New Stingray fails motor test when powered from wall wart

Nak

Hiya!

I just received my Stingray kit this evening and couldn't wait to put it together... It's all assembled per the instructions, but I've run into an unexpected snag. The "Testing the Motors" programs on p. 25 of the instructions initially failed for me. They would basically twitch one wheel and then the board would flash the red undervoltage LED and spontaneously reset! I double-checked all the connections and was about to give up in frustration when it occurred to me to try the test powered from the battery pack rather than the 7.5V 1a (Parallax recommended and supplied) wall wart.

SO, I am guessing that the motor startup is exceeding the current available from the wall wart... But should it be? The specs on p. 24 say the motors should draw ~180mA with no load. I have the wheels attached (per instructions) but the bot is sitting on top of a Digi-key catalog so the wheels can turn freely. Is it possible to measure the current drawn somehow?

ISTR having read that electric motors draw significantly more current with switched on and thus it's preferred to "slow start" the motor using PWM, but this is Parallax's test regimen... If the test isn't expected to work with a wall wart, it would be nice if that were mentioned in the instructions!

Follow up: A simple test using the PWM_32_v2.spin object to do a slow-start worked fine on wall-wart power... So it appears to be an over-current like I suspected.

Post Edited (Nak) : 12/8/2009 8:47:03 AM GMT

Chris Savage

What voltage and current is the wall-wart rated at? Another consideration is the unloaded voltage of the wall adapter. There is a warning in the manual about using a wall adapter when driving the motors if the adapter outputs more than 7.2V and most do. The motor test programs don't use ramping so the code can be more easily understood. In order to implement ramping significantly more code would be required.

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Chris Savage

Parallax Engineering
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SRLM

Testing with a bench power supply set at 7.4 v, these are the results that I get:

Unloaded, with electronics running and motors at about ~1/4 speed: 500ma
Stalled, with electronics running and motors set at about ~1/4 speed: 700ma

I suspect that if you were to test running the motors at full speed then you would get higher numbers. BTW, when you are accelerating the wheel (speeding up) it can't be considered to be unloaded. T = Ia...

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Nak

Chris: The wall wart is the 7.5v 1a unit I purchased from Parallax with my first Propeller board. I understand the issue now, but for future versions of the Stingray manual I would suggest adding a paragraph that explains why the motors will draw significantly more current when started this way, etc. And it might explicitly mention that the test won't (or might not) work on wall wart power for this reason.

Overall I am quite favorably impressed with the Stingray kit! The instructions were clear and detailed and the machined parts fit together perfectly. This is *exactly* the starter kit for robotics that I was looking for... Basic, easy to understand and build, and a nice size to work with. Easily portable but still plenty of room for hand crafted add-ons. Great work from the Parallax team!

SRLM: Thanks for the info! I am still curious to know the instantaneous current drawn from the batteries when one of the motors is driven full on with no PWM. I don't even know how to go about measuring such a thing. Is this something an oscilloscope can do?

Chris Savage

Nak,

Your feedback is appreciated and noted. I will be working on the future accessories we had planned to release for the Stingray coming up very soon and at that time I will make revisions to various code, documentation, etc. based on feedback from customers since the initial release. Take care, and stay tuned for more!

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Chris Savage

Parallax Engineering
50 72 6F 6A 65 63 74 20 53 69 74 65
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SRLM

Current is measured either with an ammeter (usually in the form of a multimeter) or a benchtop power supply. My power supply (www.bellnw.com/products/0201/), like many other power supplies, displays the current and voltage output.

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Nak

SRLM: Thanks for the pointer! If I may ask, does this thing enable you to record transient current spikes? Like the one that my motors were causing?

SRLM

I believe that the power supply that I have has a function where it will send out the measurements via RS-232 for things like data collection. I have never had a need for it though, so I don't know if the sample rate is high enough. You could make your own logger easily enough though. Use a commercial (non tool) current sensor (like this one from Sparkfun: www.sparkfun.com/commerce/product_info.php?products_id=9028), hook it up to an ADC then to a Propeller, and simply log the values as fast as possible to a terminal. You'd have to get some calibration values, but it would certainly give you a very accurate spike curve.

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Nak

Wow, clearly I am in no danger of running out of interesting project possibilities any time soon! Thanks again for all your replies SRLM. BTW what is that robot on your profile pic?

SRLM

It's from a project long, long ago: http://forums.parallax.com/showthread.php?p=741023

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