Giant Servos – Will They Work? / Are They Worth the Cost?

Due to severe weather and the death in our coach’s family, we have not been able to gather everyone to meet in quite awhile. We’re hoping to meet again either Saturday or the following Monday. I the meantime, I have been doing some research into a thought we had about how to improve our power situation. Although it would be great to have a second battery on the robot, the power situation I’m referring to has nothing to do with electrical power, but rather the power of movement. As the FTC rules state, we are only permitted a maximum of 8 Tetrix DC motors on the robot. While they are sometimes unreliable and prone to breaking and burning out, they are by far the best performing motors the rules allow us to use. currently, we arm maxed out on our motor count, with four motors on the chassis (one for each wheel), two on the lower arm (each powering a worm gear), one on the rack and pinion arm, and the last on the back of the bucket which runs the block spinner.

7 of the 8 DC motors we have positioned on the robot are visible in this picture

7 of the 8 DC motors we have positioned on the robot are visible in this picture

The 8th motor is attached to the back of the box. we're currently deciding how best to reattach the motor and box to the arm.

The 8th motor is attached to the back of the box. We’re currently deciding how best to reattach the motor and box to the arm.

One of the changes we would like to make in the two months before state is adapting the upper arm (rack and pinion, and bucket) to be wider, more stable and more evenly distributed in weight. One of the bigger challenges in this seems to be the way the spinner’s DC motor effects the arm given its current placement. Although the motor seemingly fits well with all of the other components of the arm, it seems to be the primary source of additional weight on the arm. The box itself is made of mostly 24 gauge steel, with a 16 gauge steel hook on top that allows the robot to hang. Although the box does weight slightly more than the motor, there is very little that can be done to lighten the box. There are probably some small changes to be made, but overall, the current design is rather lightweight given its role in hanging the robot. The motor on the other hand, is awkwardly placed and is the farthest from the motor controller (makes wiring it very difficult). Our hope is to possibly move this motor farther back on the arm to reduce how it affects the arm’s center of gravity. This will be dependent on how well the double arm idea goes, but assuming it does well, this shouldn’t be too hard to build.
That of course then brings us to the widening of the arm. Danny and I have already talked through this idea some, and I despite his confidence, I personally have some issues with how this will realistically work. I’m concerned that we will have issues with fitting this new design into the 18″ x 18″ x 18″ parameter, and that if we are able to, it might compromise the consistency in which we can score blocks. Luckily, I trust Danny’s brilliance enough that I’m fine with trying it out. Widening the arm should make the arm more stable and enable us to more accurately score blocks and move the arm. Assuming this addition works well, this should also give us the ability to place arm guides on the robot so that Tyler can operate the arm more fluidly, and with less guesswork involved. To do this however, we will likely need to produce another rack and pinion, and another motor. One of the thoughts we’ve had is that we may try to use a pair of giant servos on the rack and pinions, instead of a motor. This would free up a motor for the flag spinner that we took off of the robot after the first qualifier. Additionally, FTC allows us to have a maximum of 12 servos on the robot, of which we are currently using zero. Given this, we could use two servos on each rack if that was needed. One of the issues with this idea seems to be the lack of power in the giant servos. despite the fact that giant servos are certainly more powerful than standard servos, they still aren’t quite up to par with the Tetrix DC motors, and therefore many not be strong enough to power the racks (even if we have four of them together). The most promising thought we then had was the idea of moving the motor that we are currently using on the block spinner to the second rack. This would mean we would have two motors on the lower and a giant servo on the back of the box; or possibly farther back on the arm, but still running the spinner by chain. Some of my concerns with any of these ideas lie with the cost of the servos. Not only are they weaker than the motors, but the continuous rotation giant servos are near double the cost of regular DC motors. My worst fear would be that we buy a giant servo, or multiple, and find that they don’t work, or that they kind of work, but break or fail in certain situations. More research many be required, but it seems that there are only a few servos that even come close to being viable.

This HiTec servo I found at ServoCity is arguably the closest replacement to a DC motor that we can legally use. With the continuous rotation upgrade, this servo would cost us $139.99 plus shipping.

This HiTec servo I found at ServoCity is arguably the closest replacement to a DC motor that we can legally use. With the continuous rotation upgrade, this servo would cost us $139.99 plus shipping.

Last Thursday, Danny and I decided that we would want to get a second rack milled, and since the old one has undergone a fair amount of wear and tear, that it would make sense to get two milled while we’re at it. This weekend, Cheryl texted me the frustrating news that our friend at Intermec could not get them milled because the guy who does the milling (a miller I suppose) doesn’t want to do the job because some of the tools were damaged when they made the first one. I’m at a lost as to how tools were damaged given that the rack is made of aluminum, but more importantly, we now have to order two more racks from an online site. Since not all racks are the same size, we’ll have to get new gears as well. This could create a sticky situation since the gear will have to fit over the motor hubs in the same way the Tetrix gears do. This could get very interesting.

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