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Postby ome kaftoeter » Sun Jan 02, 2011 12:00 am

Hello Profilers,

Happy new "milling" year!

Today I looked into the thing about the resistor. The Vref on the colinbus board is quite straight on; just a simple voltage divider (correct me if I am wrong) see picture. Some resistors in serie to get the correct resistor value and a bypass capacitor to keep a clean Vref. Calculations show that Vref is 0,468 Volt and with the current Rsense (0,5ohm) it results in a max current of 0,94 amps. With an Rsense of 0,33 ohm (after mod) it should be a max current of 1,42 amps. Resulting in a desired Vref of 0,71 Volt.

This means replacing R43 by 1k2 ohm to obtain a max current of 1,402 amps.

I am going to try this variant of the mod soon and will post my practical findings.

Regards, Michel
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2011-01-02-18.35.13.jpg
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Postby ome kaftoeter » Mon Jan 03, 2011 12:00 am

The proposed change is made. This is really awesome. Though I had to grease my axes again for the profiler to run without blocking (since I hadn't used it for quite some time) I could achieve a maximum speed of 130 mm/sec without any blocking where it was about 70 to 80 mm/sec before the modification.
Also the profiler runs more stable now (less vibrations; I had little vibrations in the rails at higher speeds before). I could notice only the smallest vibrations anymore at speeds that I couldn't achieve before so it also sounds promising for milling. The motors still make noise when not running but much less than before. I have never seen my profiler move like this before.

Of course I have to wait for the long term results but this looks like a really simple modification that causes huge improvement. A concern seems to be a rating of 1.2 amps printed on the motor while driving it with 1.4 amps. Looks like a little overdriving so I am not sure if that will give any longterm problems. After intensive speed testing the motor was quite warm but not hot. I could easily hold my fingers on it but could feel it was warm (warmer than my skin so i guess 40 - 50 degrees). When after (or during) the first milling job from now the temp get's unacceptable (60+ degrees i guess) I still can lower the current to 1.2 amps which is still quite higher than the original design of 0.94 but within the motors specs. Fact however seems to be that the profiler has more power in it than originally meant by colinbus. So you have to release the profilers real power yourself....
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Postby scp » Mon Jan 03, 2011 12:00 am

Hi Michel,

thank you for your analysis of the circuit. However, when I follow your calculation for the standard current Vref=220/(220+2350)*5V, my result is 0.428V rather than your 0,468V. This gives somewhat lower current values of 0,85A (standard) and 1,28A (+50% mod). A 3k3 Resistor in parallel to the 2k2 R43 would do the job.

I would not consider approx 1.3A too much for the 1.2A rated motor. As long as you can still touch the motor after some time of operation, it should be ok. Of course as long as we operate the profiler for hobbyist purposes only, there should not be any impact (for the motor).

BTW: Does each axis have its own Vref voltage divider?

Regards,
Paul
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Postby ome kaftoeter » Mon Jan 03, 2011 12:00 am

Hi Paul,

You are right about the calculations. I obviously made a mistake. I was already wandering why the calculated current was quite some higher than the current measured by Rainer. However I now replaced R43 with 1k2 instead of the 2k2 it was setting the motor current to 1,4 amps. So I will pay special attention to motertemperature during the first milling jobs.

Yes, each motor has it's own Vref. So the resistor has to be replaced three times. R43 was for the Z-axis if I remember right but you can easily find the corresponding resistor for both other axes.

Regards, Michel
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Postby ome kaftoeter » Mon Jan 03, 2011 12:00 am

When we look at the datasheet for the steppermotor en.nanotec.com/downloads/pdf/1108/ST4118M1206.pdf we can see colinbus design seemed to be intended for bipolar serial connection of the steppermotor with a max allowable current of 0,85 amps. But as we look in the colinbus assembly instructions http://www.colinbus.com/profiler/prof_electronics_assembly.pdf (page 7/9) we can see that the steppermotors are connected as bipolar 1 winding with an allowable current of 1,2 amps.
This looks like a mismatch to me. So it would be an even better option to not modify the driver pcb but simply connect the motors as bipolar serial. Than you also get the higher torque. Simply switch the black wire for the orange wire and the white wire for the yellowe wire for each motor. Why didnt I think of this before modding the pcb? Now I immediately started thinking of modding the driver pcb back to original and rewire the motors. But....... According to the datasheet we can calculate a power of 4,44 Watt in bipolar 1-winding connection and a power of 4,48 Watt for bipolar series connection. The difference is probably not noticable but since on paper my newly modded configuration should be slighlty more effecient regarding power and should not differ in holding torque according to the datasheet I will keep the configuration as it is now.

