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Topic: Stepper motor sizing based on load travel time and distance (Read 269 times) previous topic - next topic

dpressm

Hello, I am a software guy who is new to using stepper motors.  Basically I have built a large moving bookcase door that rolls back and forth.  I have connected it to steel cables and pulleys into the attic where I want to place a stepper motor and gearing to open and close the door.  I had imagined using a chain drive attached to the steel cables.  Size is not a problem as all of the mechanics are up in the attic.

I am not sure where to start and do not expect you to all my work for me but could you at least point me in the right direction?

Here is what the operational parameters are: 

Distance of travel: 40 inches

Force to roll Bookcase: 25 lbs (measured by pushing on a scale against the bookcase)  Actually it was probably closer to 30 to get movement started and 18 while moving.

Expected time of Travel: about 3 seconds maybe more

Frequency of usage: No more than 45 times a day

Why a stepper?  This "Bookcase" will actually contain some fairly fragile artwork - sculptures etc. so i want to be able to accellerate and decelerate smoothly.  I plan to use ultrasonic distance measurers to confirm the position as there is some slack in the cables and I would expect some stretch.  Obviously exact positionoing is really only important at the beginning and end of the motion.  I will also use infrared motion detectors to check for obstacles (either stationary or moving - think scurring dogs trying to beat the door).

I can gear the motor as needed as there is no problem with space.

I plan to use either an arduino or a rasberry pi to control the door.

I would prefer the motor to be unbraked (?) so it can be moved manually if needed.

I have attached a picture of the 8 ft tall bookcase (before the finish moldings and shelf lighting were installed) and two pictures of the mechanism as it attaches to the bookcase and guide track and of the cables going up to the attic.  There is a second guidebar and attachment at the bottom as well as cables to pull the bookcase both open and shut.


Thank You VERY much!











Robin2

Please post a diagram (a simple pencil drawing) that illustrates the cabling arrangement. If the cable is being moved by a pulley or wound onto a drum then the radius of the pulley (or drum) is important.

Also it would best to measure the force at the place where the motor will be - pulleys etc will add friction loads.

When you have figured out the force double it (at least) to make sure your motor has plenty of power and will not be under strain.

Will it be necessary to be able to move the bookcase by hand as well as with the motor?

There is a simple suggestion for measuring torque in Stepper Motor Basics


...R
Two or three hours spent thinking and reading documentation solves most programming problems.

dpressm

Please see the picture from up in the attic as well as the two sketches along the lines you suggested.

The measurement of the force required to move was made two ways: by pushing against a scale; and by hanging weights up in the attic.  So all friction from the pulley system is included.  In fact the four pulleys in the current system will not be in the final system.  They were installed to enable weights to be hung and maintain tension on the cables - as well as test how much force was required to move the cables/bookcase.

Hopefully my envisaged future system sketch makes sense.  Basically I need to know what torque and mechanical advantage is required for the stepper selection.

So number of rotations of sprocket B (and motor) Times circumference of B divided by circumference of A times (note assumes sprocket C which is not labled is identical to A).




Paul_KD7HB

Quote: " They were installed to enable weights to be hung and maintain tension on the cables - as well as test how much force was required to move the cables/bookcase.".

If you know how much force is required, you also know the torque required for the motor.

Paul

Robin2

as well as the two sketches along the lines you suggested.
I don't see any sketches.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

dpressm

Robin2:  The two sketches were in the attached pdf  I am attaching them again here as png's


Paul_KD7HB:  So if the last pulley before the weights in the "Current System" sketch has a diameter of 2.5 inches then 25lbs * 1.25 in = 31.25 lb in or 500 oz in correct?

In terms of power I would need a motor capable of delivering:

25 lb * 40 in / 3s = 333 lb in /s /12 = 28 lb ft /s * 1.36 = 38 watts









JCA34F

From what I can figure, you will need about 3 foot lbs (4 Newton meters) of torque at 102 RPM, about 42 Watts. What size motor were you planning on?

MarkT

Hello, I am a software guy who is new to using stepper motors.  Basically I have built a large moving bookcase door that rolls back and forth.  I have connected it to steel cables and pulleys into the attic where I want to place a stepper motor and gearing to open and close the door.  I had imagined using a chain drive attached to the steel cables.  Size is not a problem as all of the mechanics are up in the attic.

I am not sure where to start and do not expect you to all my work for me but could you at least point me in the right direction?

Here is what the operational parameters are: 

Distance of travel: 40 inches

Force to roll Bookcase: 25 lbs (measured by pushing on a scale against the bookcase)  Actually it was probably closer to 30 to get movement started and 18 while moving.

Motor doesn't know anything about linear forces, it only sees a torque, so that is what you
need to work with - the gearing and the pulley radius are needed to convert linear force to
torque, and an estimate of mechanical efficiency is also needed as some gearing mechanisms
have significant frictional losses.
Quote
Expected time of Travel: about 3 seconds maybe more
Again you have to translate the linear speed into angular velocity using the gearing and pulley
radius.  But of course you get to choose the gear ratio so you can match the mechanics to
the typical motor speeds you'll get (3000--6000rpm is very common for a medium sized
DC motor, much less for a stepper (but see below why you won't be using a stepper))

Quote
Frequency of usage: No more than 45 times a day

Why a stepper?  This "Bookcase" will actually contain some fairly fragile artwork - sculptures etc. so i want to be able to accellerate and decelerate smoothly.  I plan to use ultrasonic distance measurers to confirm the position as there is some slack in the cables and I would expect some stretch.  Obviously exact positionoing is really only important at the beginning and end of the motion.  I will also use infrared motion detectors to check for obstacles (either stationary or moving - think scurring dogs trying to beat the door).

I can gear the motor as needed as there is no problem with space.
This is not a motion-control application, you'll have limit switches to detect end of travel and use a
DC motor.  The power required as others have calculated is too high for a stepper of any reasonable
size and power dissipation (steppers are far less efficient and consume full power when stationary,
their only advantage is accurate repeatable positioning which isn't appropriate here AFAICT)

Smooth acceleration is easily achievable with DC motors by ramping up and down the drive level,
and the system's massive inertia will be smoothing that even more.  The one thing that might cause
issues is a drive train with significant slop - give up the bicycle chain idea and use something like
T2.5 timing belt perhaps?

Without doing the calculations I suspect something like 6:1 ratio timing belt reduction with some
gear reduction on the motor itself will be appropriate.  Do not expect back-drivability to be easy
with significant reduction ratios like this - however with a DC motor you have a reasonable chance
for this, a stepper's heavy cogging would an issue for that.

Your typical medium power DC motor is something around 3600rpm, even with a 16mm diameter
sprocket that's about 3m/s belt velocity at full speed without any gearing on the motor, suggesting
roughly 10:1 reduction gearmotor is about right, getting down to 0.3m/s

Another reason to avoid steppers is the noise and vibration, which might be pretty objectionable,
although gear motors can be fairly noisy (you pay more for quieter precision gearing...)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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