CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Corrosion Allowance in Plate Heat Exchanger   Next in Forum: Good Read Spacecraft Launch Vibration
Close
Close
Close
37 comments
Member

Join Date: Oct 2017
Posts: 9

What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/08/2017 5:19 PM

I am reading about many braking methods employed in motors and rotors but none outline the criteria for their general applications. My project involves stopping rotors at specific locations, for example, precisely every circular arc of C/x, where C is the circumference of the circle about which the rotor rotates, and x is a predefined variable of user preference. The carry weight is relatively light (approximately 100 pounds, perhaps less, not including the rotor) and the rotor needs to rotate and stop as quickly as possible. Needless to say, minimal friction is desired, so obstruction is out of question unless the lifespan of the rotor can last for a long time. However since we are moving about a circular arc, obstructions might be necessary since each rotation would have the actual location farther away from expected location. I am using a microcontroller/PLC for the project. Given the situation, what is the best method to go about doing this?

I like idea of inductive brakes applied in the reversal of a different rotor in the project, but the /utmost/ important quality is the reversing rotor and non-reversing rotor stopping precisely at the same location on each rotation/reversal.

Any insight on some possibilities or suggested methods is immensely appreciated. Thank you.

Register to Reply
Pathfinder Tags: brake rotor
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19472
Good Answers: 1141
#1

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 5:35 PM
__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply Score 1 for Good Answer
Member

Join Date: Oct 2017
Posts: 9
#2
In reply to #1

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 6:17 PM

Like it, but it needs to be fast, carry a lot of weight on top of its rotation, and be able to move in sections up to C/30. The circumference could easily be 15 meters.

Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19472
Good Answers: 1141
#4
In reply to #2

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 6:51 PM

What would be a typical time-to-stop from speed requirement? If there is no quick stop requirement, then a gradual braking to a particular slower speed that could then be stopped at a precise point, probably a lot easier to accomplish....Something like a reverse polarity to generator load and then when a predefined speed was reached then a friction brake would be employed, the charge from the generation phase could be used to drive the motor, while the friction brake was engaged to make a controlled stop at a precise point....

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#6
In reply to #4

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 7:50 PM

It's counter-intuitive to assume that a friction brake will guarantee it to stop to a precise point every single time, especially after assuming that it will reach a particular speed after a certain amount of time. I'm making a machine for the company I work for. There will be tens of thousands of starts-and-stops per day. As long as the duration of time from start to a complete stop is about .75 seconds, the motor lands /exactly/ on the same spot for each rotation, and a friction-brake-based motor is durable, I'm fairly certain my company will be willing to purchase it.

Register to Reply
Guru

Join Date: Oct 2008
Posts: 40057
Good Answers: 1598
#8
In reply to #6

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 8:02 PM

Not by itself, of course.

Much will depend on the undefined total mass of the rotor and its diameter.

If these cannot be defined, at least within a range specifying a motor and braking system will be difficult.

Again, I say enlist the aid of your motor/controls suppliers to help define the requirements and limitations of the proposed project.

You have work to do.

Register to Reply Score 1 for Good Answer
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
Good Answers: 155
#35
In reply to #6

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/11/2017 4:59 PM

use a chip for measuring angles very finely, and use it as a feedback sensor, then have the motor/controller adjust itself after first stop if there is an error signal.

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#7
In reply to #4

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 7:50 PM

It's counter-intuitive to assume that a friction brake will guarantee it to stop to a precise point every single time, especially after assuming that it will reach a particular speed after a certain amount of time. I'm making a machine for the company I work for. There will be tens of thousands of starts-and-stops per day. As long as the duration of time from start to a complete stop is about .75 seconds, the motor lands /exactly/ on the same spot for each rotation, and a friction-brake-based motor is durable, I'm fairly certain my company will be willing to purchase it.

Register to Reply
Guru

Join Date: Oct 2008
Posts: 40057
Good Answers: 1598
#5
In reply to #2

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 7:21 PM

Is this homework? Interview question? School project? Actual, real life, work related, design assignment.

You must be able to define the objectives and the variables, if it is a real project.

Register to Reply
Guru

Join Date: Oct 2008
Posts: 40057
Good Answers: 1598
#3

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 6:29 PM

"the rotor needs to rotate and stop as quickly as possible." Could mean an infinite number of things.

Your system will need more definition. The best place to get that is to identify some suppliers and ask them.

Seems to me an optical encoder controlling the, to be defined, brake is a start. Depending on too many undefined variables a friction brake may be dictated.

