Control Newport rotational stage PM500 with Instrument Control Box

Hello,

I am trying to control the Newport rotational stage PM500 with MatLab. The following is my program.

function t = PM500_Ang_Sec(theta)

% Create a GPIB object. theta is in arc-sec
obj = instrfind('Type', 'gpib', 'BoardIndex', 0, 'PrimaryAddress', 1, 'Tag', '');

% Create the GPIB object if it does not exist
% otherwise use the object that was found.

if isempty(obj)
    obj = gpib('ni', 0, 1);
else
    fclose(obj);
    obj = obj(1)
end

% Connect to instrument object, obj.
fopen(obj);
angle = num2str(theta);

% Communicating with instrument object, obj.
fprintf(obj,'xy')
 timedelay_ms(1000)
fprintf(obj,strcat('xg',angle))

fprintf(obj,'xr?')
 timedelay_ms(500)
 t1 = fscanf(obj);
 t=str2num(t1(3:end));

% Disconnect from instrument object, obj.
fclose(obj);

% Clean up all objects.
delete(obj);

When I run this function (with theta = 1200, for example), I am always getting the stage to move about 10arc-sec away from the target value (1193). But when I re-run the function again, it gets to 1200. Can anyone tell me what I did wrong?

Many thanks!

Feng