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Topic: servos only go 90 degrees??? (Read 4 times) previous topic - next topic

retrolefty

#15
Jan 17, 2010, 03:15 am Last Edit: Jan 17, 2010, 03:18 am by retrolefty Reason: 1
Quote
They are claimed and supposed to be 180 degrees


Where is that claimed, by the servo manufacture? Link available?

Lefty


Peter Rand

Lefty; - Standard servo motors have 180 degree range almost universally

for example see the code above taken from the Arduino examples and used in tutorials and kits.

jus as an example see an array of servos here

http://creatroninc.com/products.php?CategoryID=187

I believe 180 degrees is a  universal standard.

It is said 90 degrees in each direction but I take it that just means from a central position

The microsecond pulse limits I use are  general 1000, 1500,2000 for one extreme, mid point and other limit 180 degrees away.

Am I wrong??  

retrolefty

#17
Jan 17, 2010, 04:06 am Last Edit: Jan 17, 2010, 04:13 am by retrolefty Reason: 1
Quote
"Standard servo motors have 180 degree range almost universally"  "I believe 180 degrees is a  universal standard."


I've used R/C servos for decades and can tell you that your above statements are not facts let alone universal.

R/C servo standards revolve around pulse width specifications for the PPM encoding used. The 'standard' is that all R/C type servos will have a range of movement that includes 1000 usec to 2000 usec with a nominal 1500 usec center of range. There is no standard that equates that to a given degrees of travel. 90-100 degrees were typically at first but gradually most servo manufacturer increased range (over travel) above and below the 1000/2000 specs, but not to any agreed on 'standard'.

Here is a useful servo guide: http://www.servocity.com/html/how_do_servos_work_.html

And an extract from same:

"When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide."

Some R/C servos will indeed cover 180 degrees using the pulse widths below and above the 'standard' 1000/2000 usec pulse width, but again there is no 'standard' or guarantee that the ones you own will indeed cover 180 degrees.

Also the electrical range of travel will always be less then the mechanical range of travel. The gear train has 'hard stops' to prevent mechanical damage to the internal feedback pot, but the control electronics will never drive the gear train too close to it's mechanical travel limit.

Lefty

Peter Rand

Many thanks Lefty for the information.  I was simply going by all I read and claims that I hear - I am no expert and take these things at face value.  I'm certainly surprised with my results as described - that the 90 degree range was so similar for all the hardware combinations I tried and that 90 was so far from 180.

So do I understand it that you are not surprised at my results?  And that I am doing things right and can relax my expectations?  90 degrees is rather limiting for many movements I want.

Many thanks again for the advice.

Peter Rand

Lefty - even the servocity site you send me to has 0 and 180 as the limits described?????  Surely one expects to be close to those limits???

Peter.

retrolefty

#20
Jan 17, 2010, 05:08 am Last Edit: Jan 17, 2010, 05:30 am by retrolefty Reason: 1
Quote
Surely one expects to be close to those limits???


It is an example they used, but as they state in the text, servos vary in range. Keep in mind that R/C servos were developed for the model industry. They typically use adjustable mechanical linkages from the servo horns (or wheels) to the thing they are actuating, here is a link explaining: http://www.powertorque.co.za/epf/pdf/servoset.pdf . That way they can adapt the servo a given servo rotation to the range of mechanical motion they need. There was never a need for all servos to move the exact same number of degrees per usec.

Is your application dependent on having 180 degree rotation?

Edit: Some more quotes from goggling around on "servo travel":

"Rotational Distance. If you connect a servo to your receiver, plug in the receiver battery, turn on your transmitter and exercise that servo, you'll probably find that the servo arm goes about 90º total from one extreme to the other?. This seems, also, to be a standard setup for computer radios; the Travel Adjust/ATV/EPA values are defaulted to 100% in either direction.

Most servos, however, allow for ±60º rotation. What's happening here? Why don't the servos move the entire 60º when the TA/ATV/EPA value is 100%?"

Here is uTube video showing how a guy modified his servo to increase it's travel to 180 degrees, he said it orginal only went 90 degrees prior to his mod: http://www.youtube.com/watch?v=KUvjcHdq3JA



Lefty

Peter Rand

Quote
Is your application dependent on having 180 degree rotation?


I am a woodturner/sculpture interested in kinetic art.  So several different arrangements. Sometimes I can use attachments/levers to get the movement I want, but for the most part I want rotations of about 120 degrees - and in general I want to bury the whole mechanism (microcontroller,servo,lazy susan or levers, batteries etc) out of site - often inside the piece.  So small and strong is good!

retrolefty

120 degree travel should be pretty common for many servos, if you set the min/max timing values below and above 1000/2000 usecs, however you may have poor luck with the specific servos you happen to have.  :-/ I added a few links and info to my prior posting.

Left

Peter Rand

Lefty - wow now I'm learning a lot! - the servocity site and stock is an eyeopener - I'll spend time on your other sites -  very instructive - I suppose I should be grateful that I wasn't doing something wrong at least - many thanks -

Wojtek

Hi
I figure out what couse that problem.
The most motors that claim to be 90° will happily turn through 180° if you supply them with appropriately shorter and longer pulses.
I've achieved 180° control of Hextronic HXT900 and Turnigy TG9 servos using pulse lengths varying from 1.1 ms to 5.1 ms.
So you need change line of code
myservo.attach(9,1100,5100);  // attaches the servo on pin 9 to the servo object
Thanks
and good luck

retrolefty

From the: http://arduino.cc/en/Reference/ServoWriteMicroseconds

"Note that some manufactures do not follow this standard very closely so that servos often respond to values between 700 and 2300. Feel free to increase these endpoints until the servo no longer continues to increase its range. Note however that attempting to drive a servo past its endpoints (often indicated by a growling sound) is a high-current state, and should be avoided."

Try 700 and 2300 microseconds as your end points and see how much travel you get.

Lefty

D3C3PT1C0N

#26
Jun 27, 2010, 04:55 pm Last Edit: Jun 27, 2010, 05:10 pm by Fredx Reason: 1
i think servos should never be  turned by hand especially mini servos because the inner gears are very easy to break/misplace and the teeth of the gears are also very fragile. I am telling this because i just broke my mini servo (plastic gears are broken, motor still works)

mowcius

Quote
i think servos should be never turn by hand especially mini servos because the inner gears are very easy to break/misplace and the teeth of the gears are also very fragile. I am telling this because i just broke my mini servo (plastic gears are broken, motor still works)

Well most of the ones I have used have been slightly higher torque ones with metal gears.
For higher torque you need the metal gears to stop the problem you have described.

For most servos you should be able to turn them by hand (when they are not attached as this will not go above the maximum torque ratings.
If you are trying to turn it while it's on then it is likely you might exceed the maximum torque rating.

Mowcius

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