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[DPRG] Velocity Profiling move of RC Servos

Subject: [DPRG] Velocity Profiling move of RC Servos
From: Jim Frye tech at lynxmotion.com
Date: Mon Dec 9 14:07:01 CST 2002

Hello Randy,

>You're probably right.
>
>Well, two things. I'd use higher torque servos, and metal gear
>ones, if I was looking for any kind of reliability. I suggested
>a 100 oz. in. BB servo to Jim rather than the 422 he chose. Now
>that I've seen the arm, I see what he meant about some
>components designed for a specific servo size. Assembly in
>places is critical on servo size, so I'm not sure how flexible
>substitutions could be. I'd also wonder about the plastic gears
>in the gripper and the cable delivering power to it. The cable
>gave us some problems, bending over on openning of the grippers.
>I don't know if it was we over travelled it (yep, I admit we did
>that, ...or should I say I did that) or if it is inherently
>going to be that way.

It isn't inherent to the design. I have found it to be very reliable when 
set up properly. The cable is set up as a poshrod. It will work reliable 
only if if you minimize friction when opening. If you have trouble with 
this, simply loosen the hardware associated with the gripper assembly. If 
this doesn't help then perhaps the gripper cable itself has friction. If 
that is the case then the crimp could be at fault. It's easy to fix, just 
apply a slight crimp in the opposite direction to loosen the crimp a bit. 
All of this applies to the cable bending up instead of the gripper opening. 
If however the gripper opens all the way and then the cable bends, then 
this is caused by overtravel of the servo. This can be eliminated with 
adjustment of the hardware and or software on the gripper servo. Once this 
is fine tuned it will deliver long term trouble free operation.

>We could set up a rig where it pushes buttons at opposite ends
>of its range of motion, then set it going, and record when the
>buttons stop getting pushed. Jim, you want to run one of these
>to failure to find out? What do you think?

I seam to remember a Hitec rep telling me the servos have a lifetime of a 
few hundred thousand cycles? But the most limiting factor is the 
potentiometer.

About servos.
In my testing the Hitec units perform and behave much better than their 
Futaba cousins. There has been an inrush of GWS servos on the market. My 
personal experience with them tells me they're not ready for prime time 
yet. Gears that have less than perfect quality control means they can lock 
up under normal operation, and the pots are the cheapest thing that will do 
the job. Cheap pots means low lifetime... They also have larger overall 
dimensions as compared to other comparable spec servos. This can be a real 
problem when trying to standardize mechanical components. As for using 
higher torque servos in the arm. Yes it can be done, but I'm not convinced 
that it will be money well spent. In other words I don't think that higher 
torque servos will automatically mean you can all of a sudden pick up large 
loads. I know the PID in the servos aren't designed around what people like 
us want to do. The arm with 422 servos can easily pick up as much as 8 oz. 
But the real test is when you try to put the object down in a graceful 
manor. This usually results in a slam even when the gripper is lowered 
relatively slowly. Obviously there is still much work and experimentation 
to do. Thanks, Jim




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