Register to Reply
Guru

Join Date: Oct 2014
Location: Hemet, Land of milk and honey.
Posts: 1262
Good Answers: 23
#26
In reply to #3

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/10/2017 12:10 AM

He said that the rotor needs to stop at a specific location (x). Then he said the rotor needs to stop at a variable location.

Either or I guess, maybe the desire is specific butt maybe he will settle for whatever he can get it to do.

Can you imagine what his company might say ?

Register to Reply
Guru

Join Date: Apr 2010
Posts: 5811
Good Answers: 588
#9

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 8:09 PM

Put a rotary encoder on the shaft, calculate the stopping place, and control the motor to stop there.

"Optical absolute encoders[edit]

The optical encoder's disc is made of glass or plastic with transparent and opaque areas. A light source and photo detector array reads the optical pattern that results from the disc's position at any one time.[5] The Gray code is often used. This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft.

The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft."

Simple 3 bit example:

Rotary encoder for angle-measuring devices marked in 3-bit binary. The inner ring corresponds to Contact 1 in the table. Black sectors are "on". Zero degrees is on the right-hand side, with angle increasing counterclockwise.

https://en.wikipedia.org/wiki/Rotary_encoder

Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#10
In reply to #9

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/08/2017 8:35 PM

I've thought about this. My concern is the precision of this method. My main problem in general I think is the lack of trust I have in the precision of these methods.

If for example C/x is an irrational number, how does the imprecision of the voltage output guarantee exact location after thousands of rotations?

Register to Reply
Guru

Join Date: Sep 2016
Posts: 767
Good Answers: 8
#13
In reply to #10

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/09/2017 12:52 AM

Yeah, just use rotary encoder and inductive braking. This set-up is usually done in elevators which stop it accurately at floor levels. The assembly of both could be the same concept as that of a servo. When motion is in circle its constrained within angular position I mean some angle with-in 360 degrees. The thousands of rotation can be done by counting the revolution where your position is encoded in the memory and it is counted by a counting module that you could also incorporate in your controller assembly. It is not that complicated at all. You can do this even with a cheap adruino.

Register to Reply
Guru

Join Date: Apr 2010
Posts: 5811
Good Answers: 588
#17
In reply to #10

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/09/2017 11:28 AM

You can get encoders with up to 22-bit resolution, i.e. 4194304 steps per revolution. But a lower resolution would probably be just fine, just do the math in floating point and round to the nearest encoder step.

https://ecatalog.dynapar.com/ecatalog/absolute-encoders/en/flt=1111/Optical

Register to Reply
Guru

Join Date: Oct 2009
Posts: 1206
Good Answers: 23
#30
In reply to #10

Re: What is best way to stop rotor at a specific spot without obstructing it?

10/10/2017 5:11 AM

If C/x is irrational then x is irrational. Why are you bothering with an irrational division of the circumference?

Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19472
Good Answers: 1141
#11

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/08/2017 10:40 PM

I thinking that maybe a worm gear drive is the way to go.....A DC motor with an encoder, when the power goes off it stops almost immediately....high torque start, fast stop, simple, strong...

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 19472
Good Answers: 1141
#12
In reply to #11

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/08/2017 11:10 PM
__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 9577
Good Answers: 457
#14

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 5:51 AM

I think your best bet would be a servo motor.

__________________
"Love justice, you who rule the world" - Dante Alighieri
Register to Reply Score 1 for Good Answer
Guru

Join Date: Dec 2007
Location: Haverhill, MA
Posts: 1003
Good Answers: 121
#18
In reply to #14

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 1:13 PM

I agree, this application is exactly what a servo motor was designed to do. Not only does a servo give you exact control of position, but also control of acceleration and deceleration profiles. The important thing is to match the motor with the load (reflected inertia). Most servo motor manufacturers or distributors will be able to assist you with the proper selection of motor and drives Most drivers mate well with PLC control.

__________________
The older I am, the better I used to be
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#21
In reply to #18

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 3:03 PM

There will be a nonstop changes of the weight of the load of approximately 0 - 10 pounds, since it is picking items up and placing them onto holders that move with the rotor.

I might consult manufacturers.

Register to Reply
Guru

Join Date: Dec 2007
Location: Haverhill, MA
Posts: 1003
Good Answers: 121
#22
In reply to #21

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 4:04 PM

Not that I necessarily recommend them (although they are my "go-to" supplier for controls) but you might visit Automation Direct's web site www.AutomationDirect.com Their on-line catalog provides a lot of useful information about servo motors and drives and interfacing with PLC controls.

