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DC Drives : Tuning Procedure Tuning Procedure? Rate Topic: -----

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User is offline   AmazingTrans 

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Hello evreybody,

I have questions on tuning dc drives. And always has been doubting what is the proper procedure.
I am using 2 powerflex drives. One is master (Running as speed regulator), the other one is slave (running as torque regulator). Both of this motor are coupled on a common line shaft belted.

When I first started out this project to do a auto tune on any dc drives, I disable one side of the motor as in not sending a run command to that drive, and the other motor i will do a auto tune. and vice versa.
Now the question comes to , do i tune with 100% full load? or tune with 50% load? or 0% load( motor by itself only?)

I tried tuning with 80% loads and the problems commes when the load drops to 20%, i'll start getting resonance in the speed loop. and i will have to readjust the speed loop gain again. So, is it better to tune without load, and start off that way instead, where we'll slowly increase the gain until it is perfectly controlled? What is the optimum way of doing it? please enlighten me.

Thanks!
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#2
User is offline   Steve G 

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View PostAmazingTrans, on Jul 6 2010, 11:53 AM, said:

Hello evreybody,

I have questions on tuning dc drives. And always has been doubting what is the proper procedure.
I am using 2 powerflex drives. One is master (Running as speed regulator), the other one is slave (running as torque regulator). Both of this motor are coupled on a common line shaft belted.

When I first started out this project to do a auto tune on any dc drives, I disable one side of the motor as in not sending a run command to that drive, and the other motor i will do a auto tune. and vice versa.
Now the question comes to , do i tune with 100% full load? or tune with 50% load? or 0% load( motor by itself only?)

I tried tuning with 80% loads and the problems commes when the load drops to 20%, i'll start getting resonance in the speed loop. and i will have to readjust the speed loop gain again. So, is it better to tune without load, and start off that way instead, where we'll slowly increase the gain until it is perfectly controlled? What is the optimum way of doing it? please enlighten me.

Thanks!


I have used 4 powerflex dc drive on a project with a similar setup one was the speed reg and the other three were the torque, when I auto tuned them I had no load on them and the worked fine, the only thing that was attached to them was the gearbox and small wheel.
All I have to say is YUP!
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#3
User is offline   kaiser_will 

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Are these PowerFlex700S drives? Is your local A-B distributor of any use on this issue?

It has been many years since I tuned a DC drive loop, but had to do it many times.

At lower speeds, the field is weakened which may explain the poor performance. The firmware release notes literature says they added the following parameters for this issue:
553 Flux Gain Adjust, 552 Slip Preload Val, and 553 Slip Slew Rate.

If you are using and upgraded RSLinx version (OEM or Pro), and can create an OPC topic, it is easy to setup a data logger to trend drive values. I have used WebDock DataPort (free download for a few inputs - 1 sec delta is the minimum) to dump data into an Excel spreadsheet or Access database. The PLC can port the drive parameters, RSLinx can OPC-serve the data, WebDock can archive the data to Excel or Access. I have not used WebDock in this latest iteration, but it has never let me down. You can purchase a full license for low dollars.
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You are supposed to tune it unloaded.
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User is offline   Peter Nachtwey 

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View Postpaulengr, on Jul 7 2010, 06:00 PM, said:

You are supposed to tune it unloaded.

Why. I never recommend that. You shoul tune with an average load.
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View PostPeter Nachtwey, on Jul 8 2010, 05:27 AM, said:

View Postpaulengr, on Jul 7 2010, 06:00 PM, said:

You are supposed to tune it unloaded.

Why. I never recommend that. You shoul tune with an average load.


Different tuning procedure. The PF70 manuals tell you to uncouple the load. The tuning procedure is designed to determine the characteristics of the motor rather than the system (motor+load). Your controllers are different so a different procedure is required.
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User is offline   Peter Nachtwey 

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View Postpaulengr, on Jul 8 2010, 05:26 PM, said:

View PostPeter Nachtwey, on Jul 8 2010, 05:27 AM, said:

View Postpaulengr, on Jul 7 2010, 06:00 PM, said:

You are supposed to tune it unloaded.

Why. I never recommend that. You shoul tune with an average load.


Different tuning procedure. The PF70 manuals tell you to uncouple the load. The tuning procedure is designed to determine the characteristics of the motor rather than the system (motor+load). Your controllers are different so a different procedure is required.

The physics and math do not change just because you change controllers. The PF70 procedure will only work if they assume the inertia for the load is equal to the inertia of motor. If the total load plus motor inertia is different the assumed values the PID gains will not be right.
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User is offline   AmazingTrans 

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They are Powerflex DC Drives.
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View PostPeter Nachtwey, on Jul 9 2010, 02:36 AM, said:

View Postpaulengr, on Jul 8 2010, 05:26 PM, said:

View PostPeter Nachtwey, on Jul 8 2010, 05:27 AM, said:

View Postpaulengr, on Jul 7 2010, 06:00 PM, said:

You are supposed to tune it unloaded.

Why. I never recommend that. You shoul tune with an average load.


Different tuning procedure. The PF70 manuals tell you to uncouple the load. The tuning procedure is designed to determine the characteristics of the motor rather than the system (motor+load). Your controllers are different so a different procedure is required.

The physics and math do not change just because you change controllers. The PF70 procedure will only work if they assume the inertia for the load is equal to the inertia of motor. If the total load plus motor inertia is different the assumed values the PID gains will not be right.



