torque of motor requirement??? 2

[slithers]

sorry to ask such a dumb question. but i am a newb.... anyway here goes,

i have a block of plastic which i need to move in a linear direction 10mm and back to start position. i am going to use a rack and pinion with a servo attached (the rack attached to the plastic). can anyone please give me some advice/formula's to work from, especially on which motor to use. (torque rating i.e. in-oz, kg-mm ).

thanks in advance.

 

[GBor]

Torque = Force * Pinion Radius
Force = Coefficient of Friction * Weight of Plastic
Force is also equal to mass times acceleration, so if you have a particular acceleration that you require, you will need to consider it in the equation.
Use the torque to calculate horsepower for the motor.

 

[Barry1961]

Just some basics...

Measure or calculate how much force it will take to move the assembly then multiply it by the pitch radius of the pinion gear.

For example: If it takes 5 newtons of force to move the rack and the pitch diameter of the pinion gear is 50 mm.

5 N * 25 mm = 125 N-mm or .125 N-m.

You then calculate at what the rpm needs to be.

Barry1961

 

[slithers]

hey, thanks for the response guys, do you/anyone also have an opinion that a servo is suitable or a stepper motor? the mass i am working with is only just above 1kg. i need presision movement though (to about 0.1- 0.4mm). i don't usually deal with motors so i am new to this,

any thoughts would be much appreciated.

 

[Barry1961]

If you would give the details of what you are trying to do it would help. The speed and move profile would be a good start. Is this is cycling 100 times a second or 10 times a day?

If this is a reciprocating motion you may be better off with an adjustable crank on a regular gear motor.

Barry1961

 

[slithers]

right, application: the plastic block i mentioned earlier which requires traversing is going to house a load cell with a plunger attached which will measure the force required to actuate a switch. i imagine it will be required to cycle around a thousand times a day. do you think an adjustable crank will be a better solution? easier to apply? cost is not really and issue but also i am required to do a little programming in this project which is why i choose the servo. i had the idea that i could use one of the hobby servo's available at hi-tech.
if it is not feasable to use rack and pinion i could use an adjustable crank as you mentioned. i am also keen to hear of any other alternative ways to do this preferably using a small hobby servo.

again thanks for the replies on this guys.

 

[GBor]

Sounds like a good application for a slider-crank mechanism. Forget the rack and pinion. Pin an actuating bar to the outer radius of disk. Attach the output shaft of the motor to the center of the disk (perpendicular to the plane of the disk). Since you are trying to move this object precisely, make the actuator attachment point so that the diameter of the circle that it will draw as it goes through a revolution is equal to the travel distance for your plastic block. You still calculate the torque the same way, only you used the radius of the disk. The speed of motion is then the time for one revolution.

 

[Barry1961]

If you can access the servo motor command registers you may have enough resolution to measure the force without the load cell. I have done this with Delta Tau, PMAC for double sheet diction when doing pick and place.

If you try this you will need a very efficient mechanical design and will still need to manually measure the force to scale your command to…..

Barry1961

 

[tygerdawg]

All the dynamics formulas you need are nicely gathered together in something called "Smart Motion Cheat Sheet". It's a PDF that you can search for on the web and download for free.

The responses above are correct, but they all left out gearing efficiency. You'll have to upsize your torque requirements to account for efficiency.

Your loads are light. Your repeatability & accuracy seem to be fat. There are a variety of methods to do this. You may consider timing belts, also. Do you really need a motor? Ever thought of using pneumatic cyclinders with the appropriate valving to lock the stroke in place where you need it? Stepper motors would do this task as well as servos.

TygerDawg

 

[GBor]

It is true that we didn't mention gear efficiency. From a purely theoretical standpoint, this must be considered, but if I recall correctly, these types of gears are on the order of 96% efficient. Considering that motors generally don't come in horsepower to the third decimal, I anticipated that your motor will be oversized slightly...enough to compensate for a few inefficient percentage points...perhaps I'm being overly optimistic.

 

[zekeman]

Your rack and pinion solution is perfectly suitable for this application if you use a 1.8 degree stepping motor ( again your suggestion) with an indexing drive which runs open loop , you have plenty of flexibility in terms of distance, speeds and resolution and can program the ramp up and ramp down speeds easily. It's so simple that a tech without any engineering experience could do it. Your resolution of .1" is easily achievable. For example, if you choose a pinion of 2" diameter, at 1.8 degree steps, the resolution is
1.8/360*PI*2= .034".
Don't use a silder crank for this!
As a footnote, be careful about using the load cell for very small forces, since accelerating forces on the switch mass may be of the same order as the switching force you are trying to measure.

 

[sgwhip]

Thank you, Tygerdawg. I found it just like you said.

http://www.electromate.com/technicalsupport/smartmotioncheatsheet.pdf

 

[shaun123]

Zekeman, what sensor would you suggest? and also the resolution was stated as 0.1mm not 0.1", is the pinion design still acceptable? very interested to read people's thoughts on this.

also thanks again tygerdawg, found that paper very useful.