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Re: Electric Solenoids for Shifting
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.
Last edited by MetaEngineer : 12-01-2017 at 00:30.
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