Go Down

Topic: Need some guidance for a school project. (Read 2221 times) previous topic - next topic

jensPA

Hi!

Me and two friends at school are working on a school project, the goal is to build a "robotcar" from a old RC Buggy of mine. Using a infrared sensor and a simple compass to guide it in one determined direction. I am responsible for the guidance system, first of all do you think I could get this to work? My only experience is a class where we programmed a linetracker.

I am thinking of buying a Arduino Uno, Ardumoto shield for the motor, a simple servo to steer the car and the sensors I mentioned before. Before I start I need to ask some probably simple questions.

Which motor can I use? I need a relative strong one I think because it's going to power a rather big construction, but what and how big motor can I get with this shield and powering the whole thing with eight 2500mAh NiMH AA batteries in serie?

Also when programming the linetracker one big problem that would recur was that the use of delay() would make the whole guidance very detained. Would this be a problem for me? I know the servo needs a delay to reach the desired position. If so, how usually do you solve this problem (I guess you have to program smarter in some way)?

I'm sure I have a lot more questions, I just can't recall them right now.

Thanks in advance

Jens

John_S

Using delay() in all but the simplest sketches is usually problematic. Look at the "blink without delay" example and use that timing method exclusively in your project. It will save you headaches in the future.

Look at the motor shield specs to see what the max current that it can handle is (It might be as much as 1 amp.) Select a motor with a current draw less than that.
http://jsrintervalometers.blogspot.ca

jensPA

Thanks for the answers John_S.

The ardumoto shield can drive 2 amps per channel. First of all is it possible to parallel the outputs and get 4 amp since I only have one motor. And how does this work? Will the motor when set to 100% in the code spin regardless of what voltage of the motor is? I also guess that you preferably want to power the shield now through the Arduino board with that specific current, am I right?

Grumpy_Mike

Quote
First of all is it possible to parallel the outputs and get 4 amp

No the chip uses transistors, it is only wise to parallel up FET drivers. Transistors have a hard time sharing.

Quote
I also guess that you preferably want to power the shield now through the Arduino board with that specific current, am I right?

No you can't get that much current from an arduino.

jensPA

Thank you for quick response.

But the question about the voltage, does it matter if the max voltage of the motor is for example 8 V or 12 V? If I have a motor with max voltage 12 V and i only have 9.6 V from the batterie, would that motor spin 100%?

Sorry for newbie questions, am not that good with hardware.

Grumpy_Mike

If you under voltage the motor it will spin slower and develop less torque.

jensPA


If you under voltage the motor it will spin slower and develop less torque.


That I get. But I am wondering what voltage the shield would put out if I give it a 100% PWM signal and I have VIN with 9.6 voltage?

Grumpy_Mike

You loose about 2.5V with that chip so if you supply it with 9.6V you will get 7.1V drive on your motor.

jensPA


You loose about 2.5V with that chip so if you supply it with 9.6V you will get 7.1V drive on your motor.


So if I go with a supply of 12 V, then I get about 9.5 V to the motor? Would this motor work? If a remember right shouldn't the amp be 13 W / 9,5 V = 1,4 A, am I doing this right?

Load Speed ??4800 rpm
Nominal voltage 12 V / DC
Efficiency 68%
Idling speed 5700 rpm
Type SP 3657-50
Torque (max.) 26 N mm
No load current 0.25 A
Shaft Ø 3.17 mm
Operating voltage 6-24 V / DC
Average power consumption 1.6 A
Manufacturer-SP3657-050-GFCS-5
Weight 224 g
Effect (electric motor) 13 W

Grumpy_Mike

Well calculating power dosn't quite work like that because it depends on the load.
As the spec says the motor works down to 6V them it will still spin.
Whether it is powerful enough for you in your project is another matter.

jensPA


Well calculating power dosn't quite work like that because it depends on the load.
As the spec says the motor works down to 6V them it will still spin.
Whether it is powerful enough for you in your project is another matter.


Ok, I notice the no load current is specified, why not also max load current? Anyone have any idea of what motor I should buy? Cause I have no clue right now. The chassi looks like this
The remaining part is the Arduino board + shield, servo, batteries about 260g and a motor about 250.

Grumpy_Mike

Quote
why not also max load current?

Often the stall current is quoted on the better data sheets, but motors are not rated for continuous stall. The maximum working current is hard to measure and often doesn't mean anything useful.

retrolefty

Quote
why not also max load current?


Often the 'locked rotor' current for simple DC motors can be estimated by simply taking an accurate ohm meter reading at the motor terminals. Then using simple ohms law calculations using the maximum applied voltage you will be using will give you an estimate of what to size the switching transistors and voltage source for max current capacity.


Lefty


jensPA

Ok, I should have known that you want the stall current.

But you say they often specify this on better data sheets, do you know any shop that have these better sheets?

And once again thank you for the answers.

retrolefty

Quote
But you say they often specify this on better data sheets, do you know any shop that have these better sheets?


Only the manufacture that designed and built the motor is in a position to develop and publish a true datasheet for a specific motor. A given 'shop' may be able to perform their own measurements and give you an estimate of the key motor parameters, but I wouldn't count on it.

Remember, measure twice cut once.

Lefty

Go Up