Hi there, my club has been using power expanders (pe) for a really long time now, but whenever I ask why we do so, everyone simply tells me that because our seniors did it.
I would like to ask :
What’s the advantages of using a pe?
What’s the best way to make use of the pe? (which motors and ports to connect to)
Is the trade off of a pe worth the extra weight?
Is the trade off of a pe worth risking disconnection of the pe (there have been several cases where my club has lost matches due to forgetting to plug in the pe)
Your batteries do not go dead as quickly because you have two, or the voltage doesn’t drop as badly when you run a bunch of motors under load.
Either 4 lift or 4 drive motors imo.
Depends on your robot. If you want a heavier base, then maybe.
Maybe make a prematch checklist, so you do not forget. Or leave the battery plugged in, so you do not have to remember.
As Highwayman says - your battery does not drained off so fast. Having a power expander is as good as having 2 batteries in parallel, it will definitely last longer.
The power expander has a circuit breaker inside too. That means, including the 2 breakers in the cortex, you have effectively 3 circuit breakers to distribute your current. Lesser chances of tripping the circuit breakers or PTC.
IMHO, it is always better to plug in the motors that you will not be taxing too much. Simply because there is only 1 breaker in the pe. But of course, it is also possible to split the motors (that you will be taxing) between cortex and pe. E.g. 2 lift motors on pe and another 2 on cortex.
There is no right and wrong, but the main consideration should be distribution of power.
It is definitely worth the trade off.
In any case, you wouldn’t want your base to be too light-weight as well.
As for forgetting to plug the pe… well… it is as per the pneumatics switch, etc, you just need to have a checklist or routine to make sure everything is taken care off.
As has already been said, the PE has the advantage of having a second battery, but the main advantage is to split your current draw. The motors have PTC (Positive Temperature Coefficient) thermal fuses built in, as well as the Cortex and PE. When you have stalling issues on all of your lift or drive, it is usually the PTC in the Cortex.
The Cortex has two 4 amp PTCs, one on ports 1-5, and the other on ports 6-10. Adding a PE adds a third 4 amp PTC. To prevent tripping the PTCs, you want to split the load over the 2 or 3 of them. A good way to do this is by putting all of the left side on 1-5 and all of the right side 6-10. If you add a power expander, 2 motors should come from 1-5 and 2 from 6-10.
Here is an example way to wire up your motors. Of course, this would need to be adjusted to your robot.
1 - Intake
2 - Left Drive (Power Expander)
3 - Left Drive (Power Expander)
4 - Lift
5 - Lift
Many good points in the text, but also some debatable ones.
The above suggestion is fatally susceptible to the OP’s reportedly common mistake of ‘failure to plug in Power Expander’. It also has only just 1PTC for all drive motors.
An alternate concept is to assume that functions like drive, lift, intake will most commonly be used independently; and then have a goal of limp-mode (vs dead-mode) with any one PTC disabled or unplugged. That means, put one each motor (Ldrive, Rdrive, Lift, Intake) on the power expander, like this: Now any one PTC will not disable all motors for any one function,
and there are 3PTC for drive (vs 1(!?!) above), 3PTC for lift(vs 2 above), and 2PTC for intake (same as above).
1 - Intake(Power Expander)
2 - Left Drive
3 - Left Drive (Power Expander)
4 - Lift
5 - Lift
That is a good point, however IMO, if the PTC were to trip, it would still be equally as bad. You are right to say that in the configuration you have mentioned, you are less likely to lose functionality of your mechanisms entirely. Although this is true, if some of the motors were to stop working, eg. two base motors, there would be a greater amount of stress on the other active motors. This would cause the other two base motors to stall, and the stalling would see saw between those connected to different PTCs
This year we used our power expander because if we plugged our 4 wheel and claw into the pe because if we plugged them into the cortex combined with our lift that was two motors operating the heavy lift system and sliders and a wrist to move the claw, the cortex was having trouble distributing enough power and our motors weren’t operating properly. The pe helped this problem because it allowed us to evenly distribute the power supply. This changed the game for our robot and would suggest using a pe if you near the max of 10 motors, we had 9.
As the other two said, it also helps the battery life.
Stupid error, and too late to edit it to pretend it didn’t happen…
Previous setup, PTC1-5 disables both left drive certainly will make the drive system nearly useless.
Update in Caps, put both RIGHT and LEFT drives on both 1-5 and 6-10
1 - Lift
2 - RIGHT Drive
3 - Left Drive (Power Expander)
4 - Lift
5 - Intake(Power Expander)
One correction. Posts 1 & 10 can’t go through the power expander as it is a 2 wire output vs a 3 wire PWM signal. Maybe you can but I would bet those 2 wires would fall out.
1 - Intake
2 - RIGHT Drive
3 - Left Drive (Power Expander)
4 - Lift(Power Expander)
5 - Lift
Even if you aren’t tripping breakers and your battery lasts for the entire match, I would still recommend a power expander. With heavy, high-performance robots, there is a very noticeable performance increase when you add one. I’m not sure why, I think it’s probably because the vex batteries have a low C rating, and so they can’t get enough current to the motors when they’re all drawing a lot at the same time, idk tho