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Endat 2.1 + GeoMacro: voltage stability
#1
Hi All
I have an issue on an heidenhain Endat2.1 encoder.

As you may know, this kind of encoder contains an absolute track (transferred digitally to the GeoMacro) and an incremental track (sin-cos, 1Vpp).
During runtime, I can read both variables:
- the loop is closed on the incremental track, so I can get Mx62 for that track
- Mi920 contains the endat reading of the encoder

Usually, I would expect that these the difference between these 2 tracks is more or less the difference of the resolution of the 2. That is almost always true, apart when I am moving; I could think that, during motion, the difference is higher because the Mi920 is updated at a lower frequency, because is not used in servocycle.

Unfortunately, sometimes happens that the difference of the two tracks is amazingly high, leading to motor phasing problems (other than repositioning problems, of course).

I talked to Heidenhain, and they told me that this behavior could come from a bad power supply: the encoder must be strictly powered at 5V +/- 5%, otherwise the incremental encoder could loose counts. This seems coherent with the fact that the actuator is now running on-site, where the power plant is 30 years old, and I never noticed this kind of feature in my laboratory, where the plant is 5 years old.

The question for you is: can a bad source from power plant make the 5V dc unstable at level of the primary feedback? I use a traco switching to generate the 24Vdc for the GeoMacro. Do you think it is possible that a big noise in the 220Vac can pass through the 24Vdc switching and through the GeoMacro circuitry that transform 24Vdc to 5Vdc?


thanks a lot
gigi
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#2
piefum,

In addition to the possibility of electrical noise passing through the voltage conversions and power supplies, I can think of one more explanation for this problem.
Is it possible that we are not sampling the incremental signal on the sinusoidal encoder fast enough and we are loosing counts? Have you tried increasing the encoder sample clock? You can try this by setting MI993 from default value of 2258 to a value of 2256 which increase the sampling clock from 10MHz to 40MHz. If the position discrepancy is caused by missing counts, it would be mitigated by this change.

Regards,
Sina Sattari
Hardware Engineering Manager
Delta Tau Data Systems, Inc.
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#3
Hi Sina

(06-21-2013, 09:34 AM)Sina Wrote: In addition to the possibility of electrical noise passing through the voltage conversions and power supplies, I can think of one more explanation for this problem.
Is it possible that we are not sampling the incremental signal on the sinusoidal encoder fast enough and we are loosing counts? Have you tried increasing the encoder sample clock? You can try this by setting MI993 from default value of 2258 to a value of 2256 which increase the sampling clock from 10MHz to 40MHz. If the position discrepancy is caused by missing counts, it would be mitigated by this change.

thanks for the info. I can manage to implement this later this week.
Anyway, it is possible to know the exact flowchart of the sin-cos encoder sampling implemented in the GeoMacro? We are interested in the oversampling theory of the endat 2.1 reading.

I would like also to understand why the sin-cos signal is not reset every time it sees the real mark of the encoder between counts. I expected that the maximum drift of the encoder is less than a real mark of the encoder itself; in my case, the drift is way much higher (1/5 of an encoder turn).

thanks
gigi
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#4
I faced a similar problem twice with a heidenhain sincos encoder that was used in a UMAC with Control Techniques SLM system (the encoder was sampled and converted in SLM format with some dedicated adapters) and both the times I could see huge spikes on the actual position.
And both times the system had power supply problems, so check for noise in the power supplies

Ciao
Andrea
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#5
(06-26-2013, 12:05 AM)tecnico Wrote: I faced a similar problem twice with a heidenhain sincos encoder that was used in a UMAC with Control Techniques SLM system (the encoder was sampled and converted in SLM format with some dedicated adapters) and both the times I could see huge spikes on the actual position.
And both times the system had power supply problems, so check for noise in the power supplies

thanks a lot. Next week I will be on site and I am preparing some tools to investigate the power supply of the encoder.

BTW, I have just received this measure (figure attached), gathered at 2250Hz, of the encoder readings, while the actuators are in open loop (no current flowing into motors).
It is clear that the measure on actuators 1, 2, 5 and 6 have a noise of few bits, and this is fine.
Actuators 3 and 4 however presents this strange armonics, in amplitude 10x the noise on the other axes. Do you think this is related to the missing counts? This behavior is not repeatable and it seems it affects all the axes.


Attached Files Image(s)
   
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#6
Are axis 3 and 4 mechanically coupled? their position response look very similar. Are these axis on air bearings? If they are, are we certain that these harmonics are not caused by air pressure?
Sina Sattari
Hardware Engineering Manager
Delta Tau Data Systems, Inc.
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#7
(06-26-2013, 01:25 PM)Sina Wrote: Are axis 3 and 4 mechanically coupled? their position response look very similar. Are these axis on air bearings? If they are, are we certain that these harmonics are not caused by air pressure?

the 6 axis form an Hexapod, so yes, they are all mechanically coupled. However, the joints at the extremities of each actuator are "floppy" to avoid mechanical interferences between axis.
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#8
Do I understand correctly that you have 3 Geo MACRO drives each connected to 2 of the motors and all of them slave to an Ultralite? It would be very beneficial if we have a backup of your system for checking the settings. We only need the I-variable and MACRO variables and not the PLCs, Motion programs or kinematics.
Sina Sattari
Hardware Engineering Manager
Delta Tau Data Systems, Inc.
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#9
(06-28-2013, 09:27 AM)Sina Wrote: Do I understand correctly that you have 3 Geo MACRO drives each connected to 2 of the motors and all of them slave to an Ultralite?

correct for the GeoMacro; the CPU is a TURBO UMAC CPU.

(06-28-2013, 09:27 AM)Sina Wrote: It would be very beneficial if we have a backup of your system for checking the settings. We only need the I-variable and MACRO variables and not the PLCs, Motion programs or kinematics.

ok, thanks a lot. In attach you can find the I-var configuration as well as the 3 GeoMacros conf files.


Attached Files
.zip   UMACcfg.zip (Size: 24.91 KB / Downloads: 1)
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#10
(06-28-2013, 09:27 AM)Sina Wrote: Do I understand correctly that you have 3 Geo MACRO drives each connected to 2 of the motors and all of them slave to an Ultralite? It would be very beneficial if we have a backup of your system for checking the settings. We only need the I-variable and MACRO variables and not the PLCs, Motion programs or kinematics.

I just returned from the mission on the machine: probably we fixed this problem by acting on the grounding scheme.

In particular, we replaced the filter for the +24V DC of the GeoMacro and we hard-wired the DB25 connector to ground (we cannot trust the mechanical connection of the connector's hood).
It seems now that we have 1/2 bit of noise on all the actuator without drift so far.
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#11
(06-26-2013, 12:05 AM)tecnico Wrote: I faced a similar problem twice with a heidenhain sincos encoder that was used in a UMAC with Control Techniques SLM system (the encoder was sampled and converted in SLM format with some dedicated adapters) and both the times I could see huge spikes on the actual position.
And both times the system had power supply problems, so check for noise in the power supplies

see my other reply; it seems that the stabilization with different choke modules on the DC power supply and a better grounding fixed everything.

Thanks a lot for all the answers

ciao
gg
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