Motor Shield RC Car Project

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Stubie17
 
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Motor Shield RC Car Project

Post by Stubie17 »

Hey all,

For a university project me and a couple of friends are creating an autonomous robot by butchering an old RC car. The issue we're having is when we try and make the robot spin on the spot, it starts spinning fine but quickly slows to a stop.

The motors in the car are RC 280SA-2485s which are connected to M1 and M2 on the motor shield. We're currently piggybacking the L293Ds on that side because 1 chip couldn't cope. We're using the battery that came with the RC car (9.6v). The car travels forwards and backwards fine at variable speeds for long periods of time, but when it spins from a standing start it starts fine but slows to a stop within about half a second.

Fairly new to arduino and the motor shield, so any advice at all is appreciated.

Thanks,
Stuart

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adafruit_support_bill
 
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Re: Motor Shield RC Car Project

Post by adafruit_support_bill »

You are running these at slightly more than the rated voltage (8.4v) So the current ratings must be scaled proportionally. At maximum efficiency, you would be drawing about 1.35A. That is just a little over what you could expect from a pair of stacked L293D chips. I suspect they are going into thermal shutdown.

Stubie17
 
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Re: Motor Shield RC Car Project

Post by Stubie17 »

Thanks for the speedy reply!

Any reason this would be happening when it's spinning but not moving forward? Also do you think a heatsink would counter this? Somewhat limited on supplies...

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adafruit_support_bill
 
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Re: Motor Shield RC Car Project

Post by adafruit_support_bill »

Try ramping up your spin more slowly. A heat-sink might help a bit.

Stubie17
 
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Re: Motor Shield RC Car Project

Post by Stubie17 »

Unfortunately due to the construction of the car, the spin can't really start much slower as it seems to take a lot more torque to spin the car than move forward or backward (assuming this is why the thermal shutdown is happening). Looking through the 'need more power' section there were a few different options, in your opinion is it possible to get this working (by using the SN74HC595Ns or a heatsink) or should we scrap the car and use a different chassis? Would love to find out by testing but money and time are both a bit tight, so we can't really afford to spend too much of either moving in the wrong direction.

Thanks again,
Stuart

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adafruit_support_bill
 
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Re: Motor Shield RC Car Project

Post by adafruit_support_bill »

You could go with SN74HC595N for more current capability. A heatsink should give you a little extra headroom too.
due to the construction of the car, the spin can't really start much slower as it seems to take a lot more torque to spin the car than move forward or backward
This is common - particularly with 4WD and 'sticky' tires. One technique to reduce the power required for spins or tight radius turns is to reduce the friction on some or all of the tires. If you can't swap out any wheels or tires, you can wrap some tape around the treads to reduce the traction. Usually adding this treatment to just one pair of tires (typically the front) is sufficient.

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