Brainstorming Rechargeable Pi+PiDA/Motor+Servo Supplies

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Brainstorming Rechargeable Pi+PiDA/Motor+Servo Supplies

Postby alanmcdonley » Fri Jul 31, 2015 9:25 am

Perhaps this is off topic for the Pi Droid Alpha forum - Feel free to move as appropriate.

The wireless charging industry is in full gear with inexpensive charging docks and charging "receivers" that claim to supply 700ma of 5v USB power.

There is one that even "breathes" in blue light when the receiver is properly located above the dock. This sounds like something a bot could use as location feedback when rolling over a docking plate.

Starting with this in the power chain for a Raspberry Pi (A/B/B+/2) with Pi Droid Alpha based mobile robot, and with the right choice of storage I might be able to stop having to plug in my bot every few days. (Right now my RugWarriorPro bot will "run" for a few hours, or "sit quiet" under my desk for a day or two before the whining starts.)

Estimating Power Consumption:

  • The Pi Droid Alpha power need (with external motor and external servo power) appears to be negligible,
  • The Raspberry Pi (400-600ma), with active USB wireless (100-200ma), and with active camera (260ma) is probably around 1A total,
  • A tilt/Pan setup with two SG90 micro servos (holding the Pi Camera and a Pololu 2470 IR ranging sensor ) probably requires around 800ma when moving and perhaps 1.3A when stalled at the limits (seems like needs some sort of current limiter would be smart.)
  • Motors - YMMV, mine are 600ma max 1.2A stall max each, so that is 1.2A cruising and 2.4A between the time the bot hits a wall and the processor cuts the power.


A bot with this configuration would probably sit very quiet at 500-600ma, run around the room at 3A, and draw 4.7A having a "fully aware" nervous-breakdown.

Ouch - that means even if I figure out how to supply that peak load, just sitting quietly on the recharge pad takes up most of the available wireless charging power.

On the net, I see a 12000mah 4A power bank with five USB outputs for around $60 US. That could probably power the bot for 12 to 24 hours of "active" state. It would probably recharge in 20-24 hours if the bot was in total sleep, (but I don't have a real time clock (RTC) wakeup feature in the configuration).

If the bot remained always on, just sitting on its recharger, it would probably take the rest of a week to recuperate from an "all out party night"!

Wow, robots are power hungry souls.
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Re: Brainstorming Rechargeable Pi+PiDA/Motor+Servo Supplies

Postby mikronauts » Fri Jul 31, 2015 3:46 pm

Hi,

I'll check to see what they are like. I did get a couple of 500mA inductive chargers a while ago, but have not had time to play with them.

Yes, bots are very power hungry :(

I love power banks too, and have a variety here between 5200mAh and 13000mAh :)

No matter how we slice it, 700mA will take a loooooong time to re-charge the bot, even if the Pi turns itself off.
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"That power bank"

Postby alanmcdonley » Fri Jul 31, 2015 7:06 pm

I read the comments on that particular powerbank...folks commented that it turns off power when it senses the connected device is fully charged
and does not turn back on. That's not cool at all.
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Re: Brainstorming Rechargeable Pi+PiDA/Motor+Servo Supplies

Postby alanmcdonley » Fri Jul 31, 2015 9:19 pm

I was looking at my 5200mah 5v USB power bank, and noticed it says 1A so might be borderline to power the Raspberry Pi B+ board, WIFi, camera, and Pi Droid Alpha.

Additionally, I will need loosely regulated 4-7v at 1A for the tilt/pan servos.

Solution 1: Powerbank and add a $5 5v 1A buck-boost off the six 5000mah C-cell unregulated motor power with a 5A SPST power switch (Vmotor, buck-boost-in, powerbank unswitched).

Solution 2: For $15 I could use a 5v 5A step-down-only regulator to power everything - Pi, PiDA, and servos, which simplifies recharging, lowers bot weight, but probably ends up with a shorter run time. 5A SPST power switch (Vmotor, 5v step-down regulator in) This is the "safest power" solution.

