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Analog Torque Mode Accuracy


Tuomo
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Hi,

 

I'm planning to build a high performance cnc system (3 feed axes plus 2 index axes) for my pattern and mould milling machine. I already have Siemens 1FT6 series servos w/Heidenhain 2048 encoders + Simodrive 611 converter system (power modules, regenarative infeed module + line filters & hf commutating reactor).

 

Simodrive 611 converter system has an excellent match with 1FT6 servos, and thus, I do not need to worry about the current loop tuning or torque smoothness.

 

However, I would like to use your controller (Turbo UMAC 160MHz) (instead of proprietary Sinumerik 840D) to be able to better customize the entire system. In this scanario Simodrive just takes care of the current loop and commutation (Analog current/torque mode) and the Turbo UMAC acts as the real cnc controller.

 

Dual feedback (servo and load) shall be used with Heidenhain rotary and linear encoders (incremental 1vpp signals) with your 4096 interpolator.

 

Simodrive power module + 611 U HRS drive control unit has the following specs when used as a current mode drive.

 

-analog input resolution 14bits (sign + 13bits)

-differential input

-sampling rate (8kHz or 16kHz)

-PWM switching frequency 4kHz or 8kHz) i.e. dual sampling

-current loop bandwidth ca. 1kHz

-space vector controlling (iq id)

 

I have, however, a few concerns, and I would to hear your honest opinionbefore I go forward.

 

1) Is +-10V analog torque command signal (ACC-24E2A) with the above mentioned (10V/8092) resolution good enough (accuracy and speed) for high speed machining if shielding and grounding are done correctly? Are normal unavoidable ac noise or common mode disturbances any issue with analog current mode signals when Turbo UMAC is closing the pos and velocity loops with 8096 interpolated feedback signals?

 

2) How good system I could get with an analog torque command in terms of accuracy and speed if compared with digital communication (like Sinumerik is communicating with Simodrive)?

 

3) What should be the Turbo UMAC servo update rate to give the best performance with the above mentioned Simodrive current drive? 4 or even 8kHz?

 

4) Incremental 1vpp velocity feedback signals from the servos must be "split" between Simodrive drive (which is also providing the supply voltage for the encoder) and ACC-51. Is there any problems regarding this in terms of noise disturbances etc?

 

Thank you in advance!

 

Tuomo

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Hello Tuomo,

 

You should be able to meet your goals with some care. Many people have done comparable systems successfully.

 

The UMAC's analog outputs have 18-bit resolution, which is more than enough. They will be at least as fast as the serial digital communications of the Sinumerik. Degradation of the signal from noise is a potential issue, but it is manageable with standard techniques, and typically this will not be the most noise-sensitive signal.

 

You must worry most about the 1Vpp signals from the sinusoidal encoders. The PWM signals on the power cables from the drive to the motor can inject significant noise into these signals if you are not careful. We recommend, as Heidenhain does, double shielding of the encoder cables, with the inner shield tied to signal ground, and the outer shield tied to chassis ground. The motor cable should be well shielded as well. The motor cables and encoder cables should be kept as separated as possible. You may well need to install chokes on the motor cable leads to "soften" the edges of the PWM signals.

 

Proper grounding is essential. Ground loops can destroy signal quality. Our sine encoder inputs are high impedance, and if the Simodrive's are as well, double wiring should be OK, but you have to make sure that both circuits are at basically the same reference voltage.

 

It is easy to experiment with different servo update rates. I would try both 4 and 8 kHz, and see if the performance at 8 kHz is enough to justify the extra processor time.

 

Good luck!

 

Curt

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Hi,

 

Thank's for your answer!

 

A few question more:

 

1) Simodrive 611 has 14bits (sign + 13bits) resolution in its analog input - not 18bits. Is this good enough for accurate torque command assuming that the max linear acceleration (at 10V) for servo would be, say, 3m/s2. In theory 13bits corresponds min acceleration of 3/8196 m/s2?

 

2) My educated guess is that Simodrive 611 has 120Ohm resistor between the encoder differential input lines according to Heidenhain's recommendation. This means that the input impedance is about 120Ohm. Please refer attached image!

 

Correct if I'm wrong but is there some config option (like using high input impedance) with the acc-51E to handle exactly this kind of scenario? I think with dual feedback this is more or less the only way to provide the velocity data back to the controller...

 

3) Have you used analog torque command succesfully in high precision applications? Just to make sure my idea is feasible and sound.

 

4) Best way to have Backlash and Quadrant error compensation with dual feedback?

 

Regards,

 

Tuomo

Hello Tuomo,

 

You should be able to meet your goals with some care. Many people have done comparable systems successfully.

 

The UMAC's analog outputs have 18-bit resolution, which is more than enough. They will be at least as fast as the serial digital communications of the Sinumerik. Degradation of the signal from noise is a potential issue, but it is manageable with standard techniques, and typically this will not be the most noise-sensitive signal.

 

You must worry most about the 1Vpp signals from the sinusoidal encoders. The PWM signals on the power cables from the drive to the motor can inject significant noise into these signals if you are not careful. We recommend, as Heidenhain does, double shielding of the encoder cables, with the inner shield tied to signal ground, and the outer shield tied to chassis ground. The motor cable should be well shielded as well. The motor cables and encoder cables should be kept as separated as possible. You may well need to install chokes on the motor cable leads to "soften" the edges of the PWM signals.

 

Proper grounding is essential. Ground loops can destroy signal quality. Our sine encoder inputs are high impedance, and if the Simodrive's are as well, double wiring should be OK, but you have to make sure that both circuits are at basically the same reference voltage.

 

It is easy to experiment with different servo update rates. I would try both 4 and 8 kHz, and see if the performance at 8 kHz is enough to justify the extra processor time.

 

Good luck!

 

Curt

 

Heidenhain.thumb.jpg.f8df4b5f07b9ee5ec49a90b2a51eda8d.jpg

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4) Incremental 1vpp velocity feedback signals from the servos must be "split" between Simodrive drive (which is also providing the supply voltage for the encoder) and ACC-51. Is there any problems regarding this in terms of noise disturbances etc?

 

Look at using the Heidenhain IBV 606 to split the encoder. It will provide buffering for each output and has some adjustment capabilities to improve phase amplitude matching and angle adjust to reduce subcycle interpolation errors.

 

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Tuomo:

 

It is important to understand the difference between the instantaneous trajectory commanded acceleration value in memory (which we compute to 96 bits!) and the commanded torque value we output and the drive accepts. While the acceleration is an important component of that value, it must respond to many other affects -- cutting loads, friction, gravity loads, vibration, etc. Remmeber that we are recalculating this command thousands of times per second, and your mechanical system likely has a position bandwidth of under 50 Hz, so the servo essentially "dithers" the output for for an effectively higher resolution for tracking acceleration. 14-bit resolution is sufficient for all but the highest-resolution (e.g. sub micrometer) systems. People have done very high-performance systems in analog torque mode this way.

 

With the ACC-51E, you can select with jumpers whether to use the 102-ohm termination resistors or not. The best solution is probably what Jeff suggests above, but you may want to experiment with direct connection techniques. In any configuration, you will want to look at the signals carefully with a scope.

 

The dual-feedback technique itself is the best solution to backlash and quadrant error (ballbar test) problems. As soon as you close the outer position loop with load feedback, there is no point in using our backlash compensation algorithms, which are intended to apply a correction to what the motor sensor is reporting. Minimizing the dynamic errors in crossing a quadrant boundaries is really an issue of optimizing the tuning.

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