Beacons

Each beacon transmits bursts of infra-red light at regular intervals in time.  The beacon at one end of the table transmits about every 5ms, the other about every 12ms.  Note that these times are approximate - 12ms could mean anything between 11ms and 13ms.

The beacons are mounted so that the light source is about 160mm above the surface of the table, and about 90mm behind the inner surface of the end wall.  A shield is fitted just below the light source, and extends about 85mm in front of the beacon.  Note that the shield does not overhang the playing surface, but it does overhang the end wall.  The purpose of the shield is to reduce the amount of infra-red light close to the table surface, and thus reduce interference with distance sensors.

Beacon Receivers

To use the beacon signals, you need a beacon receiver.  There is a beacon receiver built in to the Handyboard, and we also provide an external beacon receiver (see pictures below).  

Handyboard beacon receiver (arrow)	External beacon receiver - connect to port 8

If you want to be able to receive a signal while close to the beacon, you will have to mount the beacon receiver at least 160mm above the table surface.  This means mounting the Handyboard high in your robot or using the external beacon receiver.  To use the external beacon receiver, you must connect it to port 8 on the Handy-Board.

Software:

We provide software which returns the beacon time interval in micro-seconds.  This is contained in the  beacons.icb  file.  It should be saved in the lib/handyboard folder under Interactive C.

This software is written in assembly language - the native language of the microprocessor on the Handyboard.  When you initialise it, it runs in the background, constantly measuring the time interval of the signals received by the beacon receiver.  It also provides two new functions which you can call from your C program.

To use this software, put the line  #use beacons.icb  at the top of your program.  You now have two functions available to control and use the beacon receivers:

beacon_init()  is used to initialise the beacon receivers, and to stop them if you no longer need them in your program:

beacon_init(0)  disables both beacon receivers;

beacon_init(1)  enables the internal receiver, disables the external receiver;

beacon_init(2)  enables the external receiver, disables the internal receiver;

beacon_init(3)  enables both beacon receivers.

beacon()  is a function which returns the time interval between the last two bursts of infra-red light received by a beacon receiver:

beacon(1)  returns the time interval from the internal receiver;

beacon(2)  returns the time interval from the external receiver.

The time value is in micro-seconds, and is returned as an integer.  If no infra-red light has been received recently, or if the beacon receiver requested is not enabled, the function will return 0.

Note that having beacon receivers enabled puts a small extra load on the Handyboard processor.  You should only enable the beacon receiver which you are using.

Suggestion:  Make your programs easier to write and to understand by hiding the details of the beacon time checking in a function.  See the example program below.

Interference

The two beacon signals on the table can interfere with each other.  Shielding around the beacon receiver will normally prevent it from receiving the signals from both beacons at the same time.  However, the signals can bounce off objects on the table, or just outside it, and arrive at the receiver from unexpected directions.

If both beacon signals are received at the same time, the beacon software can return a wide range of time values.  As the two beacons are not synchronised, these false time values will change constantly, so your program should be able to reject them.

The distance sensors also emit infra-red light of similar frequency or wavelength to the beacons.  The beam is very narrow, and it is very unlikely to fall on a shielded beacon receiver.  If light from a distance sensor does reach a beacon receiver, it can prevent the receiver from receiving the beacon signal - the software will return 0.  It can also cause the beacon receiver software to return the time interval of the light pulses from the distance sensor (around 1ms) instead of the time interval of the beacon.  Your program should reject such values.

Test Program

This program allows you to test your beacon receiver.  It is written for the internal beacon receiver.  To use the external receiver, just change the definition of beaconRX.

// first tell compiler to use beacon software
#use beacons.icb         
// note - cannot put comment on #use or # define

// define which beacon receiver to use: 1 internal, 2 external
#define beaconRX 1

void main()   
{
    int bcn;         // define variable for beacon result
    beacon_init(beaconRX);   // turn on internal beacon receiver
    
    while( !stop_button() )   // repeat until stop button pressed
      {
        bcn = checkbeacon(beaconRX);   // get beacon value using function
        printf("Beacon: %d\n", bcn);  // print message
        if( bcn == 12 )      // double beep for 12ms beacon
          {           
            beep();
            sleep(0.1);
            beep();
        }
        else if (bcn ==5)   //single beep for 5ms beacon
            {
              beep();
              sleep(0.3);
          }
          sleep(0.4); // delay a bit anyway so display doesn't flicker
    } // end of while
    
    beacon_init(0);   // turn off all receivers
    printf("Stopped, Goodbye\n");  // print message
} // end of main function

/* Function to check beacon time, return 5 or 12 for beacon time or 0 if no valid signal.
   Pass argument 1 for internal receiver or 2 for external receiver.
   Main function must enable appropriate receiver first!   */
int checkbeacon(int beacon_receiver)
{
    int btime = beacon(beacon_receiver);  // create variable and get beacon time
    if((btime > 11000) && (btime < 13000)) return 12;  //is it about 12ms?
    else if((btime > 4000) && (btime < 6000)) return 5;  // is it about 5ms?
      else return 0;   // if neither, return 0
} // end of checkbeacon function    

Note the range of time values accepted as valid for each beacon.  This allows for some variation in the beacons, but should still reject most false values due to interference.  This simple example does not do any more complicated checking - you can test it in various situations and see how reliable it is.