On April 11, 2017 the folks at FTC announced that a new control system would be allowed in the 2017-2018 season. The Expansion Hub from REV Robotics is used already in the FIRST Global Challenge alongside their Control Hub (a separate item). As the FTC Blog says, the new Expansion Hub “replaces the functions of five Modern Robotics devices,” specifically:
- the power distribution module,
- two motor controllers,
- a servo controller, and
- a device interface module.
At the time of writing, these five modules total $387.75, compared to a $175.00 sticker price for the Expansion Hub (though teams may receive a discount)1. This alone piques interest.
The folks at REV released a fantastic technical guide for the Expansion Hub, including details about the ports, connectors, voltages, etc. that flow in and out. (Personal note: It’s one of the more detailed and helpful guides I’ve seen.) Below are a few details from the guide and some hands-on use:
- Everything is all in one place. When the Expansion Hub effectively replaces five Modern Robotics modules, that’s an obvious win for a team’s budget. However, students will find it’s a big win for space as well. At previous competitions, one could see control modules spread across the interior of robots (wherever they could fit!) or stacked 4-5 high using standoffs. Interior space is tight, so having a single* module not much larger than the old PDM is welcome.
- The module has a Bosch BNO-055 Inertial Measurement Unit built in. This replaces another $65.90 (at the time of writing) worth of Modern Robotics sensors, or $34.95 + soldering effort from Adafruit2. This should allow some teams to level-up their autonomous routines.
- Not much changes in terms of software. Teams go through the process of creating a configuration file on their robot controller phone, mapping the attached devices. After that, and an upgrade to version 3 of the FTC App, teams are all set.
- REV claims that their Expansion Hub is protected against a few issues which tripped teams up previously: reversing the battery polarity, electrostatic discharge events, and a few other exceptional cases. If this is true, it’s fantastic news for robot reliability.
* You’re probably going to need two of them, though. See below.
The Just Okay
- Given all of the great things, I hesitate to complain about anything. However, the module does support “just” four motors. Of course, each of the old Core Motor Controllers only support two motors — and now it’s built in to a single module — so it’s still a major improvement. Some teams may try to go as long as possible using only four motor ports in order to avoid adding a second hub.
- Connectors have changed. Previously, teams standardized on Anderson Powerpole connectors for battery and motor connections, however the Expansion Hub and the REV battery use an XT30 connector for power and JST-VH connectors for motors. This won’t be a problem, as REV will sell adapter cables of all kinds. It does require, however, that teams pay a little more attention to how things are plugged in. (For more on JST connectors, check this out.)
- Voltages have changed. Modern Robotics modules and sensors operate on 5V signal, whereas the REV Expansion Hub uses 3.3V signal. This means the current going through I2C sensors, motor encoders, etc. is different. Teams will need to purchase level shifting boards in order to use some Modern Robotics sensors and some motor encoders, such as those on the AndyMark NeveRest motors. That’s annoying, but workable. Compatibility information is included in the technical guide. (Note: Some Adafruit sensors, such as the RGB color sensor, have level shifting built in.)
The REV Expansion Hub is an exciting addition to the available hardware. However, just as some teams continue to use legacy Tetrix modules, we can expect to see Modern Robotics modules stick around for a while. It’s important not to confuse the Expansion Hub with the Control Hub, which is not legal for FTC teams (yet?). Make sure to think about XT30-to-PowerPole, XT30-to-Tamiya, and JST-VH-to-PowerPole adapters before you need to power your robot and your motors. You’ll also need a separate power switch for the hub and level shifter boards for old sensors and encoders.
Overall, teams that choose to purchase the REV Expansion Hub will gain some pretty cool hardware and support (e.g. the technical guide) at the cost of a little more thinking when it comes to connectors and voltages.
Edited October 8, 2017 to capitalize REV.