Tutorial Details:
Difficulty Level: Basic
Topics Covered: Motor commands for NQC.
Assumed Knowledge: THe Basics, My first program
Written By: BILL LANE

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In My First Program we saw just how easy it was to make our Spybot spin. This is what the script looked like:

task main()
{

On(OUT_A);

}

So lets have a closer look at that command line; On (OUT_A);. Spybots have two motor outputs. NQC refers to the right hand motor as OUT_A and the left hand motor as OUT_B. The On command simply tells the stated motor to run. If we wanted to run both motors we could have two commands one turning on OUT_A and one turning on OUT_B. But NQC makes it even easier than that. We can control both motors in the one command by using +. It looks like this:

On(OUT_A + OUT_B);

If you download and run that your Spybot it will move forward until it hits something or you turn it off. Which is great! But it would be much better if we could stop it too! It probably won't surprise you to discover that we can stop our Spybot by substituting the word Off for the word On. The problem is that if the first line tells it to start and the second line tells it to stop we'll never see it do anything. So what we really want it to do is to wait for a little while before executing the Off command, so we can see it move. NQC let's us do exactly that. Let's have a look:

On(OUT_A + OUT_B);
Wait(100);
Off(OUT_A + OUT_B);

Using Wait makes the program pause for the stated period of time. The time is in 100ths of a second. Which means that 100 is equal to 1 second. So in our example the robot moves forward for a second and then stops. We could also substitute Off with Float. Off acts like a break making our robot stop with a jerk. Float allows the motor to roll to a halt.

So what next? Shall we go backward? OK! To move backwards we need to change the direction of the motors. We have a few options to achieve this. The motors are currently stopped so we could reverse their direction on one line and then turn them back on with the next. Like so:

Rev(OUT_A + OUT_B);
On(OUT_A + OUT_B);

We can also combine both commands using OnRev(OUT_A + OUT_B); (there is an equivalent command for setting the direction to forwards and turning on the motors; OnFwd). Once again you'll want to add a Wait and then turn both motors Off.

You saw at the start that we could turn by turning just one motor On. But that's not the best way to turn. You'll get a much zippier turn by using both motors. Simply run one motor forward and one motor backwards. Here's an example:

Fwd(OUT_A);
Rev(OUT_B);
On(OUT_A + OUT_B);
Wait(100);

There's another command we can use that will allow us to make out Spybot suddenly spin in the opposite direction. Using Toggle makes the stated motors switch direction. Using it now makes our forward motor run in reverse and our reverse motor run forward. Hence it will spin in the opposite direction. Adding a short pause and an Off command here's the completed script:

task main()
{
On(OUT_A + OUT_B);          //move forward for 1 second
Wait(100);
Off(OUT_A + OUT_B);        //stop then reverse for 1 second
OnRev(OUT_A + OUT_B);
Wait(100);
Off(OUT_A + OUT_B);       //stop then spin left for 1 second
Fwd(OUT_A);
Rev(OUT_B);
On(OUT_A + OUT_B);
Wait(100);
Toggle(OUT_A + OUT_B);   //spin right for 1 sec
Wait(100);
Off(OUT_A + OUT_B);        //stop all motors
}

These aren't the only commands for moving Spybots. But they'll certainly get you started. To learn about the rest take a look at Dave Baum's NQC Programmers Guide. It's part of the standard NQC download so you mayalready have it . If not it can be downloaded from the NQC homepage (check the links page for the address).