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2009-10-02 18:13

CNC Abene





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CNC - Abene VHF-3 - Electrical part 4

Encoder trouble - to say the least.


Initially, when I thought I was going to use the VSD-A from Granite Devices, I fabricated some brackets and extensionshafts so I could mount some really nice SCANCON encoders I had to the motors. These encoder have differential linedriver and 3600 lines resolution, 14400 counts per revolution in quadrature - perfect for the VSD-A.

This worked very nice with the VSD-A but because of the limited acceleration I switched to the HP-UHU drive which I found, during testing, had a max encoder count frequency of ~130kHz. With 14400 counts rev that would have allowed me a maximum speed of 541rpm, or rougly 1/4 of the motors rated speed - not very good.

I decided to get new encoders, I settled for the USDigital E7P because of three main reasons. The E7P is available with differential output, it's small enough to fit inside the already fabricated covers and it was availble with 625 lines of resolution which would give me a nice round figure of 1000 steps/mm on the X- and Y-axis, 2000 steps/mm on the Z-axis and stay well below the 130kHz frequency limit on the HP-UHU. (2000rpm * 2500 / 60 = 83.3kHz). Can't go wrong with that can you?

Well, to make a very long story a whole lot shorter the USDigital E7P encoder simply refused to work reliably with my setup. The UHU servo chip has an internal counter that keeps track of any invalid transistions on the encoder input. For example the the state of the A- and B inputs can't go from 0 0 to 1 1 with out "going thru" either 1 0 or 0 1. If that happens it means that we longer know for sure where the motor is. This can happen for a few reasons. The frequency (ie the motorspeed) is to high so the chip can't keep up or it could be noise coming into the system causing false readings. I knew it wasn't a speed issue because I had verified the max frequency when testing with the SCANCON encoders. Besides it didn't matter one bit if I tested at 1000rpm or 10rpm it still lost position. So it had to be a noise problem then.

During the course of several weeks I tried every trick in the book, and some that are not in the book to get these encoders to work. I tried with long and short cables, twisted and untwisted pairs, shielded and unshielded, many different grounding strategies regarding the shields and the common point for the motor power supply and the low voltage supply of the drive. I tried with different voltages for the motorpowersupply, anything from 130VDC down to 40VDC.

I tried feeding the encoder from a battery powered supply, I tried bypassing the differential receiver in the drive, using only the A- and B-singals (not the complementary ones). I tried with a homebrew linedriver circuit, I switched motors and I switched drives - no matter what I did I got the same results. Drive complains about invalid transitions on the encoder inputs with loss of position as result - unacceptable.

Being pretty confident it was a problem with the actual encoders I tried two other encoders I had around. One Hengstler and one AMT102, both worked beautifully although the Hengstler had 3600 lines so the speed had to be held down. I tried a few of the different grounding and shielding strategies I had tried with the E7P and it all worked, even with unshielded cable. Yet nothing worked with USDigital E7P encoders.

During all this USDigital claimed that there had to be something wrong with my setup since I could verify the integrity of signals on the E7P with my scope - which I could. Rise and falltimes looked OK, voltage levels OK, dutycycle and phaseshift OK but never the less - when mounted on the motor it simply DID NOT WORK.

After telling them that my system worked with 2 other encoders but not with the E7P they offered to take them back "for investigation" but I had to pay for the shipping. At this point I was so tired of it I decided to write it off as a lesson learned:

Never ever buy USDigital crap again.

But now what, I still needed enocders. I got pointed towards Renco Encoders and found their R35i model. After E-mailing them it turned out it was available with differential linedriver, 625 lines and it would fit inside the already fabricated covers. After explaining I about my problems with 'another manufacturers' encoders they assured me I would have no such problems - guess what, they where right! The Renco 35i, the AMT103 the Hengstler AND the SCANCON all works just fine while the USDigital E7P doesn't. (Yes, I DID try more than one E7P with the same results.)

The next photo shows the final test setup with the Renco R35i encoder. The encoder cable during this test was roughly 15m long, twisted pairs and as a test the shield/screen is left floating. The cable was also wrapped around the running VFD you see on the wall there. It was left like this, cycling thru a G-code program for hours without any sign of trouble.


So again, I've learned MY lesson:

Never ever buy USDigital crap again.


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