Coming Soon: VEX Power Expander

VEX Power Expander

The VEX Power Expander is a new product that will be available in late 2008 or early 2009. In anticipation of its release the documentation for the new VEX Robotics Competition game – *Elevation *already includes references to this new product. We are working to ensure that we have sufficient stock to ensure availability for ALL teams, before we release. Our goal is to release the Power Expander as soon as possible; we want all our customers to get as much time as possible to integrate this exciting new product into their robots before they compete.

The VEX Power Expander allows the use of a secondary battery on a VEX robot. This secondary battery is used to provide an alternative power source for (4) VEX Motors or Servos. Basically this lets you run half your motors from the primary battery and half from a secondary battery (the Microcontroller still receives power from the primary battery). PWM signals are received from the VEX controller and passed through the Power Expander to a motor or servo, while power for these (4) powers is provided by the secondary battery. This does not increase the number of total PWM signals, it simply provides an alternative power source for (4) of them.

Features:

[LIST]
*]One standard VEX power input connector for adding a secondary battery.
*]Four Motor inputs that connect with extension cables to any four Motor ports on the VEX controller
*]Four Motor outputs for connecting motors or servos. Any motor connected here is powered by the 2nd battery.
*]One digital output connector used to send data to the Vex Controller. The data this port will provide is still TBD.
*]Automatically turns ON when it detects a PWM signal.
*]Automatically turns OFF after a set wait time when no PWM signal is detected. (This keeps the battery from draining due to idle power consumption.)
[/LIST]
Refer to the attached images to see what the new VEX Power Expander will look like and get a better feel for its functionality.


JVN,

Does the expander’s ability to supply current to attached devices match that of the Microcontroller?

Same amps per channel, same amps total? More? Less?

Blake

It has the same limit as the Microcontroller (spread over (4) ports instead of over the (8) in the Microcontroller). This means you can double your output by running (4) motors on the Power Expander and (4) motors on the Microcontroller.

If a team finds themselves running into the current limit, they can do some interesting things to balance load out over the two. You can also leave yourself some redundancy. It is something worth thinking about.

I still don’t think it is necessary for all designs, however some robots (like the ones engaged in heavy QQ pushing matches) find themselves in a bad loading situation.

-John

Is there a ballpark range on the price?

Should we be worrying about the “Y” cables at all or whether or not to use them with the power expander?

Understanding of course that load is still capped.

That is so cool! That should make your battery last twice as long, correct? I am excited for this.
Will it come with the cables to connect the microcontroller to the power expander?

Depends on which motors are driving more. If you run your drive train and controller off the same battery the first battery is going to die way before the second one that might just be driving arms and servos.

this is great! my controller seems to give out whenever i run more than 4-5 motors at a time, and it looks like this will help greatly. cant wait to buy one!

Y-Cables will still work with the Power Expander, and with a Microcontroller with a Power Expander attached.

this will be grat for those who have been doing lots of 3rd party integrating lately

This is cool but all this really does is add a second battery for the motors to use. But if there is no built in voltage regulator this could be a very useful part meaning that you could use higher voltage battery’s 12v and 15v max to drive the motors for more speed and torque. Although you said something about a D/O port outputing some sort of data which leads me to believe there is an internal voltage regulator for the circuitry if not using the higher voltage may damage the circuitry.

I really want to see something like this but with 4x5-10 amp 24vdc relays on it to drive other electronics.

I had posted something like that a while ago.

What this is, is a quickfix for those teams that were tripping the microcontroller’s internal breaker last season and maybe a response to the fact that the new TETRIX kit uses two 7.2V batteries.

We want new microcontroller (waaa!!!).

Folks - For better or worse it is time for one of my long essays. Hopefully slogging through it will be worthwhile. :slight_smile:

My hunch is an emphatic “Neither”. I doubt it is a response to Tetrix using two batteries in series in a single circuit to energize a fairly small and light-weight robot, and I don’t think the Vex microcontroller’s 4 Amp limit should be thought of as something to fix.

Notice that unless something changes from the Tetrix prototypes I/we used at the 07-08 FTC World Championship, the Tetrix kits used their two batteries in series to double the voltage you can supply to Tetrix devices.
This is good if you are continuously practicing for a few hours, or if you are using a single set of batteries to try to survive an entire tournament, because when you double the voltage you use to energize a DC circuit, you halve the current you must supply to it (and in a Tetrix bot you can pull that current from the charge stored in two batteries, not one double voltage battery); but the second battery is not the equivalent of the Vex Power Expander.

