::rtm:: Two questions regarding the HiTech HS-322HD servo motors we are allowed to use:
- Are these motors ‘locking’ motors?
According to the <R47> we are allowed to remove the locking pins from the window motors however not so from the servos.
“C. The locking pins on the window motors may be removed.
The intent of this rule is to maintain the maximum power level for each ROBOT, yet still allow teams to modify mounting tabs and the like, not to gain a weight reduction by potentially compromising the structural integrity of any motor. The integral mechanical and electrical system of the motor is not to be modified.”
We need a ‘free-backspin’ on a motor to release a mechanism AND we need a small lightweight motor. SO Question number two IF the answer to question number one is ‘yes’
- Is it possible to cause the servo to ‘backspin’ using programming code by building a pause or stop into the code?
The servo motors do not have a mechanical “lock” they have end of travel stops which can be broken if you push them past their limit. The servos are only intended to rotate 180 degrees.
On the servo you command it a position (from 0 to 180 degrees) and it uses a proportional control loop to maintain that position to the best of the motors ability. the only way to make the servo be “freely” back driven is to stop providing a PWM pulse to the motor. At that point the force to back drive the servo is the same as it would be when you are turning it by hand.
The stops within the servo are integral to the system and would not be legal to be removed. However the rules may allow for a constant rotation servo to be used.
If your plan is to have the motor being back driven at a fast speed, you need to determine a different method of accomplishing your goal.
If you wish to talk about specifics feel free to PM or email me.
:yikes: Thank you for the quick reply!
So here’s what we’re thinking, a spring loaded claw mechanism in which the ‘fingers’ would be closed by using a servo to wind a ‘string’ around a shaft. The springs would then force the ‘fingers’ back open which necessitates the need for the motor to be able to ‘backspin’ speed is probably not of the utmost importance being as we will have time to open the claw comletely as we drive across the field.
OK,
a bit of terminology first
Back drive: forcing the motor to turn in a manner that it would not normally, for example when you push your robot across the floor without powering the motors, you are back driving the drive train motors.
Now, it sounds like you want the motor to pull the claw closed, and the springs to pull it open. Keep in mind that all the motors in your kit (except the servo, which is different) are reversible, meaning you can control the speed, and the direction of rotation. So in this case you would simply have the motor rotate say forward to pull the string onto the drum(shaft in your case) to pull the claw closed. Then to open the claw the motor simply changes its direction and un-spools some rope from the drum.
Now to use the servo in that same operation you would need to pull enough rope in 180 degrees of rotation to close your claw, I am assuming you are using this claw to hold the tubes, in which case I suggest you look at a different motor.
For an example of the motor reversing I was speaking of, take a drill and put a pencil in it where the drill bit would go. Now wrap a string around the pencil a few times and slowly pull the trigger to have the drill spin, the string will wind around the pencil. Now hit the reverse switch on your drill and spin the drill as you pull on the string, now your claw would be opening.
For further visuals do a search for winch on Google or you tube.
Thank you for the ‘backspin’ definition (not having an engineering background I just use whatever terms make sense to me sometimes and yes I am blond all of which increases the level of confusion at times 
)
“Now to use the servo in that same operation you would need to pull enough rope in 180 degrees of rotation to close your claw, I am assuming you are using this claw to hold the tubes, in which case I suggest you look at a different motor.”
Have been looking at specs on the HiTechs and saw the ‘180’ reference just a few minutes ago so I went ‘back to the drawing board’.
We considered the Bainbots (sp?) however the gear reduction necessary has us concerned (weight wise) at that point in the arm. Have been looking at the Hi-Tec HSR1425-CR whose specs fall within the same range as the HS-322HD but is a continuous rotation servo. Thoughts?
I would actually recommend one of the bane bots motors along with one of their planetary gearboxes if your team has the ability to press the pinion onto the shaft.
Either the window motor or the fisher price motor is likely to be the easiest to work with. Remember if you are pulling a cable, with some pulleys your motor does not have to be at the end of your arm. Also use a limit switch to keep your motor from letting out too much string or cable and causing a backlash.
The reason I do not recommend a servo in the application you plan is the very little torque that it provides. Yes the continuous rotation one can be geared down using parts from www.servocity.com however you will likely spend most of the match waiting for it to close on your tube tightly enough to hold firm.
This could be a good use of a small pneumatic cylinder as well.
Hmmm you have definately given us something to think about. I have sent your thoughts and guidance to our design team and will take another look at things in the morning.
Thanks for the help!