Author Topic: Servo with Sharp IR on top, finest scan? time?  (Read 2262 times)

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Offline RebelgiumTopic starter

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Servo with Sharp IR on top, finest scan? time?
« on: September 04, 2008, 06:05:21 AM »
I have three questions:

1.
What is the smallest amount degrees you'd let your servo turn to take a new scan?
What's the finest "resolution" of scans, while it's still usefull ofcourse.
By usefull I mean that the Sharp IR doesn't overlap with it's previous scan.

2.
What is the finest movement possible of a servo? I have a Hitec HS422

3.
How long (time) would it take for a servo to perform such a small movement?
« Last Edit: September 04, 2008, 07:11:15 AM by Rebelgium »
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Offline benji

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Re: Servo with Sharp IR on top, finest scan
« Reply #1 on: September 04, 2008, 06:22:30 AM »
depends on your application , what are you going to do with the scan?
i prefer steppers to servos when it comes to scans ,as it provides precise steps
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Offline RebelgiumTopic starter

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Re: Servo with Sharp IR on top, finest scan
« Reply #2 on: September 04, 2008, 07:05:44 AM »
I'll use it to navigate my robot...
And It's not dependant on the application, the beam will be just as wide on my robot as it would be on some other scanner...
« Last Edit: September 04, 2008, 09:50:04 AM by Rebelgium »
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Offline Admin

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Re: Servo with Sharp IR on top, finest scan? time?
« Reply #3 on: September 04, 2008, 07:49:09 PM »
1) Here is my rant. I'm sure benji has read this soooo many times he wants to poke his eyes out :o
http://www.societyofrobots.com/robot_faq.shtml#robot_accuracy_tradeoff

2) Anyway, a typical servo will be off around 2-5 degrees or so.

3) Your servo datasheet says it rotates .16 sec/ 60 degrees. But thats at top speed, so I'd assume less during the acceleration period . . .

Offline brijesh

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Re: Servo with Sharp IR on top, finest scan? time?
« Reply #4 on: September 04, 2008, 08:27:26 PM »
I have three questions:

2.
What is the finest movement possible of a servo? I have a Hitec HS422



From the data on this site
http://www.servocity.com/html/hs-422_super_sport_.html

We have that from 1500 us to 600us variation in pulse width causes 90degree of rotation.
So degrees/us = 90/(1500 - 600) = 0.1 degree/us change in pulse width.

But the servo has 8us dead-band, hence finest movement possible is 8*0.1 = 0.8 degrees.

Your main problem is not angular resolution it will be the resolution of Sharp IR sensor output. You will soon find out that it is pretty noise and getting good readings with resolution greater 1cm is not possible unless you implement some sort of filter.



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Re: Servo with Sharp IR on top, finest scan? time?
« Reply #5 on: September 04, 2008, 08:32:53 PM »
Quote
We have that from 1500 us to 600us variation in pulse width causes 90degree of rotation.
So degrees/us = 90/(1500 - 600) = 0.1 degree/us change in pulse width.
Thats not rotational velocity, thats maximum travel distance (divided by two) and the range of possible PWM.

Quote
But the servo has 8us dead-band, hence finest movement possible is 8*0.1 = 0.8 degrees.
Dead-band is different for various servos. And if you apply any force, it will never be within that dead-band.

Offline brijesh

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Re: Servo with Sharp IR on top, finest scan? time?
« Reply #6 on: September 04, 2008, 08:51:01 PM »
Quote
We have that from 1500 us to 600us variation in pulse width causes 90degree of rotation.
So degrees/us = 90/(1500 - 600) = 0.1 degree/us change in pulse width.
Thats not rotational velocity, thats maximum travel distance (divided by two) and the range of possible PWM.

I am not calculating the angular velocity, only calculating number of degree the servo will travel for a 1us change in pulse width. Hence the number
0.1 Degrees/(us change in pulse width). Should have made that more clear.

Underlying assumption is that the variation is linear through out the range of motion.

Quote
But the servo has 8us dead-band, hence finest movement possible is 8*0.1 = 0.8 degrees.
Dead-band is different for various servos. And if you apply any force, it will never be within that dead-band.

8us dead band is for HS422, the servo used by the original poster. I agree other servos will have different dead band.


 


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