Isn't this actually a major design error?'
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Postby ome kaftoeter » Tue Jan 04, 2011 12:00 am

I think I made a mistake again but now about the holding torque. looked in the datasheet again and the holding torque of bipolar serial connection is of course quite higher than bipolar one-winding. So that means modding the PCB back to original and rewire the stepping motors.

But the datasheet of the motors show also bipolar one-winding >speed and bipolar serial winding
Also the datasheet mentions an allowable temperature rise of max 80 degC. This would mean an allowable temp of 100 degC at room temperature (20 degC)? But even if we would only take the 80 degC than the allowable temp is still way beyond what one can touch with ones hands.
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Postby scp » Wed Jan 05, 2011 12:00 am

Ome Kaftoeter If I only would know a ratio or something in which torque and speed are related to each other.

Hi Michel,

maybe this link might help you. The manufacturer shows the torque vs speed in diagrams for many models.

http://de.nanotec.com/schrittmotor_st4118.html#kennlinien
For our model:
http://de.nanotec.com/show_graph_imagick.php?product_id=435

I just wonder about the high voltages mentioned there (24V/48V).

Regards,
Paul
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Postby scp » Wed Jan 05, 2011 12:00 am

Ome Kaftoeter If I only would know a ratio or something in which torque and speed are related to each other.

Hi Michel,

maybe this link below might help you. The manufacturer shows the torque vs speed in diagrams for many models.

http://de.nanotec.com/schrittmotor_st4118.html#kennlinien
For our model:
http://de.nanotec.com/show_graph_imagick.php?product_id=435

I just wonder about the high voltages mentioned there (24V/48V).

Regards,
Paul
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Postby ome kaftoeter » Wed Jan 05, 2011 12:00 am

Hi Paul,

This really helps a lot. It is clear that I am going to mod the pcb back to original and change the connection of the motors. The torque is in serial connection much higher than in one-winding connection. At least up to a speed of 300 rpm for 24 volts (worst case). 300 rpm is about 900 mm/sec. No way my profiler will ever go that fast. So in serial connection the motor gives more torque at speeds suitable for the profiler.

And now I am curious. Cause since changing Vref caused a huge improvement in torque. But changing the connection to serial with the original motor current there shoudl be an improvement of at least 1/5th more torque in comparison with changed Vref and one-winding connection. Sounds promising....

The motors are 3.7 V for one-winding connection. With it's resistance of about 3 ohms this gives about 1.2 amps motorcurrent. However it takes some time to get that current running through the motor since the winding is a coil which wants to keep the current at zero as it was before applying current. A trick to get the max current faster going through the windings is to apply a high voltage and limit the current. So e.g. 48 volts are applied causing a fast current rise in the windings but when the current in the windings exceeds 1.2 amps the coltage is brought down to maintain the current of 1.2 amps. That is what the sensing resistor in combination with Vref are taking care of.
I hope I made it a little more clear now why the high voltage.

Regards, Michel
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Postby scp » Wed Jan 05, 2011 12:00 am

Hi Michel,

thanks for explaining the topic of the high operating voltage.

But please bear with me: I can't follow exactly your calculation. How can 300rpm = 5r/sec mean 900mm/sec?

We have a spindle step of 3mm per revolution on the profiler spindles. I have set a maximum speed of 30mm/s in the software (which I was not able to use reliably before the mod).
So this means 10 spindle r/sec and 600rpm (!) We finally end up in the area where the single bipolar configuration is way better (0.24Nm instead of 0.17Nm), at least for the 24V configuration.

Regards, Paul
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