So, I would like to open up a new discussion on hexapod leg design.
Currently, in most hexapod designs (MSR-1, Phoenix, etc), the femur servos are principally used to cancel out the torque on the leg created by the weight of the robot in standing position. When the robot lifts a leg, the femur servos in that leg experience minimal torque (just that caused by the weight of the leg). Since the femur servos are constantly trying to fight gravity, it causes a little shaking and the buzzing noise.
Femur servos are employed 80% of the time in standing mode and 20% in lifting mode.
What if we added springs to the femur joint to cancel out the “typical” torque caused by the standing robot. This would create zero torque on the femur servo during standing mode, 80% of the time, and place the full brunt of the load on the femur servo only in lift mode. No more buzz while standing. Way more energy effecient.
I created an excel program to model the addition of a spring and the results look as follows:
Im in the process of adding springs as it happends!
You get pros and cons with adding spings.
I have tried to use them a number of times on leg designs and first
off i will say, thay do the job.
Like you say This would create zero torque on the femur servo during
standing mode, "80% of the time, and place the full brunt of the load
on the femur servo only in lift mode. And when the robot lifts a leg,
the femur servos in that leg experience minimal torque!
You do still have to get the right sort of tension spring as an over tight
spring will make the servo get hot if used lots. but with this in mind, the
lifting of the leg is only in short periods of time.
i had used spings on an SES custom hexapod and even without applying
power the robot it could stand up just using the springs. very help full
when you have sudden power drops!
but…
… it can course other stresses if not used right.
To help things along, I whipped up this handy leg-spring calculator in excel. You input the data on your robot and the spring and it runs a simulation with a torque graph as output.
I remember for about a year ago I added springs to my original Phoenix. (my hope was to make her jump ). Well, she didn’t jump… But she got incredible strong. I removed the spring after a while because the spring wouldn’t let her stand in her normal pose on my desk when the power was off. And the spring made it more difficult for Phoenix to perform other sequences (ex. playing with a box).
Anyway, using spring is a good and simple solution for adding torque on the femur servo.
This is an interesting topic indeed. First you gotta do a case study of how much standing your bot will do and how much of the time it will be lifting the leg, since adding a spring in the lifting the leg portion would be an added “weight” sort of speak.
The advantage of it would be when it’s standing still. I think if the bot were to stand still for any long period of time, a stiffer spring, coupled with turning “off” the servo, would make the robot last longer between battery charges.
). Well, she didn’t jump… But she got incredible strong. I removed the spring after a while because the spring wouldn’t let her stand in her normal pose on my desk when the power was off. And the spring made it more difficult for Phoenix to perform other sequences (ex. playing with a box).