A range sensor is an important tool in mobile robotics, not only for obstacle avoidance, but also for features recognition (e.g., corners, halls, doors, human legs). However, the first experiments should be made in simple environments with useful toolboxes as the ones available in Matlab. A common measurement system for indoor environments is the Laser Range Sensor, as the Sick Laser model LMS 200 provided by the Sick Sensor Intelligence.

From this point forward, I identify this sensor as "Sick Laser". Since this device communicates with a computer by a serial port (RS232 and RS422), I decided to develop a simple interface in Matlab to configure the Sick Laser and to make some acquisitions. The code was tested in Matlab R13 (6.5.1).

First of all it is necessary to open a serial port to establish the communication with the Sick Laser. The serial port object returned by the function

sick_port = openPort(PORT,BAUDRATE)

must be configured according to the baudrate and other specifications (mainly the TimeOut, OutputBufferSize, OutputBufferSize for optimization purposes). For simplicity, If the user has the certainty that the serial port PORT is free and available, the previous function may be replaced by

sick_port = serial(PORT,'BaudRate',BAUDRATE)

The PORT corresponds to the serial port device (ex: 'COM1') and the BAUDRATE, the required baudrate (the possible values supported by the Sick Laser are 9600, 19200, 38400 and 500000 bps).

The next step consists on the first trial to communicate with the Laser:

testSick(sick_port)

This function tries to establish a communications with the Sick Laser at the current baudrate. If the communication fails, it tries again with a different baudrate. If the communication fails at all possible baudrates, the communication can not be established (the current serial port may be reserved/used by other software, or it must be a different serial port, or even a hardware problem, cables or the Sick Laser).

At this point, it is necessary to notice that the Sick Laser has two modes of operation: the Installation Mode and Operation Mode. The Installation Mode is defined for configurations and the Operation Mode for acquisitions. The current mode is switched by functions

chgSickInstallMode(sick_port)

and

chgSickOperationMode(sick_port)

For instance to change the current baudrate, the Sick Laser must be working in Installation Mode. By default, when the Sick Laser is switched on, it is configured to communicate at 9600 bps. The function

chgBaudRate(sick_port,NEW_BAUDRATE)

switches to the required NEW_BAUDRATE.

At Installation Mode it is possible to adopt several configurations. A wrong configuration could be solved by a hardware reset. The function

initScan(sick_port)

performs a hardware reset, similar to a power off - power on.

At this moment, the Sick Laser must be switched to Operation Mode and it is ready to make acquisitions. The function

out = getScan(sick_port)

To explain the previous functions, it is necessary to introduce the telegrams. The information or data exchanged with the Sick Laser is carried by telegrams. There are two types of telegrams,

telegrams to the Sick Laser:

where each cell contains two bytes (ex: STX = 02h = 0x02h = 0x02). The data is transferred in binary format and INTEL data format (word transfer takes place with lower address and the least significant byte first and then bytes of higher significance and higher address).

The telegrams finish with the CRC. For example, the following telegram requests an acquisition (CMD = 0x30 and MODE = 0x01):

where the body includes the 361 sampling (722 bytes):

for example

Resuming, a telegram to the Sick Laser is compiled as follows:

msg = [STX ADR LENL(length(msg_body))
     LENH(length(msg_body)) msg_body];

crc16 = crc16alg(msg);
msg = [msg LENL(crc16) LENH(crc16)];

where the message body (msg_body) contains the request (command and options) or the answers (e.g. the current acquisition values). The length of the message body is simply evaluated by

function number = LENL(number)
number = mod(number,256);

function crc16 = crc16alg (msg)

CRC16_GEN_POL = 32773; % 0x8005
crc16 = 0;
abData(1) = 0;
len = length(msg);

for f=1:len
    abData(2) = abData(1);
    abData(1) = msg(f);
    if crc16 > 32768 % (crc16 & 0x8000)
        crc16 = mod(crc16,32768) * 2;
                % crc16 = (crc16 & 0x7fff) << 1;
        crc16 = binvec2dec(xor(dec2binvec(crc16,24),
                dec2binvec(CRC16_GEN_POL,24)));
    else
        crc16 = crc16 * 2; % crc16 <<= 1;
    end;
    % crc16 ^= MKSHORT(abData[0], abData[1]);
    abData(2) = mod(abData(2),256) * 256;
    crc16 = binvec2dec(xor(dec2binvec(crc16,24),
        (dec2binvec(abData(1),24) |
        dec2binvec(abData(2),24))) );
end;

Some examples of telegrams are presented. In each telegram, the message body (msg_body) is set by the command and options (if required).

For initialization, which is equivalent to a hardware reset (a power off - power on):

CMD = 0x10;
msg_body = CMD;

To testing the device:

CMD = 0x31;
msg_body = CMD;

For changing to a new BaudRate (MODE = 0x42, 0x41, 0x40 and 0x48, equivalent to 9600, 19200, 38400 and 500000 kbps)

CMD = 0x20;
MODE = 0x40;
msg_body = [CMD MODE];

As previously referred, the Sick Laser works at two different modes: Installation Mode and Operation Mode. For switching to Installation Mode it is necessary to introduce the password (in this case, the password is "SICK_LMS", equivalent to
0x53 0x49 0x43 0x4B 0x5F 0x4C 0x4D 0x53 in ASCII)

CMD = 0x20;
MODE = [0x53 0x49 0x43 0x4B 0x5F 0x4C 0x4D 0x53];
msg_body = [CMD MODE];

For switching to Operation Mode

CMD = 0x20;
MODE = 0x25;
msg_body = [CMD MODE];

Finally, for an acquisition:

CMD = 0x30;
MODE = 0x01;
msg_body = [CMD MODE];

The next step describes how to exchange the telegrams. This goal is accomplished by two simple functions,

function sendMsg(sick_port,msg)

fwrite(sick_port,msg,'uint8','async');
delay(sick_port,0)

function delay(sick_port,time)

while (strcmp(sick_port.TransferStatus, 'read') |
    strcmp(sick_port.TransferStatus, 'write') |
    strcmp(sick_port.TransferStatus, 'read&write') |
    toc    drawnow
end

If necessary, the following function flushes the buffer.