When you crunch the numbers, you find that one Vex battery that has been properly cared-for and has been charged, has enough stored charge in it to run a non-expanded Vex robot for many minutes.

[LIST]
*]2000 mAHr = 2 AHr = 4 Amps for 30 minutes (4 Amps is the most the microcontroller will supply to attached devices) (I am ignoring the small current consumed by the microcontroller itself).
*]Let’s say that the battery is old, that even a new battery’s voltage drops too close to 5 V as the battery is supplying its last few coulombs of charge, and etc; and assume that all of those effects add up to cutting by 50% a battery’s ability to successfully energize a robot.
*]4 Amps for 30 minutes now becomes 4 Amps for 15 minutes.
*]15 minutes = SIX! matches in which the robot’s motors were consuming just as much current as they could possibly suck through the microcontroller for the entire duration of every match (plus some piddly amounts while sitting idle on the field).
*]And, I doubt many VRC (or FTC) robots pull 4 Amps continuously…
[/LIST]

OK, so the info above makes it sound like even a half-charged old battery should be perfectly fine to use in a match. Well… don’t stop reading yet. The real answer is a bit more nuanced.

Pulling a lot of current out of a battery (for example taking 4 amps out of the six NiCad cells in a Vex robot battery) generally lowers the voltage the battery can sustain while that current is flowing. In our Vex NiCads that becomes noticeable. There are at least two important effects that we care about:
[LIST]
*]According to a recent middle school science fair experiment, the robot’s DC motors spin a bit more slowly as the voltage across their inputs drops. That voltage comes from the battery. Based on what I learned in Electrical Engineering school, I’m not surprised. Based on what the student showed in his award-winning results, we all need to think about this as we write our autonomous-software instructions.
*]If the battery’s output voltage drops far enough below 5 V, the Vex Microcontroller can stop working. When/if the current drain stops and the battery’s voltage rises, the microcontroller circuitry will begin working again (but it will have forgotten that it was first turned on a while ago and is in the middle of a match now (can you say “re-run autonomous and then do nothing more”?)).
[/LIST]

It’s time for initial conclusions:
[LIST=1]
*]A well-charged battery that has not been abused and can consequently hold a lot of coulombs (a lot of electricity), and that can supply that electricity at a relatively stable and high voltage, is a good thing.
*]Even if a non-expanded Vex bot is often pulling the 4 Amp max through it’s microcontroller, a good battery can energize that bot for more than one match.
[/LIST]

So, … Why sell the Vex Expander???
A second battery (plus the expander) in a Vex bot doubles the amount of power (current) that at any one moment can reach the devices in the bot (mostly the motors).

[LIST]
*]The expander and the heavy battery that you attach to it, lets you send a total of 8 Amps instantaneously to your motors (4 to one batch and 4 to another). That means that even if your drive train motors are working as hard as they can using one battery, you can still have plenty of current available from a second battery to operate an arm, a winch, a shooter, the microcontroller, etc.
*]Or, you can put almost the entire 8 amps into a drive train that can out muscle drive trains that only have 4 amps at their disposal.
*]Or you can ensure that no matter how current-hungry your motors become or how exhausted the battery energizing your motors becomes, your microcontroller will not get reset by a voltage drop because it and some sensors are only nibbling at the current available from a second battery.
*]Or…
[/LIST]

Blake
PS: I want the Qwerk for VRC!!!
PPS: Thanks to RWSMAY for correcting the mistake in my original version of this message.

Yes, yes, nice post.

TETRIX = more battery life
Vex Expander = More available power

Well written Blake…

I do believe however that the Tetrix kit utilized the batteries in series for a total of 14.4 volts. Essentially the 12VDC gearhead motors were overdriven. I am not sure if this will continue to be the case when the new “blackbox” is issued with the kit.

Power expander looks great.

On another note, why did you feel the need to quote that entire page-long post:confused:

Yikes - Did I get that backwards! - I was out of the room when the batteries got connected and I never needed to pay attention to them afterwards - I obviously thought they were connected in parallel - I’ll revise the message.

Thanks

Blake

There was no need… just the circumstance of picking eeene… meeene… mineeee… mo… when I hit the reply with quotes button, and just let it fly.

What is the Dig/Int port at the bottom of the Power Expander (below the motor ports)?

Good question. There also appears to be a small button or LED adjacent to it. Perhaps it is a “low battery” warning signal to the micro controller?

  • Dean