Go to Post Holy 5-year-old-thread-revival Batman!! - Mr. Freeman [more]
Home
Go Back   Chief Delphi > Technical > Motors
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
Closed Thread
Thread Tools Rate Thread Display Modes
  #16   Spotlight this post!  
Unread 11-05-2011, 22:53
Ether's Avatar
Ether Ether is offline
systems engineer (retired)
no team
 
Join Date: Nov 2009
Rookie Year: 1969
Location: US
Posts: 8,125
Ether has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond repute
Re: how to model motor+gearbox

Quote:
Originally Posted by Clem1640 View Post
Here's a link to a white paper and math model I posted last year:

http://www.chiefdelphi.com/media/papers/2405
Thanks Clem! Interesting paper. I don't know how I missed it.

For grins, I checked your analytical solution to eq_14 using my favorite* CAS, Maxima. I got the same answer as you, but I don't like the way Maxima factored it [see attached PDF]. What CAS did you use?

You modeled the gearbox by using a constant parasitic torque. I'm looking for something just a bit more realistic.


*it's my favorite because it's free and therefore anyone can afford it:-)


Attached Files
File Type: pdf Maxima.PDF (15.6 KB, 14 views)

Last edited by Ether : 11-05-2011 at 22:59.
  #17   Spotlight this post!  
Unread 12-05-2011, 09:53
James Critchley James Critchley is offline
Registered User
no team
Team Role: Mentor
 
Join Date: Apr 2011
Rookie Year: 2010
Location: Lake Orion, Michigan
Posts: 45
James Critchley is an unknown quantity at this point
Re: how to model motor+gearbox

Quote:
If I understand correctly, there are two separate forces acting on the gearbox: One of them (windage) is loosely proportional to the speed of the motor, caused by churning of the lubricant and friction we can't get rid of. The other is loosely proportional to the forces on the gear teeth, and thus to the torque on the gearbox.
They are all simplified models of very complex interaction forces... There are many many more than two forces/torques at work. Every model has it's boundaries.

This one (load dependent plus windage) actually has some undesirable behavior about zero speed. You would expect the gearbox to stay stationary until the torque exceeds some threshhold torque boundary. This won't happen here and the slightest torque still transmits 90% and will result in motion. This aberant behavior may or may not be important to your application. A "constant parasitic torque" model you've described does prety well in this case and can even approximate some non-back-drivable gearbox behaviors.

The "constant parasitic torque" is often the dominant force in tight fitting pin joint type rotational systems. Here the coulomb friction is a function of the "clamp load" in the joint and approximated as constant. Unless you have an air bearing, this behavior is always present to some extent. A "next best" model could be to apply all three. As you combine effects you need more measurements/observations to determine the values of the parameters.

The bottom line is that we can say "bad things" about any model. A good model minimizes complexity of understanding, implementation, and/or measurements while giving a desirable approximation. A bad model is one which gives inferior results to models of similar or lesser complexity. I haven't seen any bad models in our discussions, and we certainly don't want to steer anybody away from what has been proven to work!

Hopefully that about does it for the "simple" models discussion.
  #18   Spotlight this post!  
Unread 12-05-2011, 15:25
Ether's Avatar
Ether Ether is offline
systems engineer (retired)
no team
 
Join Date: Nov 2009
Rookie Year: 1969
Location: US
Posts: 8,125
Ether has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond reputeEther has a reputation beyond repute
Re: how to model motor+gearbox



Attached screenshot "tetrix.png" shows Tetrix data gleaned from Richard Wallace's posts.

The purple dots are the two data points from post #1. These are actual dynamometer data on a Tetrix motor-plus-gearbox.

The blue dots are eyeballed from the dyno data graph attached to post #15 for the same motor, but without the gearbox. To facilitate visual comparison, this data was then adjusted for a "perfect" (100% efficient) 52:1 gearbox so it could be compared graphically to the data with the actual gearbox.

It's quite apparent that, for this gearbox at least, the "threshold torque boundary" consideration can be ignored, and a "constant torque loss" assumption is not valid1. The torque loss in this gearbox is essentially zero with no load torque, and increases linearly2 as the load torque is increased. This data suggests that modeling torque loss as a linear2 function of load torque would work well for this gearbox.


Assuming this data is typical, the answers to the questions raised in the original post would be:

Quote:
...suppose I bolt a CIM to a gearbox that has ratio 10:1 and efficiency 90%. Then:

- the free speed at the gearbox shaft should be slightly slower than 1/10th of the CIM's free speed, because of the friction in the gearbox... but how much slower? answer: for practical purposes, no slower.

- the free current should be slightly more than the CIM's free current (because of the torque load of the gearbox friction), but how much more? answer: for practical purposes, no more.

- the stall current should be exactly the same as the CIM stall current answer: of course.

- the stall torque would be less than 10 times the CIM's stall torque, but how much less? answer: assuming that "90% efficient" means "the maximum power with the actual gearbox is 90% of the maximum power with the ideal gearbox", and assuming that the speed vs torque curve is linear, the answer is "10% less" (see attached sketch "efficiency.png" which assumes linear speed vs torque with actual gearbox)

- assume a linear behavior between free and stall points? answer: probably a good enough approximation.


1 the assumption is not valid, but it may still be possible to come up with an average value which can be tweaked to give useful results for certain problem domains

2 probably roughly linear, but can't tell for sure from only two data points :-(



Attached Thumbnails
Click image for larger version

Name:	tetrix.png
Views:	15
Size:	6.9 KB
ID:	10691  Click image for larger version

Name:	efficiency.png
Views:	16
Size:	11.1 KB
ID:	10693  

Last edited by Ether : 12-05-2011 at 19:46.
Closed Thread


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -5. The time now is 15:01.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi