Jan 4

The snake will consist of many segments that are in principle all the same as previously defined when investigating possible battery solutions. Each block needs to be investigated further, let’s start with the servo that will act as the muscles. I have no idea what kind of force the servo must be capable of producing. I can try to make some educated guess, but perhaps the most practical is to use the ones with the highest force and get hands-on experience of the actual torque generated during movements. Searching the internet shows the most simple one is T=Kt*I were the Kt stands for Torque Constant and I for the current but there is a lot of discussion on this.

Searching thought the many, many servo’s at Conrad, there are two main topic to decide upon. Since a servo works with a small DC-engine and a gearbox, the way to increase torque is to make the gearbox ratio larger. As a result, the possible torque is increased but the turning speed is decreased. Second topic is the material used for the gears and bearings. If it is plastic, at high torque the teeth of the gears will simply snap. Same for the bearing but than more wearing out of the bearings that finally leads to the problem that only the top of a tooth is used from the gear which again results in breaking the teeth.

For speed, there are special servo’s used for controlling the lines of a sailing boat. For example the SW1200 can supply a lot of torque (115Ncm) but are slow (1.6sec for 360 degrees or 0.26sec for 60 degrees). Since they can turn 360 degrees it seems there is no stop and they can spin around several times which makes sense if you are pulling a line. Improving speed will decrease torque to 72Ncm but increases the speed to 0.1sec for 60 degrees.

I worked with the AK-12digital servo’s in the past, they provide a torque of 160Ncm and a speed of 0.2sec for 60 degrees and I was not impressed with the torque they delivered. The most powerful servo that Conrad has available is the RS-1501 that provides 180Ncm of torque and a speed of 0.2sec for 60 degrees, I’m not sure this is enough.

Another option is to use a DC-motor and a gearbox in combination with some position measurement. Based on a snakes vertebrae, it can make an angle of + or – 20 degrees compare to the next. However, my vertabraes will be longer so this should be a bit bigger, assume + and – 40 degrees. Than moving forward using a sinus pattern were  ones per second the servo would go from far left to far right would mean a minimum speed 1 sec per 80 degrees or 0.75 sec for 60 degrees.

While writing this (and that is exactly why I write this) it might also be an option to use the SW1200 and add an extra gear on to of it to reduce the speed by a factor of (0.75/0.26)=3. That would mean the circumference of the large gear would be 3 times larger than the small gear. Assume the small gear has a radius of 1 than the circumference is 2*PI*1=2*PI. Than the large one has a circumference of 6*PI so the radius is 2*PI*R=6*PI, so R=3…. Force = mass * arm length, so when the radius is 3 times longer than the force that can be applied is also 3 times longer. Normally the force is less when the arm is longer but here the gear reduction reduction increases the force. Not sure that all above is correct but this would mean that the than the force of 115Ncm can be increased to 345Ncm.

The other option is to use the RS-1501 in combination with an extra gear. The speed is 0.2 sec for 60 degrees so the reduction can be (0.75/0.2)=3.75 or 3 for short. Than using above assumption the force can be increased by 3 to a massive 540Ncm. It would require a modification of the servo since it can not turn 360 degrees but that should not be a problem. Most of them use a potentiometer connected to the axe of the final gear to give position feedback. It would be simple to use a new potentiometer for the larger gear and feed this back to the original control. Since this servo also uses metal gears and bearings I will order one to do some initial experiments.

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