Electric Solenoids for Shifting

Hey all.

I’m not sure if this is the proper sub, but here goes anyway.

Our team uses the AM Super Shifter gearbox. We normally use the attached pneumatic cylinder for shifting. However, this year we are trying to save weight by not including a pneumatic system, and AM also sells a servo based shifter for the gearbox. The problem is that the servo based shifting is slower and requires drivers to slow down and wait for 1.5-2 seconds to shift. Does anyone have any suggestions for an electric solenoid that we could use to shift instead? The cylinder has a 3/4" bore and a 1/2" throw.

Here you have encountered the limitations of FRC-legal solenoids and servos, and the advantage of pneumatic actuators.

At 60 PSI, the shifting cylinder can provide quite a bit of force.
0.75 in / 2 = 0.375 in radius.
Pi * (0.375)^2 = 0.442 sq in area
60 PSI * 0.442 sq in = 26.5 lb or 424 oz of force.

Getting that kind of force over a full 1/2 inch stroke from a solenoid limited to only 10 watts (as per the actuators list in Table 8-1, R32) is not possible. The gear reduction of the shift servo allows it to provide adequate force, but at the cost of increased shifting time, and still not enough force to shift gears under load. A solenoid would require more power to accomplish the same feat in less time.

COTS servos costing less than $75 are allowed per Table 8-1, R32, so a large high torque servo at the upper end of this price range may give faster shifting, though still not as fast as pneumatics and may not handle shifting under load. Watch out for hitting the roboRIO’s current limit of 2.2 amps at 6 volts with high power servos.

A custom actuator using a larger motor and gearbox is possible, which would allow even faster shifting including under load. However, this could negate a fair bit of the weight savings you got by removing the pneumatics system, will take up much more space than a servo, and require more sensors and programming to manage than a servo or pneumatic cylinder.

If you will not be shifting gears often, consider an onboard air tank, regulator, solenoid valve, and air cylinder shifters. This loses the weight and space of the compressor. (See R90 and R91). Some experimentation would be in order to determine how fast you would use air and at what point the system would fail to shift under load.

Also, consider running shifting at <60 PSI. Some experimentation may be necessary to find an optimal lower-pressure shift, but it would be legal, in my non-official opinion, to use the R90 setup and exploit R87 to run lower pressure in the entire shifting system, making a single (plastic) tank last longer without an extra regulator.

I will say that if you aren’t shifting often, you can probably get away with one 120PSI tank, for one match, at least. 1197 ran that setup (offboard compressor, 120PSI stored) last year with no issues.

My apologies for the ninja edits.

I read rules R90 and R91 closer and realized that there is no way to remove the regulator from the robot. The scheme in R91 doesn’t really save any weight and severely limits your air storage to 60 PSI max instead of 120 PSI.

The diagram in Figure 8-14, R90, makes more sense, as it allows 120 PSI storage regulated down to whatever you want <= 60 PSI. Shifting may still reliable at lower pressures. If you do go this route, please report back what you find so others can more easily tune their shifters to use air more efficiently.

Keeping air tubing short between solenoid valve and cylinders will save some air as well.

@MetaEngineer

Thanks for the breakdown of the FIRST rules, I’ll report these findings to the students working the shifters. We’ll discuss them and see what they say. We are debating whether or not to include a pneumatic system, probably w/o a compressor.

@Both
If we do so, we’ll run a few tests to see how many shifts we get with a few different tank configurations.

Thanks to both of you for the quick responses. I’ll come back to this thread when we have results.

One speculative method is a custom-made screw-type actuator, which uses a (relatively) small motor to turn a screw, which drives a nut into or out of the gearbox to effect the shift.

This is essentially a tiny linear actuator, and can be far faster and more powerful than a servo.