3 axis chopper board

[rashdan]

hi
Any body have explanation for the 3 axis chopper board in this site Tach42 3 axis board .
And i have stepper motor 3.7Amp 2.5Volt and i will run it with 5Amp 19.6Volt power supply. Can this chopper work with me.
thanks

[KOC62]

I am not clear as to what you want explained. The chopper board can work with your motor.
Do you also understand that this chopper board only works with the unipolar board? In other words, this chopper board only replaces the power resistors for the unipolar board.

For a 3-axis system, your power supply can work. However, the work power required is determined by how fast and how hard you are cutting something. Hence I can't tell you what power you will be using. So it is possible that for some of your work you may need more current from your power supply than 5amps. For now just try it, if you already have the parts.

Also read the wiki... 3axis - PMinMO.com for further explanations.

[rashdan]

hi
I mean i want know how it work. I do not know how to calculate what power i need in my system. How can i calculate it?
thanks

[KOC62]

The link I provided explains how it works.
What part are you having difficulty with?

The cutting forces are very complicated to work out mathematically. You will need to know the weight of various moving parts, including your spindle. You need to know the desired acceleration, cutting speeds, depth of cut, diameter of cutter, properties of the material being cut, etc. Most of these questions can't be answered by the casual CNCer.

We just use a guide, a rule of thumb, when it comes to power supply selection. Part is based on people's experience and part is based on basic motor coil inductance along its maximum current rating and which configuration you will use like UNIPOLAR, PARALLEL, SERIES, etc.

You can fine tune the power supply selection by connecting a voltmeter to your available power supply output. If your cutting load causes your power supply voltage to drop by more than 20% you may want to increase the current capacity.

As a guess, start with 32 times the square root of the motor's inductance in mH for motor voltage calculation. e.g. motor inductance is 2.8mH for the configuration you are wired. Then 32*sqr(2.8) = 53V. If your motors are different then use the lowest inductance for power supply voltage calculation.
For current selection add up all your motor full current ratings divide by 2. e.g. motor current peak = 3.7amps. Two coils per motor times 3 motors divided by 2 = 11amps. Please note that if your driver can only handle 50V you will have to lower the power supply voltage but may keep the current capacity the same. This is not exact science but a ballpark. Others may use a different guide, like dividing by 3 instead of my 2.

[rashdan]

hi
The difficulte part is how the circuit limit the current, based on what?
I want build wood engraving cnc machine.
How can i calculate motor's inductance?
The last part of your massege not clear for me
thanks

[KOC62]

The current limit is based on a sense resistor. Once the desired current is reached it shuts of the motor current. It is a chopper circuit.

You need to have some electronic training to understand circuitry.

Your motor inductance is obtained from the motor datasheet.

The last part tells you how I determined the power supply voltage.
Secondly, it tells you how I got a power supply current rating.

Is my english hard to follow?

[rashdan]

hi
I understand that it can be used the ADC in microcontroller to do this function, Is this true?
I am very bad in english?
thanks

[rashdan]

hi
One more question i will run my cnc with 5Amp power supply for each motor, Is this enough?
32*sqr(2.8) = 53V I dont understande this.
And wht does it mean e.g. ?
thanks

[KOC62]

Rashdan,

Yes, the ADC can be used.
To make it work correctly may be difficult with short duration pulses. You will need a sample rate much faster than the pulse duration you are measuring. I would think at least 16 times. You also need to be good at programming.

Yes, 5A per motor is good enough.

32*sqr(2.8)=53.
You asked how to know what power supply voltage to use. Gecko has come up with a formula to answer this question. The maximum power supply voltage you should use is 32 times the square root of the motor inductance. Where the motor inductance is in millihenries. The motor inductance is obtained from your motor datasheet. It also depends on your connection like UNIPOLAR, SERIES, PARALLEL, etc. These connections change the motor inductance and thus the power supply maximum voltage calculated.

e.g. means "for example". It comes from Latin "exempli gratia"

(If this is not clear then you need to ask someone, who knows your language and english, to explain it to you.)

[rashdan]

hi
Sorry for the large number of questions.
I will conect my stepper motor like unipolar.
I understand your words thank you for your explanetion.
thanks

[H500]

An a/d converter is not easy to use for this application. The waveform is not steady. You need to do the conversion at exactly the right time. You will also need an amplifier.

A pair of comparator is much simpler.