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battery powered robots
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Please be positive and constructive with your questions and comments.

battery powered robots

by fmorton on Fri May 22, 2020 10:19 am

I keep having this problem in various forms.

For example, I have a robot using circuit playground with a crickit powered by a vibrating motor that is steered by pivoting two brushes using servos (https://www.adafruit.com/product/155). Using makecode to make it reachable for kids.

When testing just the movement of a single servo (nothing else running) everything works fine using a 2A 5V (https://www.adafruit.com/product/276) power supply. But, when changing to a fully charged usb battery (https://www.adafruit.com/product/1566), most of the time the servo does not move, but vibrates in sync with the triangle warning led blinking.

I assume this means there is not enough current, but that does not seem possible giving that the usb battery is supposed to deliver 5V 2A power.

What am I misunderstanding and is there a better alternative to the usb battery?

Thanks.
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fmorton
 
Posts: 12
Joined: Tue Aug 29, 2017 7:48 pm

Re: battery powered robots

by adafruit_support_bill on Fri May 22, 2020 10:54 am

Nice robot! Never seen a steerable 'brush bot' before.

Servo power draw is fairly irregular. Tower Pro doesn't publish current specs for any of their servos, but a rough rule of thumb estimate for 'Standard' size servos is 400mA + 10 * rated stall torque in oz-in. For those servos, that works out to be about 1.16A each. So it is possible that the servo peak current draw is sufficient to trip some protection circuit in your USB supply.

One thing that might help is to wire a 'BFC' in parallel with the servos to help handle that peak load: https://www.adafruit.com/product/1589

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Re: battery powered robots

by fmorton on Mon May 25, 2020 2:21 pm

I will look at adding the capacitor, but I still have the basic question.

The plug-in power source is 5V 2A and the battery (which is fully charged) is also rated at 5V 2A.

The plug-in power source never fails to move the robot. With the battery as the power source, the servo moves about 50% of the time. I just don't get the discrepancy. Either the plug-in really delivers more than 2A or the battery does not really deliver 2A.

Anyway, is there a better battery power source to use other than the capacitor idea?

fmorton
 
Posts: 12
Joined: Tue Aug 29, 2017 7:48 pm

Re: battery powered robots

by adafruit_support_bill on Mon May 25, 2020 5:02 pm

The 2A rating is the current level that manufacturer claims it can reliably source. What happens above that value is not guaranteed. In some designs, the voltage may start to drop as you exceed the rated current output. In other designs, the excess current may send some components into thermal overload and shut down the circuit entirely.

The capacitor is not a power source per-se. It is just a reservoir to store power and release it to help meet momentary peak demands.

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Re: battery powered robots

by fmorton on Tue May 26, 2020 9:14 am

I am determined to resolve this. On a whim, I used Circuit Python on the exact same robot doing the same thing as the makecode approach. The robot functions without any issues at all. In fact, I can drive both servos without any issue. This points to how makecode is driving the servos. Any thoughts about how to resolve that?

fmorton
 
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Joined: Tue Aug 29, 2017 7:48 pm

Re: battery powered robots

by adafruit_support_bill on Tue May 26, 2020 9:36 am

The blinking warning light on the power supply is a pretty clear indication of an overload. The power supply has no way of knowing what programming language you are using.

It is possible that your MakeCode code is asking the servo to make more extreme movements so the peak current draws are higher.

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Please be positive and constructive with your questions and comments.