__________________
The older I am, the better I used to be
Register to Reply
Guru
Australia - Member - New Member

Join Date: Feb 2008
Location: Australia
Posts: 1748
Good Answers: 203
#15

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 6:38 AM

You might like to investigate presses as used in metal stamping process. These are set and adjusted to stop precisely at fixed locations on every revolution. The process separates the motor movement from the actuator movement.

Otherwise there have already been suggestions about rotary encoders and it's only a matter of getting one with sufficient resolution for your needs.

Finally, you could turn your rotor into the encoder by fitting necessary scale/graduations and sensors.

__________________
Just an Engineer from the land down under.
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#20
In reply to #15

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 2:59 PM

The one-dimensional movement is driven by rods attached to rotators, which unfortunately doesn't solve the multi-positional stopping about a circle.

Register to Reply
Guru
Engineering Fields - Mechanical Engineering - New Member Fans of Old Computers - TRS-80 - New Member Popular Science - Weaponology - New Member Safety - Hazmat - New Member Hobbies - Fishing - New Member United States - Member - New Member

Join Date: Apr 2012
Location: Detroit MI, USA
Posts: 2109
Good Answers: 213
#16

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 9:11 AM

We have a machine that spins a 24" basket that can weigh 400lbs when loaded at 400 rpm. It uses an air powered friction brake to stop very quickly and a good size detent system to align it in the same spot every cycle, so that another part of the machine can pull the basket out of the dip/spin booth on rails.

It is simple but effective. It can all be done in a couple of seconds from full spin to alignment.

If that is not fast enough for you then as others have said, a servo may be what you need. Fanuc makes some rather large and powerful servos that can start / stop on a dime with great accuracy.

__________________
How we deal with death is at least as important as how we deal with life. --CAPTAIN KIRK, Star Trek II: The Wrath of Khan
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#19
In reply to #16

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 2:42 PM

400 stops per minute is astounding! Can I have more details on this, please? What device are you using? Any further details on the system would be appreciated as well. Thank you.

Register to Reply Off Topic (Score 3)
Guru
Engineering Fields - Mechanical Engineering - New Member Fans of Old Computers - TRS-80 - New Member Popular Science - Weaponology - New Member Safety - Hazmat - New Member Hobbies - Fishing - New Member United States - Member - New Member

Join Date: Apr 2012
Location: Detroit MI, USA
Posts: 2109
Good Answers: 213
#23
In reply to #19

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 5:34 PM

Where did you get 400 stops per minute? I said 400lbs at 400 rpm..... The spin cycle can take several minutes, We spin / stop / reverse spin / stop, up to 4 times, I only mentioned the stop spin / lock in cycle.

Please re-read my reply and if you still understand it, never mind......

If you do understand my reply, I will give you more details on how it works, but from what I read, you want something faster than what I have.

__________________
How we deal with death is at least as important as how we deal with life. --CAPTAIN KIRK, Star Trek II: The Wrath of Khan
Register to Reply
Member

Join Date: Oct 2017
Posts: 9
#25
In reply to #23

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 7:45 PM

Yes, sorry. I must have gotten so excited at the sight of 400__ that I misread.

Register to Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20033
Good Answers: 753
#24

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/09/2017 6:02 PM

Maybe a vector drive motor/system might work for this. I don't have experience with it, so I'm mostly speculating.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply
Power-User

Join Date: Dec 2013
Posts: 170
Good Answers: 5
#27

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 12:31 AM

Does it matter if the rotor turns an extra 360 degrees before it stops. You can always make the rotors outer edge castellated, barcode each castellation and read the code of the particular castellation. This would cause a rapid deceleration of the rotor and as the decoded tooth reached the required position the speed would be slow enough to transfer momentum through a friction clutch and come to a halt where required. On a large enough rotor 30 minutes of arc should be achievable.

Register to Reply
Guru

Join Date: Nov 2006
Posts: 701
Good Answers: 2
#28

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 2:37 AM

Rotor inertia is a major factor in precise stopping and quick acceleration.

__________________
To avoid crticism do nothing,say nothing,be nothing
Register to Reply Score 1 for Good Answer
Commentator

Join Date: Jul 2017
Posts: 60
Good Answers: 2
#33
In reply to #28

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 8:59 AM

nesubra, you are absolutely correct and, in that vein, a great approach is to get the iron out of the armature. GN-z11 should check out:

http://www.designworldonline.com/ironless-dc-motors-deserve-a-second-look/#_

Sometimes ironless motors are called "coreless" or "printed circuit armature" motors and NASA has done a lot of work on these(see old NASA Tech Briefs). A review of PID controller tuning can dial in the final phase angle with a strong "I" component during the last part of the decelerations like after the last index pulse. Absolute encoders are not *absolutely* necessary for control if a good indexed incremental encoder and PID controller are used. Absolute encoders will potentially give you confirmation that you actually stopped at a certain point but they are more costly to produce and will likely not provide the resolution of a high resolution incremental encoder of the same price.

