Maybe a shutdown circuit? Delivers enough power to get started, reach speed, then coast the remaining distance. Also what is the goal of this competition? Speed? Innovation?

It appears that finishing at the target time is the goal. Maybe the OP could clarify.

 

My origional take was for car to stop on goal line after a given time, but on re reading decided that car could still be moving on crossing line. Wasted time planning a measuring device to put on brakes at 1 meter, wheel diameter, threaded rod, & switch.

 

Using a time based system will only give good repeatability if it also uses a voltage regulator for the motor drive, otherwise battery voltage will affect the trip speed.

And if using a voltage regulator, then it can be adjusted to control trip speed so the timer is not needed.

 

Suggest fresh batteries, or fully charged batteries for oficial runs. Still do not know what batteries are specified.

 

Using a time based system will only give good repeatability if it also uses a voltage regulator for the motor drive, otherwise battery voltage will affect the trip speed.

good point.

And if using a voltage regulator, then it can be adjusted to control trip speed so the timer is not needed.

See below:

the other problem with ... "thrust adjustment" methods [is] At the long end of the required time (above, say, 9 sec) ... 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.

 

Ahh! Thanks Strantor I missed that part.

For the ultimate winning strategy you could put a opto sensor on the vehicle wheel, and do a simple closed loop of pulse freq inverted to motor voltage. It could be done with a few transistors and gives closed loop control of actual vehicle speed.