Has anyone ever attached a planetary gearbox such that the ring gear wasn’t mounted to some “fixed” part in order to create an unbalanced differential* effect? This means either as a prototype which failed or as a product. If so, how did it work out, or not?
Here’s why I ask. I’m trying to design an over-the-bumper top-roller intake, which can intake from the floor and eject game pieces at the top of travel. This is common in many years; I’m thinking about something similar to 2022 Everybot’s intake and low goal scoring arm, though with different actuation on the arm. I noticed that intake was only needed at floor level, and ejection only at altitude, because the intake could physically only hold two CARGO. This made me consider controlling the arm and intake with the same motor and controller - using a differential gearbox, the arm would rotate when it could, but when it hit a stop, all the torque would go to the wheels. The rotational speeds are significantly different, so a (probably multistage) gear reduction would be needed between them. Not being aware of any FRC ecosystem differential gearboxes, I gave this up for a bit, until I remembered that a differential gearbox moves at three different speeds, and their speeds are a linear combination - an “unbalanced differential” gearbox.
My concept is still developing (it changed while I typed the paragraph above, and again as I typed this one). First of all, the arm lift is slightly over-sprung, so that it tends to rise if nothing acts on it - it has to be slightly forced to the floor intake mode. The motor drives the intake wheels, probably through a single stage planetary gearbox that drives some 4" diameter compliant wheels at about 2400 to 3600 rpm (20-30fps). This same rotation will also drive the arm rotation, though at a much lower speed/higher torque through a second planetary gearbox; the wheel rotation will drive the planetary, with the ring gear mounted to the arm and the planetary carrier (output shaft) to the chassis. The expectation is that the arm will rotate first, followed by wheel rotation when the arm is at end of travel.
* Unbalanced differential- not a simple 1:1 average, but a different linear average. The term is rather capriciously borrowed from the concept of mismatched antenna impedance interfaces.
And some of the steps along the way were even stranger, with the three parts of a planetary gearbox mounted to another gearbox, the arm, and the pulleys.
I remember that Built On Brains (B.O.B. 85) had a gearbox like that in 2007 or so. They took a gear and put it on the case of the planetary and used a CIM to spin it. they claimed that it allowed them to be able to use high power settings to have very little speed, thus good pushing properties. I don’t know if the claims were substantiated or not though performance.
Edit, looking at TBA, I might have the wrong team or year.
The absolute neatest application of using a planetary gearset as a differential is the Prius’s “Hybrid Synergy Drive.”
Basically, by attaching two electric motors to a planetary gearset, they created a continuously variable transmission between the gas engine and the wheels. It also lets them do the usual hybrid trick of putting excess energy into a battery and then providing it when needed.
I can’t find a fantastic source on the internet explaining it, but this does an OK job: Prius Hybrid Drive Explained - YouTube
The explanation including the planetary starts at about 10:00, but the leadup is interesting too.
I believe 217 in 2002 or sometime thereabouts used a planetary differential for their drivetrain. I think they used it as an IVT, similar to a Prius.
EDIT: here we go: 2002 Team 217 ThunderChickens Robot
@marcusbernstein, thanks, but I’m thinking of something much simpler. BTW, that $30 motor will require a $50+ controller, or a $100+ controller if you want controller-based feedback loops. Not to mention gear reduction. @balink, again, thanks, but I was thinking of something MUCH MUCH simpler. @sanddrag, @ThaddeusMaximus and @asid61, @Schmools thanks much! Not enough detail for lessons learned, but enough to keep me going!
I’m now looking at an over-center spung arm - that is, it’ll stay up or down unless driven, with zero, one, or two CARGO. Then, likely a light duty brake on the intake wheels to send the torque to the arm raise/lower function - most likely a servo with a cam (probably just an off-center circle) engaging an intake wheel or spacer.
On the arm’s rotational shaft, I’m picturing a motor and single stage gearbox driving the intake wheels by a belt. This same rotation feeds a planetary gearbox (ring gears hard mounted to the chassis), and the planetary output of the second gearbox drives the arm rotation via the opposite branch of the arm. A light duty servo-driven brake will keep the intake wheels stationary so the arm can be moved between high and low positions. A follow-up topic asking about over-center is on the way.
You could do it with something that will hold the roller, but to do that you need to reverse the planetary output that runs the roller. In a planetary gear set if the sun is the driving element, the planets are the reactionary and the ring gear is the output the ring gear will turn in the opposite direction.
However I’ve been thinking about this and I do think that it is possible using common FRC parts and just one motor and planetary transmission with a minor mod.
Needed parts,
AM 57 Sport (Not CIM Sport) gear box in the ratio of your choice
AM 57 Sport 3" or 4" ouput shaft (optional depending on arm thickness and support for the pivot bearings.
