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types of motors.. stepper.. servo..?

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haleemul

New Member
Just wondering if anyone can explain me what are stepper motors and servo motors?

what is the difference between the two? and if i were building a hexapod robot which motor would be better suited?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
haleemul said:
Just wondering if anyone can explain me what are stepper motors and servo motors?

what is the difference between the two? and if i were building a hexapod robot which motor would be better suited?
A servo motor is a normal DC electric motor, complete with gears and servo electronics - it only moves over a limited rotation in response to an input pulse between 1-2mS. They are small and very powerful for their size (due to the low gearing).

A stepper motor works in a completely different way, they provide rotation a step at a time, and move an accurate amount on each step. However, they are large, expensive, heavy and have relatively little power.

Hexapod robots commonly use servo's, steppers are unlikely to have sufficient power and are too big and heavy.
 

Rescyou

New Member
Servos

I would say servos are the only way to go.
Hobby servos (the ones in model planes etc.) usually don't have 360 degrees of continuous motion.

You can open them up and do some simple modification to rip out the limited rotation mechanics and even the servo electronics to make a decent mini-gear motor that will run from normal unpulsed dc. There are lots of tutorials around for modification.

Resc.
 

toowie2uk

New Member
I'd like to counteract a few things above, no offense intended.

Most servos need 1-2ms pulse every 18ms. You can get off shelf controllers, but not the easiest thing to control from a microprocessor, cos they are effectively analogue, but everything is possible.

A stepper motor is very useful, because with 4 transistors and 4 resistors and 4 diodes, you can control a stepper from the printer port of your computer, or using a CMOS counter (about half a dollar IC - (I'm UK - work in £s usually!) fairly easily. Stepper are very accurate.

Have a look at www.epanorama.net , or if you want some more help. reply and I will see what I can do.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
toowie2uk said:
I'd like to counteract a few things above, no offense intended.

Most servos need 1-2ms pulse every 18ms. You can get off shelf controllers, but not the easiest thing to control from a microprocessor, cos they are effectively analogue, but everything is possible.
No offense taken, but:

On the contrary, servo's are very easy to control from a micro-controller, all you have to do is provide them a pulse from 1-2mS at fairly regular intervals (the 18/20mS isn't at all critical). This is very easy to do with a micro-controller, even the BASIC STAMP finds it easy!.

You can buy pre-programmed PIC's for controlling servo's, they typically accept an RS232 input and provide outputs to feed 8 servos (and are commonly used in hexapod robots).

I wouldn't really describe a 1-2mS pulse as 'analogue', it could just as well be described as 'digital' - considering that 'digital' amplifiers work in a similar way (with varying width pulses), perhaps 'digital' would be more accurate.
 

toowie2uk

New Member
see your point

I own about 60 stepper motors and one servo! so you can see where my opinion lies! I see what you mean re the digital. I dont know enough about them to argue my case - I was tinkering trying to drive NE555 based PWM from a DAC on my parallel port.

In terms of steppers being low power, that isn't strictly true, there are some very large stepper motors around, which arent going to cost a fortune. Admittedly trying to drive 500mA - 1A at 12V isn't always easy. My advice to you is go and find some huge old dotmatrix/daisy-wheel printers and pull them to pieces - usually 2 motors a piece, and some sensors, gears and a IC or two if you like unsoldering. I got about 10 printers from a doctors surgury the other day - they upgraded to lasers. If you get a stepper from a shop it will cost $20 or something. If you need little motors, good for eyes etc in animatronics, you can pull the ones out of floppy drives (don't forget the holder and bearing (wont be much good without) - these are more difficult to drive though, as they are bipolar.

hope this is of help to someone - not much in they way of a reply though - sorry.

Anyone got any good translator circuits in there head for stepper control?
USB - Useless Serial Bus
 

Nigel Goodwin

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Most Helpful Member
Hi toowie2uk,

I agree that steppers are wonderful things - for the right purpose!.

The question here was for a hexapod walker, these use two servos per leg to produce a six leg 'insect type' walker. Do you have a stepper motor among your collection which will lift 12 motors like it self, batteries, chassis, sensors, and all the electronics with just 90 degrees of rotation?.

Somehow I don't think so :lol:
 

falleafd

New Member
I don't think the characters of frequencies, or something you said are the important differencies between a stepping and a servo motor.

I think the most important difference between the two types of motors is that their physical characters which are:

1) The stepping has hold torque, so that, as you drive a stepping, you can hold it at a right position, while as you turn off a servo, you will make free running.

2) As their structures, a stepping will be much bigger than a servo.

3) You can use PWM to both types of motors

4) You can use H brigde to both

5) Servo is driven in a close loop controller, while they usually use stepping in an open loop with an external motor model.

6) Servo motor is run continuously, while stepping run with its steps. You may use good stepping with 0.72 deg per step, and drive it under half step mode, it provides 0.36 deg per step. And as you run it in microstep, you may get better results about 1/8 step, that is 0.09 deg per step. It seems to be continuous.

