The 3pi+ 32U4 kit and robot are with available three different motor options:
|3pi+ 32U4 Version
|Micro Metal Gearmotor
|No-Load Performance at 6 V
|Stall Extrapolation at 6 V
|Top 3pi+ Speed
|30:1 MP 6V
|720 RPM, 40 mA
|0.33 kg⋅cm, 0.67 A
|75:1 LP 6V
|180 RPM, 20 mA
|0.64 kg⋅cm, 0.36 A
|15:1 HPCB 6V
|2100 RPM, 100 mA
|0.25 kg⋅cm, 1.5 A
The hyper edition is very difficult to control and fast enough to damage itself from impacts, so it is only recommended for advanced users. We strongly recommend keeping motor speeds below 50% on this version. A spare gearbox is included with this edition and instructions for installing it can be found here.
You can also assemble the 3pi+ chassis and 3pi+ 32U4 Control Board with different motor and gear ratio combinations to make your own custom 3pi+ 32U4 robot. Please keep in mind that using faster or lower-torque motors will make your robot more difficult to control.
Two on-board motor drivers power the 3pi+ 32U4’s two Micro Metal Gearmotors. Four Arduino pins are used to control the drivers:
- Digital pin 15, or PB1, controls the right motor direction.
- Digital pin 16, or PB2, controls the left motor direction.
- Digital pin 9, or PB5, controls the right motor speed with PWM (pulse width modulation) generated by the ATmega32U4’s Timer1.
- Digital pin 10, or PB6, controls the left motor speed with PWM generated by the ATmega32U4’s Timer1.
The Pololu3piPlus32U4 library provides functions that allow you to easily control the motors, and it can optionally take care of flipping a direction signal for you if you accidentally soldered in a motor backwards or are using a gear ratio with an odd number of stages, so the output turns the opposite direction from the input (see Section 7).
The 15:1 motors used on the Hyper Edition of the 3pi+ 32U4 robot have gearbox output shafts that rotate in the opposite direction from the motors’ pinion gears (unlike the motors used in the other editions, where the directions are the same). For consistency, we install the motors with the positive terminal forward on all assembled 3pi+ 32U4 robots, and we recommend building kits the same way.
This means the same inputs will produce different motor directions on a Hyper Edition robot compared to a non-Hyper robot, so a program written for one might need to be modified to work well on the other (by using the direction flipping functions provided by the Pololu3piPlus32U4 library, for example).
Batteries and motor performance
As your batteries discharge, the voltage they supply will decrease. However, since the 3pi+ 32U4 uses a regulated motor voltage (see Section 5.7 for more details), battery voltage does not typically have a major impact on the performance of the motors; they will be powered with 8 V as long as the motor voltage regulator is operating normally.
Even with a regulated motor voltage, the condition of the batteries starts to matter more as the motors draw more current (such as when accelerating, reversing, or stalled). The increased current draw of the motors causes the regulator to draw more current from the batteries in turn, and if this causes the battery voltage to drop below the regulator’s cutoff voltage, the regulator will turn off and stop powering the motors.
With the motor voltage regulator no longer drawing current, the battery voltage usually recovers a little, at which point the regulator is able to turn on again. As it starts drawing a high current to power the motors once more, the regulator enters a cycle of repeatedly turning off and back on many times a second, which effectively results in a kind of current limiting for the motors.
This behavior occurs more often with batteries that are drained than with freshly charged batteries, which means you might notice the performance of a 3pi+ 32U4 decreasing as its batteries start to run out. (For example, it might accelerate more slowly or even be unable to reach as high of a top speed.) It can also come into play with older or lower-quality batteries, which tend to have higher internal resistances that cause more significant voltage drops. The Hyper Edition of the 3pi+ 32U4 is most likely to be affected due to the greater current demanded by its high-power motors.