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Author Topic: Servos and Steppers  (Read 1365 times)

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Offline DaithiSp01Topic starter

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Servos and Steppers
« on: February 01, 2012, 07:20:50 PM »
hi im using servos as part of a project I am building and I have to lift a weight of 10kg I have sourced servos which are able to lift 10kg from their point of rotation however as my weight is mounted 8cm from the point of rotation I am afraid I will not be able to lift the weight using servos.

I am considering using stepper motors as I feel they may also work, id be grateful if someone  could inform me of a method for changing a stepper motor control system to a servo control system via a black box of some sort. I need to change this from a stepper to a servo due to the type of reciever and controller I am using to move the robot. thanks

Offline Soeren

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Re: Servos and Steppers
« Reply #1 on: February 01, 2012, 09:46:00 PM »
Hi,

I am considering using stepper motors as I feel they may also work, id be grateful if someone  could inform me of a method for changing a stepper motor control system to a servo control system via a black box of some sort. I need to change this from a stepper to a servo due to the type of reciever and controller I am using to move the robot. thanks
Why not a regular DC gearhead motor?
A DC motor is easier to control than a stepper (needs less extra components).

A servo is a (ridiculously small) DC motor with a gearbox and control electronics with feedback.

The black-box that you need, no matter which motor you end up choosing and using, will be a microcontroller plus some power switches (eg. transistors).
If you already have a microcontroller in your application, just program it to receive the servo signals and convert them into H-bridge signals.

Main things to consider for selecting a motor is, how much weight do you want it to lift, at what distance from the center of the output axle and at what speed.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Pogertt

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Re: Servos and Steppers
« Reply #2 on: February 02, 2012, 10:25:14 AM »
You could possibly convert the servo to continous rotation, design a geartrain or pulley and lever system and lift the load.

How high must the load be lifted?
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Offline Soeren

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Re: Servos and Steppers
« Reply #3 on: February 02, 2012, 02:24:01 PM »
Hi,

Using a geared down servo will be slow.
An average servo (modified) will be around 50 RPM.  As I gather the OP, he wants to lift 14kg by a string (or similar) wound onto a pulley with a radius of 8cm, Assuming a fairly standard servo of around 3.5kgf-cm (and disregarding all losses in the following), the gear ratio would need to be ~1:32 and that  leaves it a t the blinding speed of 1.5625 RPM or a lift of ~13mm/s - less in the real world due to losses.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Pogertt

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Re: Servos and Steppers
« Reply #4 on: February 02, 2012, 04:14:19 PM »
I suggested converting the servo to continuous rotation, not to modify it's gear ratio.
This modification will allow the servo to produce the same torque and at the same speed that it was originally designed and built to.
The modification will allow it to move it's output shaft greater than the common 180 degrees.

Slow is relative. 
Consider the life span of a Pansy compared to an Oak Tree.
The OP has not replied with a time constraint yet.
He hasn't provided details with the required height to lift the load either, which will affect time to lift.
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Offline Soeren

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Re: Servos and Steppers
« Reply #5 on: February 03, 2012, 04:17:59 AM »
Hi,

I suggested converting the servo to continuous rotation, not to modify it's gear ratio.
Yes, but since he already tested it, he might have modded it already and since he needs the power at a radius of 8cm, the gear ratio will have to be changed, or it won't have the power to lift the weight.
Although I somehow had got it at 14kg (probably crept in from another post), it is only 10kg, so my calculations was a bit off (it will be about 30% faster).


This modification will allow the servo to produce the same torque and at the same speed that it was originally designed and built to.
Obviously, but since he need 10kg force at 80mm or 80 kgf-cm, it will need to be geared down - or did you imagine that it would work without a gear reduction?


The modification will allow it to move it's output shaft greater than the common 180 degrees.
Yeah well, modding servos, or the purpose of doing it, aren't exactly secrets.
Not all servos are build for 180° though. Some only goes 90°, some goes 2..3 revs and then there's the continuously rotating types that's made without the end stops from the factory.


Slow is relative. 
Consider the life span of a Pansy compared to an Oak Tree.
Who's faster then, a pansy or an oak tree?
Of course slow is relative and (well, I better not go there... ;))

Comparing a geared down servo to any old gearhead motor of almost whatever size able to lift the weight, the servo will be relatively slow.
Comparing it to a killer snail, it would be fast, of course, but since the snail would be quite impractical, I assume the OP can deduct this, if not... Lucky I gave him the numbers, making him able to judge for himself, right?


The OP has not replied with a time constraint yet.
He hasn't provided details with the required height to lift the load either, which will affect time to lift.
Precisely... Like I didn't said that he shouldn't use a servo, I just told him the likely speed he'd get.
Apparently you chose to consider it a personal insult - well, that's really your problem and I don't see how that is helping the OP.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline georgeecollins

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Re: Servos and Steppers
« Reply #6 on: February 05, 2012, 11:59:42 AM »
80 kg/cm is a lot of torque.  Gearing down a motor or stepper motor as suggested can work.  Also you could attach a linear servo like this

http://servocity.com/html/heavy_duty_linear_servo__115__.html
to an arm and it would work. 

But not be very fast at all. 

 


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