RE: [CAD_CAM_EDM_DRO]
Posted by
Parker, Darcy
on 2003-04-02 08:26:37 UTC
Are you familiar with modeling dynamic systems and control theory? I will describe it a little bit since I think it will help to understand this to understand the difference between a positioning servo and a velocity servo.
(If this paragraph is too much, don't worry too much... the other paragraphs should be helpful. And there is a short answer at the end.)
Typically you have a 'plant' which is the servo motor (or other servo device such as voice coil, hydraulic cylinder/valve...) It will have particular dynamics - for example a typical system will be 2nd order system.... For a brushless servo motor, the 'plant' in the s-domain (laplace domain) looks like G(s)=w(s)/V(s)=1/Ke/[(s*Te+1)+(s*Tm+1)], where Te is the elec time constant (equal to approx L/R (winding inductance/winding resistance)) and Tm is the mechanical time constant (equal to approx R*J/(Ke*Kt) (where R is the winding resistance, J is the inertia of the load, Ke is the EMF constant and Kt is the torque constant...) A derivation of this should be in most controls text books. I am referencing 'Design of Brushless PM Motors' by Hendershot. Notice the plant is the output speed, w(s), divided by the input voltage v(s) in the laplace domain... it could be converted to the time domain, but it is easier to work with in the laplace domain... This has a lot to do with why the input is a velocity command.
The dynamics of the plant effect things such as 'overshoot', 'settling time' etc... Typically when you say you want to get up to a certain speed - say a step response, it will take some time to get there and it may overshoot past the speed you want and then settle down... The characteristics can be described as overdamped, underdamped and critically damped. The dynamics of the plant are never as desired... so you need to bias the system to create the desired effect. Often the system has complex dynamics... so the servo amp, which drives the power to the servo is tuned with 'filters' that bias the system to look like a simple 2nd order system with the desired 'overshoot' and 'settling' characteristics. The biasing does not change the input/output parameters of the system. It is still an output velocity divided by an input voltage. So if you want a particular velocity you command the system with a particular voltage... So the most fundamental component of the drive is a velocity command. (A torque command could be derived as well...)
A servo is typically used for positioning.... so you need another controller that sends a velocity command to the velocity servo drive. This controller keeps track of position, and receives an input of the desired position. The difference between desired and actual position is calculated, run through another filter (typically lead/lag filter) and differentiated to get velocity command which then drives the velocity input of the velocity servo amp.
Short Answer:
If you see a velocity/torque servo drive, they are designed to be used with an extra controller for positioning. If you see one with positioning servo drive marked, then they are integrated. I like to buy them seperate because I want more control in tuning the system. I can get a velocity servo drive (which typically uses a PID type control law which behaves well in 'noisey conditions'). I can then use a position controller to talk to an already tuned velocity loop and focus on just tuning the desired positioning characteristics. Also if you tune a position controller once for a particular system... you only need to tune the velocity loop for each new machine so it behaves the way the position controller expects it to. It is easier to tune a velocity loop than having to tune both the position loop and the velocity loop at the same time.
Darcy
-----Original Message-----
From: Dave Lantz [mailto:dlantz@...]
Sent: Wednesday, April 02, 2003 10:40 AM
To: 'CAD_CAM_EDM_DRO@yahoogroups.com'
Subject: [CAD_CAM_EDM_DRO]
I am interested in understanding the difference between position servo
drives and velocity/torque servo drives. I have a wasterjet here that needs
a replacement controller, and I need to know what I've got. Its a yaskawa
servopack 200v, p/n: sgda-08ap, it appears to be an AC drive, and is labeled
"position" as a type on a web page i found. another interesting thing is
that the encoders appear to be wired to the servopack... does this mean the
servopack is a servo amp taking step/direction pulses from the controlling
computer, or that its a ±10V input from the controlling computer, thanks in
advance, Dave L.
Addresses:
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(If this paragraph is too much, don't worry too much... the other paragraphs should be helpful. And there is a short answer at the end.)
Typically you have a 'plant' which is the servo motor (or other servo device such as voice coil, hydraulic cylinder/valve...) It will have particular dynamics - for example a typical system will be 2nd order system.... For a brushless servo motor, the 'plant' in the s-domain (laplace domain) looks like G(s)=w(s)/V(s)=1/Ke/[(s*Te+1)+(s*Tm+1)], where Te is the elec time constant (equal to approx L/R (winding inductance/winding resistance)) and Tm is the mechanical time constant (equal to approx R*J/(Ke*Kt) (where R is the winding resistance, J is the inertia of the load, Ke is the EMF constant and Kt is the torque constant...) A derivation of this should be in most controls text books. I am referencing 'Design of Brushless PM Motors' by Hendershot. Notice the plant is the output speed, w(s), divided by the input voltage v(s) in the laplace domain... it could be converted to the time domain, but it is easier to work with in the laplace domain... This has a lot to do with why the input is a velocity command.
The dynamics of the plant effect things such as 'overshoot', 'settling time' etc... Typically when you say you want to get up to a certain speed - say a step response, it will take some time to get there and it may overshoot past the speed you want and then settle down... The characteristics can be described as overdamped, underdamped and critically damped. The dynamics of the plant are never as desired... so you need to bias the system to create the desired effect. Often the system has complex dynamics... so the servo amp, which drives the power to the servo is tuned with 'filters' that bias the system to look like a simple 2nd order system with the desired 'overshoot' and 'settling' characteristics. The biasing does not change the input/output parameters of the system. It is still an output velocity divided by an input voltage. So if you want a particular velocity you command the system with a particular voltage... So the most fundamental component of the drive is a velocity command. (A torque command could be derived as well...)
A servo is typically used for positioning.... so you need another controller that sends a velocity command to the velocity servo drive. This controller keeps track of position, and receives an input of the desired position. The difference between desired and actual position is calculated, run through another filter (typically lead/lag filter) and differentiated to get velocity command which then drives the velocity input of the velocity servo amp.
Short Answer:
If you see a velocity/torque servo drive, they are designed to be used with an extra controller for positioning. If you see one with positioning servo drive marked, then they are integrated. I like to buy them seperate because I want more control in tuning the system. I can get a velocity servo drive (which typically uses a PID type control law which behaves well in 'noisey conditions'). I can then use a position controller to talk to an already tuned velocity loop and focus on just tuning the desired positioning characteristics. Also if you tune a position controller once for a particular system... you only need to tune the velocity loop for each new machine so it behaves the way the position controller expects it to. It is easier to tune a velocity loop than having to tune both the position loop and the velocity loop at the same time.
Darcy
-----Original Message-----
From: Dave Lantz [mailto:dlantz@...]
Sent: Wednesday, April 02, 2003 10:40 AM
To: 'CAD_CAM_EDM_DRO@yahoogroups.com'
Subject: [CAD_CAM_EDM_DRO]
I am interested in understanding the difference between position servo
drives and velocity/torque servo drives. I have a wasterjet here that needs
a replacement controller, and I need to know what I've got. Its a yaskawa
servopack 200v, p/n: sgda-08ap, it appears to be an AC drive, and is labeled
"position" as a type on a web page i found. another interesting thing is
that the encoders appear to be wired to the servopack... does this mean the
servopack is a servo amp taking step/direction pulses from the controlling
computer, or that its a ±10V input from the controlling computer, thanks in
advance, Dave L.
Addresses:
FAQ: http://www.ktmarketing.com/faq.html
FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
Moderator: pentam@... indigo_red@... [Moderators]
URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
http://www.metalworking.com/news_servers.html
http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
bill
List Mom
List Owner
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/