motor speed variable using FPGA

What kind of motor? (stepper, DC, ....)

What kind of control for motor driver? (pulse, pwm, analog, ... )

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What kind of motor? (stepper, DC, ....)

What kind of control for motor driver? (pulse, pwm, analog, ... )

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Hi Cris72, I plan to use DC motor and pwm as a driver. Can u give an idea?

If you are only supposed to generate a pwm signal for open loop drive control, it's an easy task with fpga.

You basically need a counter+comparator and possibly some extra logic for setting the pwm level.

Whiile if you want a complete closed loop controller, your system must provide some sort of feedback signals

in order to adjust the pwm level in real-time.

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If you are only supposed to generate a pwm signal for open loop drive control, it's an easy task with fpga.

You basically need a counter+comparator and possibly some extra logic for setting the pwm level.

Whiile if you want a complete closed loop controller, your system must provide some sort of feedback signals

in order to adjust the pwm level in real-time.

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I have some different opinion about this.

If the stepper motor controler is supposed with PWM,the stepper motor's speed is relation with the pwm's frequency.That is to say,if I want to get the

variable stepper motor speed,I should get variable frequency of pwm.

If the input frequency of FPGA is 50MHz,using the counter++,may get those frequency,like 25MHz,12.5MHz,6.25Mhz,....and so on.

But,there is some frequency can no be reached,like 20MHz,or 999999Hz.

I do not kown how to deel with this.

Do you have any opinion about those?

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I have some different opinion about this.

If the stepper motor controler is supposed with PWM,the stepper motor's speed is relation with the pwm's frequency.That is to say,if I want to get the

variable stepper motor speed,I should get variable frequency of pwm.

If the input frequency of FPGA is 50MHz,using the counter++,may get those frequency,like 25MHz,12.5MHz,6.25Mhz,....and so on.

But,there is some frequency can no be reached,like 20MHz,or 999999Hz.

I do not kown how to deel with this.

Do you have any opinion about those?

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generate clock enable using modulo accumulator. add your word non-stop. then at overflow generate a pulse The equation for frequency becomes:

f = clk speed * accum word value/accum resolution

e.g. if accum is 32 bits and word is 2^30 and clock is 50MHz then:

f = 50 * 2^30/2^32 = 50/4 Mhz

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generate clock enable using modulo accumulator. add your word non-stop. then at overflow generate a pulse The equation for frequency becomes:

f = clk speed * accum word value/accum resolution

e.g. if accum is 32 bits and word is 2^30 and clock is 50MHz then:

f = 50 * 2^30/2^32 = 50/4 Mhz

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I konw what you mean.

e.g. if accum is 32 bits and word is 2^30 and clock is 50MHz then:

f = 50 * 2^30/2^32 = 50/4 Mhz.I can get 12.5Mhz frequency pwm,this is surely.

But if I want to get stable frequency of 12.4Mhz,or 12.6MHz,this way can not work.

There's another way to generate a frequency with a (almost) continous range.

You must use an adder with an overflow detector.

The registered adder output is fed back to one of the inputs, while the second input is a value proportional to the desired frequency.

For example, let's suppose a 32bit wide adder, with inputs A and B and output (registered) Q. Let P be the overflow output.

You connect Q back to A. At each clock pulse you have Q = Q + B, then the overflow pulse is generated every 2^32 / B clocks.

In other words, the frequency of P signal is Fclock x B / 2^32 which can be modulated with a good resolution, provided you don't get too close to Fclock.

This is easy, if you only need a frequency synthesizer.

Generating the actual pwm could be more tricky, especially if you need an exact duty cycle rather than fixed width pulses,

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I konw what you mean.

e.g. if accum is 32 bits and word is 2^30 and clock is 50MHz then:

f = 50 * 2^30/2^32 = 50/4 Mhz.I can get 12.5Mhz frequency pwm,this is surely.

But if I want to get stable frequency of 12.4Mhz,or 12.6MHz,this way can not work.

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you will get any frequency.

In many cases the exact pulse frequency wouldn't be as a clean clock but over a window of time it will be correct and for stepper motor it should be more than enough.

example 50MHz * 2333456/2^32 = ?

Anyway from 50MHz you cannot get exact frequencies except for cases like half/quarter etc unless you use variable analogue PLL

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There's another way to generate a frequency with a (almost) continous range.

You must use an adder with an overflow detector.

The registered adder output is fed back to one of the inputs, while the second input is a value proportional to the desired frequency.

For example, let's suppose a 32bit wide adder, with inputs A and B and output (registered) Q. Let P be the overflow output.

You connect Q back to A. At each clock pulse you have Q = Q + B, then the overflow pulse is generated every 2^32 / B clocks.

In other words, the frequency of P signal is Fclock x B / 2^32 which can be modulated with a good resolution, provided you don't get too close to Fclock.

This is easy, if you only need a frequency synthesizer.

Generating the actual pwm could be more tricky, especially if you need an exact duty cycle rather than fixed width pulses,

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Yes,maybe my expression is wrong.

In fact,I need generate fixed width pulses.

Your reply seems feasible.I will try it.