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Author Topic: NEMA 17 stutter  (Read 2318 times)

AradoKYSky

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NEMA 17 stutter
« on: February 13, 2018, 16:13:51 »

Hello,

I am experience some strange behaviour managing NEMA 17 motors with the AstroEQ. What worked a few days ago, now is stuttering/chatter as it the motor is not capable of managing the load. If I assist, the motor will work. The motors are both NEMA 17 with these specs. The chatter is sufficient to loosen set screws and hex screws.

Electrical Specification:

* Manufacturer Part Number: 17HS19-1684S1
* Motor Type: Bipolar Stepper
* Step Angle: 1.8
* Holding Torque: 45Ncm(64oz.in)
* Rated Current/phase: 1.68A
* Phase Resistance: 1.65ohms
* Recommended Voltage: 12-24V
* Inductance : 2.8mH+/-20%(1KHz)

I have increased Vref to 1.2 from .9. Using EQMOD at 3, the motor does not stutter. At 4, the DEC motor cannot get out of the gate. The RA does not show any problem. This acts like the load is too great. When 5 days ago, the whole assembly was working fine. With any motor, V and I are important. If 12V and 2 amps is insufficient, I am seeking better motors within the limits of the AstroEQ. I have spare motor. I will see if this is just a bad motor. But, the loads are different between the RA and DEC. That is obvious. Time to do some mathematics.

Please review my setup. 





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TCWORLD

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Re: NEMA 17 stutter
« Reply #1 on: February 13, 2018, 16:52:20 »

Is the mount well balanced?

Alternatively, if it is vibrating, it is possible there is a broken connection on one coil. This can allow it to rotate, albeit with massively reduced torque.
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Tom Carpenter (AstroEQ)

AradoKYSky

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Re: NEMA 17 stutter
« Reply #2 on: February 13, 2018, 21:23:49 »

Hello Tom,

Thank you for the reply. Yes. The scope is balanced well on all positions. At this present time, I am functional on one driver. The replacements come tomorrow.

I also have ordered a set of NEMA 17 motors with more torque capabilities. The scope is an Orion 6" RC. This is not my SP-C6 Newt which is a steel tube. Here are the specs.

Electrical Specification:

* Manufacturer Part Number: 17HS19-2004S1
* Motor Type: Bipolar Stepper
* Step Angle: 1.8 deg.
* Holding Torque: 59Ncm(83.6oz.in)
* Rated Current/phase: 2.0A
* Phase Resistance: 1.4ohms
* Inductance: 3.0mH+/-20%(1KHz)
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AradoKYSky

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Re: NEMA 17 stutter
« Reply #3 on: February 14, 2018, 03:31:14 »

Hello,

I think I figured the stutter out. The 56Ncm motors are sufficient when the clutch is not as fully engaged. I loosened the clutch and everything started to work.

Also, I requested assistance from Pololu with their 8825 driver. I received assistance and need to make a strong suggestion. ;) The drivers are rated to 1.5 A. At 2.1, some serious cooling has be in place. Being cooped up in an enclosure and beyond >2.0 will lead to irreparable damage to the driver. Notice the .6 difference. Sustained 1.6 < X <= 2.1 is shortened life of the driver. Beyond that is catastrophic loss of the driver. 

So, lessons learned. Stay within specs of the driver manufacturer. Heat is a killer of electronics. A well balanced telescope does not take excessive power to move. Pick motors that have at least 15% head room. The closer to the head room top, the more amps are needed. But everybody knows that... 

I am waiting to hear back what options for cooling are available. Case modifications would be required to power a NEMA 23 or higher motor as they have higher amperage requirements.   But that would need a new design. Gravity still works. Enjoy!
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AradoKYSky

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Re: NEMA 17 stutter
« Reply #4 on: February 18, 2018, 04:21:13 »

Hello,

That was refreshing. Without the Lord's help, I was ready to stop the effort. I built a truth table to map between the AstroEQ, RJ11, and the windings of the motors. Once the table was completed, I realigned all wires to the proper positions for than RJ11/12 setup. This time no chatter and all is well. 

RA   Driver   RJ-11      Motor      
                  
   B2-   3      1   Red           A1+
   B1+   4      2      
   A1+   2      3   Blue           A2-
   A2-   5      4   Green   B2-
            5      
DEC   Driver      6   Black   B1+
                  
   B2+   3      OutPut Jack      
   B1-   4      1      
   A1+   2      2   Red           A1+
   A2-   5      3   Green   B2-
            4   Black   B1+
            5   Blue           A1-
            6      
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TCWORLD

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Re: NEMA 17 stutter
« Reply #5 on: February 18, 2018, 09:46:40 »

Both of the RJ11 connectors should be wired as xABBAx. So pin 2=A, 3=B, 4=B, 5=A, where A is one coil, and B is the other coil.

It doesn't matter which side of the coil is which (i.e. 2 and 5 can be swapped, and 3 and 4 can be swapped), so long as the coils don't get mixed between them.

If one of the coils is reversed (e.g. 2/5 swapped), it will reverse the direction of the motor, which can be corrected using the Axis Reverse setting in the config utility.
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Tom Carpenter (AstroEQ)

AradoKYSky

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Re: NEMA 17 stutter
« Reply #6 on: February 19, 2018, 23:42:26 »

Hello,

Yes! The RJ11 has 2-5 as P2+, P1-, P1+, P2-. This matches the pins on the board.
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AradoKYSky

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Re: NEMA 17 stutter
« Reply #7 on: February 21, 2018, 01:53:57 »

Hello,

I did find the source of the stutter. The RJ-11 cable I was using was schizophrenic in its operation. It would work then wouldn't. Power then no power. I have cable tester and found it to be a cross-linked cable. Pins 2 and 5 were cross linked. Ouch! this explained to stutter. Now, all things are working.

The only problem has been Windows 10 and Windows 7 applications with drivers. .NET 4.7 is the next level.  ;)
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