Servo motor for cutter

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Servo motor for cutter

Post  Pete on Wed Sep 02, 2009 1:20 pm

Hi,
I have been lurking here for a while, and there is some really great info here. I am looking at using a high speed servo for the cutter mechanism, in particular the HS-81 from Hitec, its speed is 0.11 sec/60 deg and has a torque rating of 36.10 oz/in (2.6 kg/cm). Do you think this would be fast and/or strong enough for a cutter? Any help would be appreciated.

Pete

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Re: Servo motor for cutter

Post  John on Wed Sep 02, 2009 2:58 pm

Hi Pete! Nice to meet you!

I'm working on my cutter mechanism right now and it is the most difficult part of the project (at least for me).

I first looked into using a Servo motor when I first started looking into the project. I went and purchased a simple servo motor just so that I could learn how to use it and quickly found out that it was not quick enough. Now granted this is just a regular servo not a high speed one.

I've used solenoids in the past, and currently using a stepper motor like Magic-Nozzle's approach. Those have been the tried and true methods of cutting as of now. It would be nice for someone to test something else and add it to the forum. Just remember that speed is everything in the cutting mechanism!!!! My stepper motor would be able to do a 60deg angle in about .04 seconds. The other thing is that you don't NEED the 60 deg. If you simply attach a blade to the armature then all you need is just enough space to clear the water and then block the water. You might end up just needing 15deg.

As far as the torque rating goes that is pretty good. My current stepper motor only has a 15 oz-in rating (.11N-m). That seems to be pretty good for cutting water.
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Servo cutter update

Post  Pete on Sun Dec 19, 2010 10:47 am

Just to give everyone an update on the servo cutter I just didn't feel it was fast enough. It was pretty easy and cheap to implement. Here is video of my results


I think I will try a stepper motor for my next attempt.

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Re: Servo motor for cutter

Post  chaser2440 on Sun Dec 19, 2010 5:59 pm

What are you using to control your stepper motor. I'm currently working on the electronic part of my project. The cutter is my next step, then tying it all together should be fun.

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Re: Servo motor for cutter

Post  John on Mon Dec 20, 2010 9:20 am

It seems like the servo motors are pretty strong. Have you considered gearing it up (or down) so that it moves faster? What I like about the servo motor solution is the fact that it knows where it is. Stepper motors have to have some sort of homing position.

John
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Servo control circuit

Post  Pete on Mon Dec 27, 2010 7:54 am

Sorry for the slow response, been quite busy with Christmas and all. The servo used for the cutter requires a 0.9 to 2.1 millisecond pulse repeated every 20 milliseconds (or 50 Hz) to control its position. A 1.5 millisecond pulse would put the servo at the midpoint position. So not to tie up the microcontroller with this I used a standard LM555 timer to generate our control pulses. These pulses used in conjunction with a digital output from the microcontroller provide two position control of the servo.



In this circuit an LM555 is configured as an astable multivibrator. The main timing capacitor, C1, is always charged to the voltage level which is determined by the voltage on LM555 pin 5 around 1.6V. When the timing capacitor is charged only with current coming through 33KΩ resistor, the charging takes around 2 milliseconds. When control voltage is applied from the microcontroller, the charging current increases when control voltage increases. When the capacitor is fully charged, LM555 starts to discharge the capacitor through 100K ohm resistor. The discharging continues until the voltage has reached the half of the control voltage (1.6V/2 = 0.8V). The discharge time is determined by the resistor between LM555 pins 2 and 7. The 100 KΩ resistor in this circuit makes this time to be around 15 milliseconds. When the capacitor is discharged, the circuit start charging it again.

You can fine tune the pulse width and therefore the servo position by adjusting the trim pot connected to pin 5. Overall the circuit is very simple and only requires a digital output from the micro to trigger the cutter. As far as regearing the servo I haven't tried that yet. Also I am looking at shortening the cutting arm itself. As you can see in the video it is about an 1.25 (31.75 mm) too long. Hopefully, this will speed it up some as well. Right now I am working on a larger nozzle (6 in) and I don't have the other nozzle assembled to try and see how much regearing would help.

