Cheap, small, stepper Pic Driver - Developing

[Ragnarok]

Hi, i just want a little help, i have just finish a pic based controller,
this is for Unipolar motors, with chopper, and microstepping (calculated in realtime for more precision or in contants 4-64 microsteping), change steps on the fly(bia the RX pin, not in schematic, so i deside to use a manager board), I have test this design in discrete components (48hrs continusly, with a fan, if not IRLZ44 gets hot... and the motor just a little) and it works wondelfull in every config, and i'd like to make it in SMT, and I have a little questions.
1 - how much does the IRLZ44 get hot, with a 3A paused (HOT!!! i Guess), and in a D2Pak?
2 - discrete is better than SMT for this kind of aplications?
3 - I'm a programmer not electronic, as you see the design is a little simple, can someone please help me to check posible design error in the electronic side, protections, noise etc.
4 - Does anyone know a method for sense the current other than checking the voltage passing thoung a Resistance??
5 - for PWM of microstepping which frecuency is adecuated 40Khz <- the one I use, 20 Khz <- half for a diferent cristral, or 7.8Khz <- to use the internal osillator, bye bye cristal.

This board is for 1 motor, I'm planning to have a Interfaz board with a pic to control everything, I dont know how g-codes works, but I was planning to used the serial conector or USB or a kind of interface to mach2, 3 or something like that, and pass a bunch of g-code to the pic proccess it and send every thing to N motor modules, instead of limit us with the parallel port, the same goes for the inputs, and is kind of cheap.

well is everything I will apreciate your help, I have send a rudimentary schematic, the PIC should be PIC16F88(is another 'cause i want the QFN 28 encapsulated to apper in the SMT image, and doesnt apper , the 16f88 has a pin to pin compability and more with QFN pak ), the Power transistor is IRLZ44 (the same but for the D2pak), SV1 = Vref, Dir, Step, Sv2 and SV3 for motor and power conection (but planning to cominicate just for RX), the SMT image is just a "It should be like this".

Thanks Everyone.

PD i dont not much Eagle and where the hell are the components, resistences and conector i'd like,........ or I dont know their correct names

[jeffs555]

Have you measured the temperature of the IRLZ44's? At 3 amps, they should be dissipating less than 1/2 watt. The TO220 package has a Tja of 65 deg C/W so at 1/2 watt they should not be getting extremely hot, even without cooling. Also, with a Tjc of 1 deg C/W they can be run at case temperatures of 100 deg C with no problems, but at 1/2 watt, they shouldn't be getting that hot.

I suspect that you are not fully driving the gate to the required 4 volts above the source. The voltage out of a 74ls00 is typically only 3.5 volts, so it is too low by it self. You should at least be using CMOS to drive the gate, and probably a dedicated MOSFET driver. What value are your current sense resistors? Since you are running the sense voltage to the PIC without an amplifier, the voltage drop across the sense resistors is probably rather high. The source on the MOSFETS will be raised above ground by the voltage drop across the sense resistor, so this has to be subtracted from the 3.5 volts out of the 74ls00. You might be only getting 2 volts of drive to the gate. This would cause the IRLZ44's to get much hotter than necessary.

Here is one alternative to the current sense resistors. http://www.allegromicro.com/hall/currentsensor.asp

[bill south]

Gosh these folks impress me with their knowledge. I'm from the old school. We design, plug in a "smoke test".
Keep up the good work guys.
billyjack

[pminmo]

ragnorok,
How are you doing microstepping with the posted circuit? I'm missing something............

[posix]

well spotted phil. this would achieve half-stepping only, in software. unless RA0 and RA1 have something to do with it?

[pastera]

How did you do the board image?

[H500]

Probably Eagle.

[pastera]

Probably Eagle.

I didn't see anything in eagle to create a ray traced image or 3d model

Aaron

[jeffs555]

Posix,
Im sure RA0 and RA1 have something to do with it. He is probably using the a/d inputs on the PIC to measure the current to determine the switching time. It might work on high inductance motors with low supply voltages. I dont use pics much, but believe the A/D conversion time is around 10 or 20 uS. A motor with 24 mH inductance driven with 24 volt supply might ramp up only 0.02 amps during one conversion period, and could give you good control over the microstepping. However, a motor with 1mH inductance driven with a 50 volt supply might change a whole amp during one conversion period, and be totally unuseable.

Jeff

[Ragnarok]

Hi, thanks for your replies, pminmo, im doing microsteping using the CCP1 mudule in PWM config, I send the apropiated PWM and AND it with the signal as if it were a Full step 2 phases bia the demux.
the A0, A1, A2(vs1, vs2, Vref) are for the chopper circuit, they are comparators not ADC ( they are alot faster), in software if any phase goes high than Vref, in firmware it turns of the phase bits for that pic cycle and the next until it rech a lower voltage, the posibles configs are full 1 phase, full 2 phase, half step, and the micros i have mentioned.

About the SMT image it was render using POV file generated by a ulp, a thread in this comunity, i been looking for it but i dont find it, let me search some more.

Thank you very much for the info jeffs555!!!!

I will try to impruve the design!

[pminmo]

Posix,
Im sure RA0 and RA1 have something to do with it. He is probably using the a/d inputs on the PIC to measure the current to determine the switching time. It might work on high inductance motors with low supply voltages. I dont use pics much, but believe the A/D conversion time is around 10 or 20 uS. Jeff

I think it would be a real stretch to measure the current in real time to do microstepping. Takes roughly 20us to just get the sampling cap charged, then you have to add the a to d conversion cycle. Plus during that 20us acquisition, the current isnt just setting steady state. At microstep rates of 1/8th step you have to be able to resolve roughly 2%.

I'm missing something, but it wouldn't be the first time.

[vladdy]

Eagle 3D from ? Matthias Weber? generates PovRay compatible images from Eagle PCB software, Povray can save to bmp, etc..