Difference between revisions of "Talk:Servomechanisms (aka "Servos")"

From AIRWiki
Jump to: navigation, search
m
Line 15: Line 15:
 
Hitech gives a generic torque specification on the box of their servos and on their internet site:  contacted by us, one of Hitec's service managers has denoted that spec as "stall torque", and told us to assume its 80 percent as continuous torque.
 
Hitech gives a generic torque specification on the box of their servos and on their internet site:  contacted by us, one of Hitec's service managers has denoted that spec as "stall torque", and told us to assume its 80 percent as continuous torque.
  
In order to test our servo, we hatest Before our test we have follo
+
In order to test our servos we have done the following:
Before testing, we have done the following:
+
  
- opened the case of our servo, fitted thermocouple near the circuitry, then closed it back, with the four screws
+
- opened the case of our servo and fitted a thermocouple near the circuitry, then closed it back
- mounted a horn on the shaft, to attach a wire.  Then the wire's end will be used to apply a force, which, using the horn as the lever's arm, will apply a moment on the shaft
+
- mounted a horn on the shaft, to attach a wire.  The wire's end will be used to apply a force, which, using the horn as the lever's arm, will cause the desired moment on the shaft
- prepared some objects (like water filled bottles, wired together) with adjustable weight: the final weight, divided by the length of the winch's arm (19mm in our case) will equal the tested torque.
+
- arranged some objects (like water filled bottles, wired together) with adjustable weight: the final weight, divided by the length of the winch's arm (19mm in our case) will equal the tested torque.
  
 +
And assumed this:
 +
- we can apply a moment using a single force at an arm's end, thereby obtaining something different from a "pure" torque:  our force induces rotation and translation, while a pure torque (a couple) causes only rotation.
 +
- in the context of our robot's functioning, a mere 10 minute's test time should be enough to characterize the tested torque as "continuous".  In fact we may prevent prolonged stress for our servos via behavior rules - the basis for this is that we see no reason to keep the body in a completely bent configuration for more than a minute or two (VERIFY THIS).
  
Assuming that we can apply a moment using a single force, not a couple of forces equal in modulus (the real torque), we've started our test, using Hitec's designation of continuos torque () as initial guess.
+
So we've started our test, using Hitec's designation of continuous torque (76.80 kgf*mm) as initial guess, and we've seen the measured temperature rising rapidly from 35° to 40° in the first three or four minutes of our test, so we've decided to abort this test, guessing that the actual temperature in the machinery/circuitry of our servo could be higher.
  
We've tried to provide that force to the horn of one the two servo models, to see what happens when the servo has to lift the mass and keep it still for some time:  the measured temperature has risen from 25° to more than 40° in 4 minutes, and we've guessed that actual temperature could be so high as to suggest a lower value of continuous torque.
+
We've prepared a second run with a lower torque (59.185 kgf*mm) and this time we've reached the same high temperature in 10 minutes, thereby deciding to adopt this second value as a better estimate of the "continuous" torque.
 
+
We've made a second test using ... as the torque, and the same temperature rise has occurred in 10 minutes, which is a reasonable result to take this new value as our continuous torque estimation.
+

Revision as of 21:03, 24 January 2011

The E-2? Robot project has adopted two kinds of servomechanisms as a means to drive its mechanical parts.

Here they are:

HS-645MG this is the smaller one, with 90° shaft rotation (in the simplest configuration) or 180° with a special RC controller or using Hitec's servo stretcher.


HS-785HB this is the bigger one, with shaft angular movement ranging from (-360°*2.5) to (-360°*2.5), two complete turns and a half per direction.


Here we describe how we've tried to assess the continuous torque of one of these devices.

Hitech gives a generic torque specification on the box of their servos and on their internet site: contacted by us, one of Hitec's service managers has denoted that spec as "stall torque", and told us to assume its 80 percent as continuous torque.

In order to test our servos we have done the following:

- opened the case of our servo and fitted a thermocouple near the circuitry, then closed it back - mounted a horn on the shaft, to attach a wire. The wire's end will be used to apply a force, which, using the horn as the lever's arm, will cause the desired moment on the shaft - arranged some objects (like water filled bottles, wired together) with adjustable weight: the final weight, divided by the length of the winch's arm (19mm in our case) will equal the tested torque.

And assumed this: - we can apply a moment using a single force at an arm's end, thereby obtaining something different from a "pure" torque: our force induces rotation and translation, while a pure torque (a couple) causes only rotation. - in the context of our robot's functioning, a mere 10 minute's test time should be enough to characterize the tested torque as "continuous". In fact we may prevent prolonged stress for our servos via behavior rules - the basis for this is that we see no reason to keep the body in a completely bent configuration for more than a minute or two (VERIFY THIS).

So we've started our test, using Hitec's designation of continuous torque (76.80 kgf*mm) as initial guess, and we've seen the measured temperature rising rapidly from 35° to 40° in the first three or four minutes of our test, so we've decided to abort this test, guessing that the actual temperature in the machinery/circuitry of our servo could be higher.

We've prepared a second run with a lower torque (59.185 kgf*mm) and this time we've reached the same high temperature in 10 minutes, thereby deciding to adopt this second value as a better estimate of the "continuous" torque.