If *designing* an absolute encoder rather than using a commercial OTS one you may want to consider binary chain codes ala Don Lancaster's:

https://www.tinaja.com/text/chain01.html

Register to Reply Score 1 for Good Answer
Guru

Join Date: Aug 2005
Location: Hemel Hempstead, UK
Posts: 4182
Good Answers: 237
#29

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 5:02 AM

To get the required control and accuracy: if it won't get in the way, would you be better off having the motor and control mechanism at the end of the rotor instead of the hub?

__________________
We are alone in the universe, or, we are not. Either way it's incredible... Adapted from R. Buckminster Fuller/Arthur C. Clarke
Register to Reply
Power-User

Join Date: Sep 2013
Posts: 149
Good Answers: 2
#31

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 6:41 AM

I'd look at rotary indexing tables if I was you.

You could probably get one built to your exact requirements.

Something like this:-

http://www.weiss.uk.com/Type-CR.286.0.html

Register to Reply
Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
Good Answers: 117
#32

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 7:23 AM

As you have already discovered starting movement is easy, stopping movement where you want it to stop is the hard bit. You don't have one problem here you have at least two. Splitting the problem into small bits will help you to solve it.

Your base load is too large and it is variable. Stopping approx 50kg "instantly" and very frequently requires a shaft that will stand up to the duty. Driving directly from a motor shaft either stepper, servo or permanent magnet will either need a very much oversized motor which will be harder to control because of it's own inertia or a gearbox. I would suggest a planetary motion box with anti backlash gears. With a gear ratio between 20:1 and 50:1 you can achieve great accuracy with the rotor using much simpler motor control. Smaller cheaper motor and smaller cheaper controls will more than compensate for the cost of the gearbox.

All motors will slip to some extent so you either have to build in a fixed zero reference point on the driven rotor and do a correction each time the rotor passes or install an absolute encoder on the motor so that any motor error is divided by the gear ratio when measured at the rotor. Program the system to read the start position and auto correct each time it moves so that there is no cumulative error. Personally I would would add both.

You can stop motors with DC injection but for your extreme duty I would go for an independent brake. Either way will generate large amounts of heat so build in heat dissipation from the start not as an add on afterthought. The brake shoes will wear but replacing them will be much simpler and can be incorporated into a planned maintenance schedule.

Register to Reply
Guru

Join Date: May 2016
Posts: 922
Good Answers: 44
#34

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/10/2017 10:44 AM

Based on the description, you want a 1 axis CNC type control so that you can program in acceleration/deceleration driving through a worm gear box. You could also potentially use a stepper motor with a microstep type feature. I'm currently retrofitting a Bridgeport Series II mill with NEMA 42 steppers and controls with 4000 oz-in torque and using an 80 volt DC power supply supplying up to 9.8 amps, which gives a pretty healthy hi speed traverse capability.

The big plusses are repeatable positioning, ability to program in accel/decal, holding without additional brake and reasonably low cost. The stepper motor and driver I am using goes for about $250 per axis. The complication is that you would need to interface the PLC to a CNC controller to drive the axis and select the number of stopping increments.

Register to Reply
Guru

Join Date: Nov 2006
Posts: 701
Good Answers: 2
#36

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/12/2017 3:21 AM

By the way what is the KW rating and polarity of motor under discussion.

__________________
To avoid crticism do nothing,say nothing,be nothing
Register to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13882
Good Answers: 155
#37
In reply to #36

Re: What is the Best Way to Stop Rotor at a Specific Spot Without Obstructing It?

10/12/2017 11:24 AM

Yes, that will be key to the control system.

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Register to Reply 37 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

DodgySolenoid (1); GN-z11 (8); gringogreg (2); gutmonarch (1); James Stewart (2); jhhassociates (1); JohnDG (1); Jpfalt (1); JPool (2); Just an Engineer (1); lyn (3); nesubra (2); phph001 (1); prof peanut (1); Randall (1); Rixter (2); SolarEagle (4); thewildotter (1); tonyhemet (1); Tornado (1)

Previous in Forum: Corrosion Allowance in Plate Heat Exchanger   Next in Forum: Good Read Spacecraft Launch Vibration

Advertisement