It is different than most other drives. By determining the motors no load characteristics the drive forms a running parameter that is used to evaluate the response of the speed of the motor to the I/V input. It creates somewhat of a fuzzy logic function based on the motors other parameters besides inertia (coil groupings, iron density, air gap, cemf, ect).
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User is offline   Peter Nachtwey 

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View PostShiner, on Jul 9 2010, 11:45 AM, said:

It is different than most other drives. By determining the motors no load characteristics the drive forms a running parameter that is used to evaluate the response of the speed of the motor to the I/V input. It creates somewhat of a fuzzy logic function based on the motors other parameters besides inertia (coil groupings, iron density, air gap, cemf, ect).

The load is still ignored and changes in the load change the response. How can anybody ignore this?
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View PostPeter Nachtwey, on Jul 10 2010, 09:54 PM, said:

View PostShiner, on Jul 9 2010, 11:45 AM, said:

It is different than most other drives. By determining the motors no load characteristics the drive forms a running parameter that is used to evaluate the response of the speed of the motor to the I/V input. It creates somewhat of a fuzzy logic function based on the motors other parameters besides inertia (coil groupings, iron density, air gap, cemf, ect).

The load is still ignored and changes in the load change the response. How can anybody ignore this?


First off, DC motors are used almost exclusively in servo applications. They automatically have both speed and torque control by nature (except in some degenerate cases). So keep in mind that the control algorithms running in a Powerflex DC drive (whether speed, torque, or position control) are automatically set up for a servo controller. Although you can certainly do some tuning with an "average" load, DC drives usually operate at millisecond cycle times so that they can achieve very high bandwidths, which is one of the expectations of DC motor users.

Quite simply it is not ignored. DC motors have the wonderful characteristic that you can model them with just the armature resistance (and perhaps a few other minor players).
http://zone.ni.com/d.../cda/ph/p/id/46

This is NOT true of pretty much any other motor, electric or hydraulic for that matter. The load is not part of this model and isn't necessary to know because the load torque is simply T=KI where I is the armature current and K is a constant (that can be determined regardless of whether the load is coupled to the motor or not). The other key characteristic is the armature resistance. If the armature is not moving, then the counter-emf is 0 so you can measure it knowing the voltage and current across the armature (R=V/I). This quantity is responsible for the slight slope that you see between 0 RPM and base speed at the peak power point.

That does it for strong field mode. In field weakening mode, there is another constant related to the field strength that has to be taken into account. This has to be measured with the motor rotating and having a load coupled to it just gets in the way. Field weakening mode is represented on this web page:
http://www.globalspec.com/reference/10785/...peed-and-Torque

The armature resistance also changes slightly with temperature, and there are two other minor resistances that are necessary to measure in a high performance motor model. The stall speed is another critical point in field weakening mode. In actual practice, a DC motor isn't very linear at the stall point and the nice linear equations break down, but again, it is important to measure this point.

Granted there are definite advantages to tuning a feed forward/backward system instead of relying on pure feedback but DC motors are frequently used in applications where the load is so variable that measuring it is futile such as in winders, rolling mills, cranes, and robotics.
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View Postpaulengr, on Jul 10 2010, 08:22 PM, said:


Yep, but that is just the motor. What about the motor+load? You keep dodging this issue. If the motor+load inertia increases and then more torque is required to accelerate at the same rate. τ=I*α. This is simple physics. Now where does all the extra torque come to keep the accelerations the same if the gains are the same for motor only and motor plus load? Inquiring minds want to know.
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View PostPeter Nachtwey, on Jul 11 2010, 12:57 PM, said:

You keep dodging this issue. If the motor+load inertia increases and then more torque is required to accelerate at the same rate. τ=I*α. This is simple physics. Now where does all the extra torque come to keep the accelerations the same if the gains are the same for motor only and motor plus load? Inquiring minds want to know.


I'm not dodging anything. If the drive+motor loop bandwidth is sufficiently high then this can be treated as a cascaded control system with the inner loop defined by motor+drive and the outer loop determined by PLC controls+load. Very high gains such as this are common in servo systems.

NOT so in this case. In fact I stand corrected. There are two operational modes for this drive, as a speed regulator or as a torque regulator. See pagte 2-15 if tuning it as a speed regulator where it says that the autotune system measures intertia of the motor shaft, friction of the motor+load, and P+I gains. The instructions clearly state that you need the load coupled to the drive for this test. There is no equivalent for torque regulator that I could see.

This post has been edited by paulengr: 11 July 2010 - 10:28 PM

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User is offline   Peter Nachtwey 

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View Postpaulengr, on Jul 11 2010, 08:19 PM, said:

I'm not dodging anything. If the drive+motor loop bandwidth is sufficiently high then this can be treated as a cascaded control system with the inner loop defined by motor+drive

The inner loop drives the load too. In fact the inner loop is the only loop where the load and motor parameters need to be known. The outer loop only needs to know the desired response.

Quote

and the outer loop determined by PLC controls+load. Very high gains such as this are common in servo systems.

What? The inner loop controls velocity and the outer loop controls position. A PLC isn't fast enough nor does it have the motion profile generator required.

Quote

NOT so in this case. In fact I stand corrected. There are two operational modes for this drive, as a speed regulator or as a torque regulator. See pagte 2-15 if tuning it as a speed regulator where it says that the autotune system measures intertia of the motor shaft, friction of the motor+load, and P+I gains. The instructions clearly state that you need the load coupled to the drive for this test. There is no equivalent for torque regulator that I could see.

Torque regulations doesn't really need a PID since all one is doing is controlling the current which approximately is proportional to the torque. Close enough for a lot of applications. However there is a velocity mode and a torque mode. In velocity mode the speed is proportional to the control output. In torque mode the torque is roughly proportional to the control output.
Even in torque mode the load needs to be coupled. The laws of physics don't change when the modes change. Auto tuning in torque mode is easy from a math point of view but it takes a different excitation.
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