Fast Response Low Pass Filter

rjenkinsgb said:

Back to the basics - what is the source of the PWM and the duty cycle control input to that?

Real-world machine factory machine tools & robotics systems in general either use a DAC to directly produce the command voltage for an analog-setpoint servo drive, or the "servo drive" is digital with the setpoint transferred as data.

Either way, the only PWM is in the output stages that drive the motor.

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The pwm will be produced by a microcontroller circuit or a control center such as a plc. The speed of the motor will be decided with dutycycle and various processes will be done. this was entirely considered for the simplicity of the procedure. otherwise, there are many methods available, but it was concluded that this is the easiest way for the user to experience the least difficulty.

Yes, I did some design work over the weekend and I decided that this process would be best done using DAC and microprocessor. I've tried all of the circuit simulations and suggestions above, but didn't get exactly what I wanted. I will try to set up the circuit with a dac as soon as possible.

Nigel Goodwin said:

Well now you've explained what you're trying to do - all becomes clear. You're trying to control a slow mechanical process, so you have no speed issues at all - use a low-pass filter, job done. Your original question makes little sense, because there's no context, and your original idea is mistaken - you've no need (or use) for such speed as you envision.

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In fact, it will sometimes be a slow and sometimes fast process, but it will perform very sensitive processes. That's why I want it to respond to sudden speed changes as quickly as possible. The fastest possible response time also means that it can respond to every pwm pulse. The process speed will depend entirely on the user's programming and the tool to be processed. This circuit structure can be used either in a lathe or in a vertical machining center. For this reason, it would be best to find a universal solution. I guess that's why there is no solution but to use a high resolution dac. Do you think it is suitable for my goal?

v1.5 said:

In fact, it will sometimes be a slow and sometimes fast process, but it will perform very sensitive processes. That's why I want it to respond to sudden speed changes as quickly as possible. The fastest possible response time also means that it can respond to every pwm pulse. The process speed will depend entirely on the user's programming and the tool to be processed. This circuit structure can be used either in a lathe or in a vertical machining center. For this reason, it would be best to find a universal solution. I guess that's why there is no solution but to use a high resolution dac. Do you think it is suitable for my goal?

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Completely pointless - you've no need, and no use, for such high speed - it will offer no improvement whatsoever, and could easily make things worse. And it will certainly NEVER be a 'fast' process, it will always be slow (VERY slow in electronic terms).

There's no point in fast electronic responses for slow mechanical purposes.

You might try studying P.I.D. which would give you advantages - and P.I.D. is deliberately NOT fast.

Nigel Goodwin said:

Completely pointless - you've no need, and no use, for such high speed - it will offer no improvement whatsoever, and could easily make things worse. And it will certainly NEVER be a 'fast' process, it will always be slow (VERY slow in electronic terms).

There's no point in fast electronic responses for slow mechanical purposes.

You might try studying P.I.D. which would give you advantages - and P.I.D. is deliberately NOT fast.

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understood. I guess I've been thinking wrong all this time. I always thought a quick response would give me an advantage. That's why I put a lot of emphasis on this speed issue.
I want to benefit from your experience. How much response time do you think would be appropriate for this kind of realization. Is a classic low pass filter enough or should I design a fast one (like 3rd order)? If yes, how much cutoff frequency should it have for 10khz pwm?

gophert said:

you are confusing instantaneous (single pulse) PWM power with motor speed In your machine. if you want instantaneous voltage control to a 10kHzto control a motor spinning at 10kRPM, do you really expect to change the rpm 60 times during each revolution of that motor? Motors have inertia and take some finite time to slow down or speed up.

Also, your desire to convert a PWM power control to a voltage control just disables the slow speed capabilities of your controller - a hallmark of PWM vs voltage control. Specifically, a 5v motor won't turn with any smooth motion at 0.5v but it will run perfectly smoothly at 10% duty cycle on a 5v supply.

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You wrote the same comment as Nigel Goodwin , but I believed in speed so much that I did not evaluate your comment. sorry.
I would also like to read your comments on the subject. Thanks.

v1.5 said:

understood. I guess I've been thinking wrong all this time. I always thought a quick response would give me an advantage. That's why I put a lot of emphasis on this speed issue.
I want to benefit from your experience. How much response time do you think would be appropriate for this kind of realization. Is a classic low pass filter enough or should I design a fast one (like 3rd order)? If yes, how much cutoff frequency should it have for 10khz pwm?

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A classic low-pass filter should be all that's required, it's really a very simple and easy requirement.

Going back to my previous life as a TV engineer, in 'later' years tuning was done via varicap diodes in the tuner, fed from an accurately regulated 33V supply (using special regulator IC's like the ZTX33B). This 33V supply was fed through a simple resistor to the collector of an NPN transistor, who's base was fed with PWM from the tuning circuit. This obviously produced a 33V PWM waveform on the collector - this was fed through two simple low-pass filters, resistor capacitor, followed by another resistor and capacitor - and considering you started with 33V P-P I was always dubious about the ripple - so I scoped it, with the scope on maximum range there was no visible ripple at all.

in my experience, there's usually a lot of inertia in the motor, shaft, and especially the chuck of a tool head. changing the speed of these takes a very long time in comparison to the period of your PWM. you are looking to change the speed of a motor (with other attached masses) in 100 microseconds, and in an actual milling machine a speed change for the tool motor takes much longer (maybe as long as between 10 and 100 milliseconds or more). most of the other motors in the system (whether it's XYZ positioning of the tool head or the work piece) are often stepper motors, and their speed control is done by other means (changing the repetition rate of the H-switches that drive the motors). if i remember correctly (i used to work in a place where we had a machine shop as part of the operation) they would actually back the tool off from the work piece when they wanted to change the speed of the tool head motor.