|Jul4-12, 05:10 AM||#1|
Help building a Torque limiting motor control for guillotine
I have an old electric paper guillotine.
Guillotine have two esential functions apart from those associated with positioning the paper ready to cut.
1. Using power (usually electric motor) to operate the blade which cuts the paper stack.
2. Using power (an electric motor or manual turning of a wheel) to put pressure on the stack of paper immediately prior to cutting. Some types of paper need more pressure than others. For electric operation the amount of torque can be varied by the operator setting the level of torque deemed appropriate. When operated manually the operator simply adjusts the amount of force he/she applies to a wheel which directly turns a shaft on which the pressure plate is mounted.
My old electric guillotine has manual operation for applying pressure. I am retired and use the guillotine for personal projects. Manual operation of the wheel is slow, inconvenient and leads to inconsistent results.
The pressure plate must be able to move vertically to any position between its topmost point and a fraction of a millmetre above the cutting table. The wheel is mounted on top of an extrenally threaded shaft which passes through an internally threaded fixed plate placed horizontally above the cutting table.
1. I want to build an additional electric motor drive to turn the head of the shaft so the pressure can be automatically applied when a button is pressed. This would be most easily accomplished using a belt drive but a gear driven arrangement would not be too difficult to arrange. The pressure plate would need to be lowered until such time as some form of variable torque limiting controller caused the motor to stop applying additional torque. Until released (see next para) pressure would need to be maintained whilst cutting occured. After the cut has been made pressure should not be released until the blade has had time to retract to its rest position. If that is not done then the retraction of the blade can disturb the stack causing the operator additional work tidying the stack before undertaking an additional cut of the same stack. This happens frequently if each sheet of paper is printed with multiple copies of the same image.
2. Ideally it would be nice if the the pressure could be automatically released after the blade has done the cutting. I have no doubt I could fit a microswitch which may be tripped according to the position of the blade. However some form of time delay would be needed to allow for blade retraction.
There are two types of release needed.
(a) Default-Partial release: Simply releasing the pressure sufficiently to allow the operator (me) to withdraw the paper, turn the stack of paper so I can make a series of subsequent cuts c with a different orientation an a stack of the same size.
(b) Full retraction: Probably triggered by activating another control after the default action has occured: This would fully retract the pressure plate to its topmost position at the end of the job.
I can design and carry out the physical engineering (I have a myford lathe and other engineering tools) but I have no idea how to I should set about choosing a motor or controlling it. The design would need to ensure that there could be no mechanical force applied which would tend to raise the pressure plate above its topmost position or touch the cutting table
Is anyone willing to help me here?
Thanks in advance
|Jul4-12, 02:08 PM||#2|
Hmmm us retired guys gotta stick together.
i have this obsessive drive to adapt local materials
an automobile electric window drive motor seems the right flavor. Most have a worm gear attached so produce rotary motion which i guess is what you need?
The window motor is torque-limited for safety and is reversible, might be about right. All you'll need is a power supply. An old 6 amp battery charger would work for that.
With a permanent magnet motor torque is in proportion to applied current. One could control current with resistance wire in series, or an electronic device. It is not difficult to in concept to measure DC current, but you mihgt want to find ahobby kit rather than start from scratch.
I have no idea how much torque you need.
A Ford windshield wiper motor is difficult to stall by hand.
Automotive starter motor seems impractically oversize to me but a lawnmower starter might do..
Lastly, some reclining easy chairs have a leadscrew mechanism with electric motor attached.
food for thought?
|Jul6-12, 03:50 AM||#3|
Thank you for sharing a very sensible approach. You have set me thinking. Sorry not been able to respond earlier as have been away for couple of nights.
Instead of measuring torque would it be possible to measure how much pressure is being applied to the stack? What kind of device could be in the pad which, when it makes contact with the stack could measure the degree of pressure applied,communicate with an electronic switch, so the motor could be turned off when pressure has reach the desired level?
1. How is pressure to be maintained once the right level has been reached?
2. What kind of device could be used to release the [pressure when the blade has cut the stack? - probably needs to lift the pad by a some predeterminable (possibly variable) distance?
3. Presumably a similar device could be used to retract the pad to the topmost position - it should not be too difficult to have the motor turned off by a microswitch.
|Jul6-12, 02:17 PM||#4|
Help building a Torque limiting motor control for guillotine
Torque can be measured directly via a motor mount that allows some twist opposed by a spring,
or inferred by measuring the motor's armature current.
DC motors with permanent magnet field have one very nice trait - torque is pretty linear with armature current. So it's possible to derive a voltage proportional to motor current and compare against a threshold.. One needs to be aware that the motor's brushes always have minor arcing so noise rejection must be incorporated in the threshold comparator.
Check your Mark's Mechanical Engineer's Handbook section on machinery threads.. There is a formula for what pitch will not overrun. If you used an electric window motor with worm drive and were fortunate enough that it had such a pitch, simply removing drive current would hold it in last position. Alternately you could reduce armature current to a value that prevents back rotation.
I assume you'd reverse the motor to drive back up.
To my recollection the worm-drive windshield wiper motor can be overrun(turned bacward by hand) , but the friction is noticeable.
I would figure out what sequence of currents were needed than build a logic circuit to produce them via old fashioned ladder logic. It'd take me a few days. I still use 555 timer IC's for building blocks - they do analog comparison, simple logic elements AND OR INVERT & Time Delay, and will drive 1/10 amp which is plenty for an interposing relay coil.
But the younger guys here would program a PIC or Arduino microcontroller in about fifteen minutes. I am a fossil these days. Should change my handle to 'Chambered Nautilus'.
Old printers are another source for drive gears and cog belts. The motors are usually steppers which would excel for your variable speed and hold functions. But they are not so simple to drive , nor very powerful. There are hobby sites aimed at robotics that would be a rich source for you, poke around this one for terms to search further. You might find some plug&play hardware.
Perhaps this thread will catch the interest of somebody who's more current than I on hobbyist motion control.. I still remember the days of Steve Ciarcia converting automobile alternators into stepper motors to manipulate his home workshop machine tools, he called tham "Flutterwumpers".
post a picture of your gizmo?
|Jul6-12, 04:37 PM||#5|
HI Thanks Jim
In haste leaving for Wimbledon very early in morning to see some good tennis!. Back Monday.
Quick piccy attached - you can see the wheel above the cross beam
|Jul9-12, 03:41 AM||#6|
Thanks for your links. The Hobby engineering site led me to http://www.tekscan.com/flexible-force-sensors and sensors to measure the pressure being applied to the stack. I think the design principles are beginning to take shape. I hope to get some time next week to begin putting some flesh on the design "bones"!
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