CIM with 16T CIM Sport pinion
AM 4245 57 Sport mount
The reason for using the 57 version of the sport with a CIM is to provide the space needed to mount the motor to the AM 4245 mount instead of the transmission and you need the 57 input plate to contain the gears. The CIM mounting block is .65" thicker so that the CIM’s long shaft does not need to be cut. That should give more than enough room for the .125" bracket and button head screws to mount the motor with the pinion at the proper depth.
The transmission would mount to the arm and the hex output would drive the roller and the end of the shaft would go through a pair of hex bearings supported in line with the motor. The tapped shaft will allow you to use that to keep the arm in the correct place.
Now for the tricky part. Essentially you want to turn it into a limited slip differential.
In the typical automotive clutch type limited slip the effect is caused by friction discs splined to the axle gears alternating with steel plates keyed to the differential case. Normal force is applied by a spring or springs pressing the axle gears apart and into the case.
I think this can be achieved in the 57 sport with a nylon washer of the correct thickness, placed between the housing and the planet carrier plate. On the other end of the transmission a wave washer between the thrust washer and the 57 motor mount plate. The trick of course is the right balance of friction that the planet carrier will transmit enough torque to the housing and thus ring gear.
The downsides of course is that you are adding friction to the roller drive, but with a CIM as the drive there should be torque to waste and the rate of wear of the “clutch” disc causing a loss of torque transfer to the housing through friction.
You are thinking of team 84, CHUCK. They used a planetary gearbox as a form of CVT in at least 2007 and 2008, and won the Philadelphia Regional as third pick both years by virtue of their tough defense.
First, let me make sure I’m parsing your second sentence correctly (it seems right):
Agreed.
Further, for a single planetary stage, the magnitude of the gear ratio will be one less than when used with a fixed ring gear and the planets being the output. If I’m thinking it through correctly, for multi-stage, the last stage reduces by one less, so (for example) a 20:1 Sport (4:1 primary, 5:1 output) will reduce 16:1 and reverse.For multi-stage, actually solving the equations shows that holding the last sun gear (rollers) fixed results in an n:n-1 overdrive; On my mental picture, I hadn’t taken the ring gear rotation into account for the lower torque stage(s). When the arm hits the stop, the roller shaft will be reducing 20:1, with the rollers going the same direction as the motor (that is, the opposite of the arm.
This is problematic two ways - the direction is reversed from the needed geometry, and the relative gear ratios are off - I expect that significantly more reduction will be required for the arm than the rollers.
Arranging the terms of the speeds so that there is addition rather than subtraction seems to imply that if you want both outputs to go the same direction (without a spur gear stage), the planetary carrier (normally output shaft) must be the input. To get greater torque on the arm and greater speed on the intakes, the ring gear (normally fixed) must be mounted to the arm, and the sun gear (normally input) to the rollers.
So the geometry that seems to work is: Motor in fixed gearbox, normally configured, reducing slightly slower than the ultimate arm ratio. This drives the “output” shaft of a second gearbox (less “reduction” than the first) whose ring gear is mounted to the arm. Finally, the sun gear of that second gearbox drives the intake roller.
Unfortunately the planets as the input and the sun as the output results in an over drive situation and doesn’t solve the issue of the arm rotating the opposite direction of the roller and motor when the roller is held stationary.
You are correct that the reduction to the ring gear when the planets are stationary is -1 from the advertised reduction of the stage. So a 4 to 1 stage becomes a 3 to 1 stage reduction to the ring gear when the planets are held stationary.
I still think the limited slip method could work and gives the desired effect of only needing one motor and transmission to both deploy the arm and spin the roller.
You can solve both problems by using a winch, but that assumes you have an elevated point to which you could connect the rope. Also be sure to add a bar next to the winch that can contact a stop at the bottom to prevent backroll. One concern about this approach though is holding the cargo while the arm retracts. Maybe a kicker bar at the top end of travel when the intake is fully lifted to control launch time. The cargo would rest back a touch til then.
Understood - not optimal! I’m not on a team this year, so I’m going to forge ahead and see if I can make this work.
Doesn’t it? Letting n be the usual low-torque sun==input and high-torque carrier==output gear ratio, and \omega_c being the high torque carrier speed, \omega_s being the low-torque sun speed and \omega_r being the ring speed, I get:
n\omega_c=(n-1)\omega_r+\omega_s
This means that if \omega_r is fixed at zero [arm at stop], \omega_s has the same sign as \omega_c, and if \omega_s is fixed at zero [a brake on the rollers], \omega_r has the same sign as \omega_c.
The use of a wave washer to force limited-slip sounds good, but as I’m looking at a significant overdrive situation in the second gearbox, it feels more like a problem than a solution.