Solution 3: For the same $15 I could use a 5v boost-buck 2A regulator to power only the servos, which keeps the longer runtime of two power sources, but also requires dual recharging connections, and extra weight. 5A SPST switch needed (Vmotor, 5v 2A boost-buck in, power bank is unswitched)

Solutions 4: Run everything except the main motors on the $15 5v 2A boost-buck regulator and only use the bot's C-cell power. This should get longer run time than Solution 2, with the simpler recharge, (if 2A is really enough). 5A SPST switch (Vmotor, 5v 2A boost-buck in)

The 5A SPST could also be replaced with a $7 "10Amp push button power switch", with low voltage cut-off and signaled cut-off.

Solution 2 is the safest.
Solution 4 gives longer runtime
Solution 3 is the fallback if Solution 4 won't run, and gives the longest runtime.
Solution 1 is cheaper but has no fall-back if the Powerbank proves marginal.

So Solution 4 with the fancy push button gets the nod.

Whew, buying stuff is hard work.
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Re: Brainstorming Rechargeable Pi+PiDA/Motor+Servo Supplies

Postby mikronauts » Sat Aug 01, 2015 8:30 am

For Pi bots, I have been using:

- 5V powerbank with 2A output for powering Pi, RoboPi etc
- 4-6 NimH 1.2V cells to power the gear motors / servos

I've received some 3.7V LiON cells, and will be seeing if I am better off powering the motors from those.

Personally, I am leery of running the Pi & motors off the same batteries.

It may be OK if the battery pack can handle the largest possible surge current, and have separate UBEC's / switching regulators for Pi & motors, but it is definitely safer to use separate batteries, as you cannot brown out the Pi from heavy motor current draw.
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Re: Brainstorming works

Postby alanmcdonley » Mon Aug 03, 2015 4:17 am

The goodness of having a forum to discuss - reading the prior reply got me thinking outside of the obvious.

My bot has "4-6 NiMH 1.2v cells" exactly if I just bring out a new tap at 4 cells for the servos. The tilt/pan servos don't require regulated power. They are spec'd at 3.2-7.2v, so a tap at four cells should be safely below 7.2 volts.

At first I thought - "six 1.2v cells - that's 7.2 volts, no problem", but when I measured the six cell voltage at light load fresh off the recharger - they start out supplying 9v. Also, with my current bot, I can recharge the cells with the bot on, which feeds higher than 7.2volts at the end of the cycle. The battery voltage is brought out to an ADC input (through a resister divider), and the bot watches the recharge pulses. It "announces" when the recharge cycle is complete (for me to unplug "him" so he can run loose some more).

Alan
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Re: Brainstorming works

Postby mikronauts » Mon Aug 03, 2015 4:48 pm

If you use the Vservo terminal, the protection diode will drop 0.7V, so your servos would be fine until the battery voltage dropped below 3.9V under load.

Try measuring the 6 cells with a 100 ohm load, and see what that does to the unloaded voltage, then subtract 0.7V for the diode.

Five cells nimH cells might be just about perfect for the servos!

alanmcdonley wrote:The goodness of having a forum to discuss - reading the prior reply got me thinking outside of the obvious.

My bot has "4-6 NiMH 1.2v cells" exactly if I just bring out a new tap at 4 cells for the servos. The tilt/pan servos don't require regulated power. They are spec'd at 3.2-7.2v, so a tap at four cells should be safely below 7.2 volts.

At first I thought - "six 1.2v cells - that's 7.2 volts, no problem", but when I measured the six cell voltage at light load fresh off the recharger - they start out supplying 9v. Also, with my current bot, I can recharge the cells with the bot on, which feeds higher than 7.2volts at the end of the cycle. The battery voltage is brought out to an ADC input (through a resister divider), and the bot watches the recharge pulses. It "announces" when the recharge cycle is complete (for me to unplug "him" so he can run loose some more).

Alan
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