Sorry for such a long post but I hope it helps. If anyone wants the Multisim 10 file for the circuit simulation let me know.

Pete

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cutter idea

Post  gwoodward on Wed Feb 23, 2011 11:53 am

Has anyone looked at using a linear actuator for the cutter motor? Here is a link for one. It moves 2" per second, not sure if that is fast enough but the speed could be increased using a bell crank.
http://www.servocity.com/html/25_lbs__thrust_linear_actuator.html

Glenn

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Re: Servo motor for cutter

Post  liteglow on Thu Feb 24, 2011 5:34 am

gwoodward wrote:Has anyone looked at using a linear actuator for the cutter motor? Here is a link for one. It moves 2" per second, not sure if that is fast enough but the speed could be increased using a bell crank.
http://www.servocity.com/html/25_lbs__thrust_linear_actuator.html

Glenn

I think that one is to slow !
Any one know where to find a decent solenoid to use ?
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cutter idea

Post  gwoodward on Thu Feb 24, 2011 8:25 am

Hi Liteglow,

You are right. I have seen some that are much faster. I just started looking into ways to make a cutter. I did find this linear solenoid at Mc Master Carr http://www.mcmaster.com/#linear-solenoids/=b696jk. It is continuous duty, 70 inch oz. of pull force, 1" (25.5mm) stroke @ 110 mil seconds. I would not need 1" of stroke to pull the cutter blade across the stream so I should be able to speed up the cutter action. Do you think it is worth a try? I bought some aluminum round stock to make several nozzles yesterday. Does aluminum stand up to water abrasion or should I use brass? I think I will make a 12.5mm, 19mm and 22mm hole size. Should I buy a 1600 gph pump? Will that be enough water for the 22mm nozzle? I'll be buying a length of 8" (20.3cm) diameter pvc pipe to make the housing. No sence in wasting money on acrylic since there is nothing interesting to see inside.
Glenn

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Re: Servo motor for cutter

Post  liteglow on Thu Feb 24, 2011 12:29 pm

I think this is the best cutter I have seen:


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cutter

Post  gwoodward on Sun Feb 27, 2011 4:07 am

Hi Liteglow,
Thanks for the link. I agree, that is a good very simple cutter. I will use that idea for my cutter although I may have to use a pull/pull system with two solenoids. I ordered two tubluar solenoids 19mm @ 100 ms. I don't know enough about electronics to make a reversing system, I'm better at mechanical stuff. I have seen his and your video's and will follow his and your leads. I am planning on making a few nozzles with different sized chamfered holes. Once I have my lath setup it is pretty easy to make more than one piece. I want to buy a pump but don't want to waste money. I mentioned to you that I am more interested in height of the stream than distance. If I end up using a 19mm or 22 mm hole will a 1600 gph pump be enough? I'll be purchasing a 203 mm pvc pipe tomorrow and start to the project.
Thanks
Glenn

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making servos move faster

Post  gryphern on Wed Apr 25, 2012 3:08 pm

To make your servo "move faster" you don't need a high speed servo, you just need your cut to happen far away from the servo.

Like on a record or CD, the outer edge spins faster than spindle hole when you spin it. So why not just make a longer cutting arm and use a servo that can generate a lot of torque?

(I'm thinking in terms of the simplest mechanism possible because I want to do this project with a bunch of kids.)

Obviously the water pressure would put strain on the drive shaft of the servo, since it would be pushing the cutting arm upwards, but I figure running the cutter arm on a track or under a ledge would prevent straining the servo.

Servos are easier to mess with than steppers, especially since you can play a sin wave off your cellphone or computer and connect your audio out the PMW wire on the servo and test the sucker without any logic. I have more fun than I can describe making digital servos dance as i play different sin waves that they interpret as a PMW (slightly more expensive digital servos are better at reading the nasty audio signal than their more analog cousins. Even if you produce a "perfect" square wave out of an audio card on a computer or phone it has nasty artifacts in it a nice clean square wave generating microchip doesn't produce. That's why most people only test servos with audio generators and actually drive them with, like, an Arduino or a 555/556 chip set up.)


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