7) There many types of stepping motors such as unipolar, bipolar, multiphase or permanent magnetic motors, hybrid motors, and variable reluctance motors... but they are all DC motors. But servo is only one type of control, and they have DC servo and AC servo.

8) You may use a stepping to work as a servo motor if you add an to it an encoder. You may use higher level driver such as L297 with H brigde L298 to drive stepping as easy as a DC motors. Important to note that servo is a one type of control theory, and they use it with DC motor, you will have DC servo motors, they use it with AC motors, you have AC servo motors.

Goodluck
 
Personally,I prefer servos.

For robots with legs,the torque of the motor output and the weight of the entire system is very important.A servo is small,light,powerful and integrated with a close loop controlling circuit.Thus no exernal transistor driver is required.Though it may cost more,it's application will cut the cost of other extra parts that will otherwise be used to hold a heavy body built with steppers.Little steppers can be used in accurate controlling units.
By the way,I have a question too:if the signal I give to a servo is somehow incorrect and it greatly exceeds the limit of 1~2ms' pulse width,will it cause damage to the servo circuit?
 

Nigel Goodwin

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Most Helpful Member
Re: Personally,I prefer servos.

Alex_rcpilot said:
By the way,I have a question too:if the signal I give to a servo is somehow incorrect and it greatly exceeds the limit of 1~2ms' pulse width,will it cause damage to the servo circuit?
I don't really know, but it is common practice to get more movement out of them by extending the pulse limits - the potentiometer inside can only turn 270 degrees though, if you try and exceed that it could break the pot, there's certainly enough power to do so - and I think it's unlikely that servos have protection against it.
 
Re: Personally,I prefer servos.

Nigel Goodwin said:
I don't really know, but it is common practice to get more movement out of them by extending the pulse limits - the potentiometer inside can only turn 270 degrees though, if you try and exceed that it could break the pot, there's certainly enough power to do so - and I think it's unlikely that servos have protection against it.
Thanks for your reply.I better be more careful while debuging my programs :lol: .
 

falleafd

New Member
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There are many good books for engineering, especially for mechanics and electronics engineering...

Goodluck to all.
 

ivancho

New Member
You can damage the gears if you go over the 1ms-2ms range. Not even all the servos have a full 1ms-2ms range... some have more or less range.... you must make sure that you don't hear the gears "slipping"... as there is a mechanical stop inside the servo.

I have modefied different types of servos to allow for 360 degrees of rotation. The idea behind this is to fool the electronics to think that the pot is at the middle position while giving a 1ms or 2ms pulse to rotate right or left. In order to do this, you have to remove the pot, and replace it with a voltage divider equal to the valu of the pot in the middle position, and then removing that physical stop inside the servo.

Most people will choose servo's over steppers for a hexapod... the reason being is that you will not have to get a driver for every motor, and how simple is to drive a servo more than it is to drive a stepper. Stepper motors are very precise motors... use where you need to be accurate on where you position the motor, just like in printers, etc. There is an inverse relationshiop between the torque and the speed of the stepper... the faster the stepper goes the less torque it has, and viceversa, not to mention the you must ramp up and down each time you want to speed up the motor.... stepper motors don't like sudden movements or it will lose steps.

You can drive a U$10 servo with 4.8V-7.2V (smaller battery) and have a 57in/oz ... while a stepper will cost you anywhere between U$5-U$20 and you will need the driver's for it, and more control lines and a bigger battery (most have to have 12Vdc) and by the time you add all the weight that adds to your circuit, you are better off with the servos, for this application in particular. Stepper are way better for other applications for sure.

Ivancho
 
About servos used

ivancho said:
You can drive a U$10 servo with 4.8V-7.2V (smaller battery) and have a 57in/oz ... Ivancho
ivancho,thaks for your post. :)
I'm an R/C fan,and as you know,we need servos on your aicrafts.The most widely used servos here are Futaba&JR regular ones.I also found a local website selling there own servo products for robots.But since there're few people bulding their own robots here,fewer have enough experience about how to chooes robot servos.I wonder what types(or brands)of servos you often choose to build a robot in the US,and their price.Would a servo used on R/C chopper,like a futaba S9602,be suitable for a small hexapod robot?
 

ivancho

New Member
Perhaps you mean the S9206.... yes a servo is a servo, kind of thing... but I would not recommend having those only because of their price U$90 each is a little bit too much for the motors. Instead you can just go for the normal standard version like the S3003 that runs for like U$12.95.

Since you are into the R/C stuff you probably know of towerhobbies. THey sell their own servos which are normally cheaper than the "brand" ones. And I believe Futaba manufactures them anyhow :wink: . Also you probably know of ServoCity they have a special right now for 5 servos for $51.00 (no S/H w/in US).... unless you have the S9206 readyly available I would not recommend them.... heck you may as well put them on ebay, buy the S3004 deal and end up with cash in your pocket to get a PIC board :wink:

Ivancho
 
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