would think that they would be using some kind of gear reduction to reduce the amount of torque required by the motor. Isn't tensile strength sometimes measured in hundreds of pounds for some materials?

 

[be80be]

This Is good you'll need some

gear reduction

Ultimate tensile strength - Wikipedia, the free encyclopedia