School Project

[Piddler]

Hello All,
This is my first post on this forum. I am in a science competition next month and have to create a electric, propeller-driven car that travels a meter in an amount of time that won't be divulged until after our vehicles are impounded by the judges. Therefore, we must be able to adjust the car on the fly at launch time. We have tried a million things and haven't had a lot of luck. We used a rheostat to adjust the voltage to the motor. We adjusted the air intake area of the propeller and even the angle of the propeller relative to the car but the results aren't near as constant as we need, sometimes varying by 30% between runs. We had an epiphany last night but don't know how to implement it.

At full speed, our car can travel the meter in about 2 seconds. We thought we might be able to use an adjustable circuit (maybe an RC with an adjustable pot) to create a delay between cutting our car on (the timers start then) and when enough voltage is delivered to the motor to make it go (around 6.5 - 7volts). We don't have a lot of electronics experience so any help would be greatly appreciated!

Here are the details:
Time requirement: 5 to 15 seconds
Voltage: 9V DC
current: App 4A
Motor/prop: standard speed 400 brushed motor and 5" prop for small hobby planes.
NO ICs allowed (I guess that means we can use resistors, capacitors, inductors, diodes etcetera).

We tried studying up on simple RC circuits and even experimented a bit but we couldn't get the motor to turn for some reason. We built a simple series circuit with a 10 ohm resistor and 1000uF cap. We attached the motor in parallel with capacitor and calculated that it should've taken app 12sec to start the motor but the prop just barely turned. Again, we appreciate whatever you can do for us.

 

[canadaelk]

You should propably go with a pulse-width-modulation circuit, I googled "transistor pwm circuit" and got plenty of results. One is in the attachement. You will need to work out the parts values, not too hard, once you understyand the principle. E

 

[cowboybob]

Welcome, Piddler!

If I understand the competition parameters, you could use transistors (which are arguably NOT ICs), configured as a Schmitt trigger comparator (plus, perhaps given your current requirments, a transistor switch):

https://en.wikipedia.org/wiki/Schmitt_trigger

With an RC timer input circuit, you could control the interval between a "start" competition command and an instant transition from zero to full power to the motor.

Here are some sample schematics (note output delay intervals):

This circuit will need a diode protected transistor switch at the output in order to handle the current load of the motor.

 

[Piddler]

Thanks for your posts!

Canadaelk: When I look at the PWM circuits, they seem to be good for speed controls which would essentially be a thrust adjustment. If so, I didn't mention the other problem with the rheostat and other "thrust adjustment" methods in my original post. At the long end of the required time (above, say, 9 sec), a weird thing happens. The amount of thrust required to move the vehicle at all (i.e. overcome the static friction of the car) will then move the car way to fast to reach the required time. Conversely, the setting of the Rheostat that will provide the precise amount of thrust needed would then require a slight nudge of the car (a big violation) to get it moving. Therefore, we abandoned the thrust-adjustment methods.

Cowboybob: That looks good. As I said, I'm a little green when it comes to transistor theory. The graphs got me thinking though. The top graph in the second picture would basically translate to complete silence for 15 s after the judges shout "GO". I'm not sure the judges would consider the car as having been "started" if it sits there initially as if it is completely turned off. I wonder if, instead of a square wave where Vcc is suddenly present after a period, if there could be an initial, slow ramping up of the voltage. That way, the propellers would immediately start turning (indicating to the judges that we have started) but not go anywhere until the voltage gets to the required level.
Thanks again for your posts!

 

[cowboybob]

OK. I'll have to admit that I am not clear on the precise timing and requirements of the competition.

...I'm not sure the judges would consider the car as having been "started" if it sits there initially as if it is completely turned off. I wonder if, instead of a square wave where Vcc is suddenly present after a period, if there could be an initial, slow ramping up of the voltage. That way, the propellers would immediately start turning (indicating to the judges that we have started) but not go anywhere until the voltage gets to the required level.

The circuit would, of course, have an on/off switch. That said, we could just add an LED (a "System ON" indicator) for the judges?

I I may not have made it clear that the delay between circuit power on and output power up is infinitely adjustable (by the "Trimmer pot") between 750 mSec and 15 Sec.

It would help (me, at least) if you would list in order (1. through x.) the exact sequence of expected events, one by one and in as much detail as you can provide, from when you place your car at the starting line (or "launch time") and what would be considered the end of the event. Also add anything that the judges may vary from one competitor to the next.

And, of course, what would be considered a win, or whatever would occur such that your team gets the trophy.

 

[alec_t]

I think you should aim for maximum acceleration, in case the judges give a 2 sec time or less. A 5" prop seems a bit small. Can your motor manage a bigger one to give you more thrust? PWM would enable good control to reduce the thrust if necessary, but it would be useful to have plenty in reserve.

 

[heydonms]

I wonder if, instead of a square wave where Vcc is suddenly present after a period, if there could be an initial, slow ramping up of the voltage. That way, the propellers would immediately start turning (indicating to the judges that we have started) but not go anywhere until the voltage gets to the required level.

If you just want the prop to start turning when you flick the switch, you don't need a ramping up of voltage. You have already established that you can use a resistor to slow down the motor to a point where your prop turns but not fast enough to move the car, and you have a circuit that provides full power after a delay. So run them in parallel

 

[ben7]

9V? I hope that isn't from a 9V battery... those can't provide enough current for your needs.

Do you have any sort of shroud around the propeller?

 

[Pommie]

Wouldn't a simpler solution be to turn the motor on at full power for a set amount of time? I'm guessing that a 1 second burst will complete the course quicker than a 0.5 second burst. Google monostable multivibrator for suitable circuits.

Mike.