Moving the Robot Arm. A Brief Introduction to Servo Motors

Transcription

1 E5: 2015

2 Moving the Robot Arm A Brief Introduction to Servo Motors

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4 Servo Motors (1) Output shaft of motor turns to angle specified by input pulses. We s a stream of pulses to servo through wires connected to it. Width of pulses defines angle of output shaft. Image adapted from:

5 Servo Motors (2) Relatively precise, but not accurate Images adapted from:

6 writeposition(s,position) Controlling the Servo Create an E5Shield object >> s = servo(a, pin) >> writeposition(s,position) S commands to I/O board, and sets angle between 0 (minimum) and 1 (maximum) The I/O board controls the motors by varying the width of pulses sent to the servo

7 Control Format Examples >> writeposition(s,0.5) This will set the pulse width on motor 1 to 1.5mSec >> writeposition(1,0.25) This will set the pulse width on motor 1 to msec

8 Define a shape by x and y vertices: %% Define a shape theta=0:10:360; %theta is spaced around a circle (0 to 360). r=1; %The radius of our circle. %Define a circular magenta patch. x=r*cosd(theta); y=r*sind(theta); myshape=patch(x,y,'m'); axis(20*[ ],'square'); grid on; Key line in code. Work together: Make radius twice as large Change color to blue

9 For loop example (3) Make the shape move: %% Move it across the screen. For loop example 3 T=0.25; %Delay between images for x0=-15:2.5:15, set(myshape,'xdata',x0+x); %Translate by x0 pause(t); %Wait T seconds Key line in code, change the x-values of shape. Work together: Make the spacing between steps 1, and the delay 0.1. Does it move noticeably faster or slower?

10 For loop example (4) Make the shape move (another method): (altered code is in red). %% Move it across the screen (different technique). For loop example 4 T=0.25; %Delay between images x0 = linspace(-15,15,20); for i=1:length(x0), set(myshape,'xdata',x0(i) + x); %Translate by x0 pause(t); %Wait T seconds Work together: Make the motion smoother, but approximately as fast.

11 For loop example (5) Make the shape move by giving it velocity: %% Move it by giving it a velocity in x. Example 5 vx = 5; %velocity in meters/sec. dt = 0.25; %time step x0 = -10; for t=0:dt:4, set(myshape,'xdata',x+x0); pause(dt); x0 = x0 + vx*dt; %update x position %Translate by x0 %Wait dt seconds Work together: Make sure everybody in the group understands the code. Make the motion smoother, but approximately as fast.

12 For loop example (6) Give it velocity in two directions: %% Move it by giving it a velocity in x and y. Example 6 vx = 5; %velocity in meters/sec. vy = 10; dt = 0.25; %time step x0 = -10; y0 = -10; for t=0:dt:3, set(myshape,'xdata',x+x0,'ydata',y+y0); %Translate by x0.y0 pause(dt); %Wait dt seconds x0 = x0 + vx*dt; %update x position y0 = y0 + vy*dt; %update y position Work together: Make sure everybody in the group understands the code. Make the initial velocity the same in the x and y directions.

13 For loop example (7) Add gravity: %% Add gravity. Example 7 vx = 5; %velocity in meters/sec. vy = 15; dt = 0.2; %time step x0 = -15; y0 = 0; for t=0:dt:4, set(myshape,'xdata',x+x0,'ydata',y+y0); pause(dt); x0 = x0 + vx*dt; %update x postion y0 = y0 + vy*dt; %update y position %Translate by x0, y0 %Wait dt seconds vy = vy - 9.8*dt; %update y velocity Work together: Make sure everybody in the group understands the code. Change the initial angle to 45

14 Making Decisions in Matlab: if Syntax: if expression, statements expression has to evaluate to TRUE or FALSE Example: if a>b, disp('a is greater than b'); if (a>b) & (a>0), disp('a>b and a is positive');

15 Logical expressions expression has to evaluate to TRUE or FALSE a>b a<b a>=b a==b a~=b TRUE when a is greater than b, otherwise FALSE TRUE when a is less than b, otherwise FALSE TRUE when a is greater than or equal to b, otherwise FALSE TRUE when a is equal to b, otherwise FALSE TRUE when a is not equal to b, otherwise FALSE (a>0) & (b>0) TRUE when a is greater than 0 and b is greater than zero, otherwise FALSE (a>0) (b>0) TRUE when a is greater than 0 or b is greater than zero, otherwise FALSE (a>0) ~(b>0)true when a is greater than 0 or b is not greater than zero, otherwise FALSE Be careful if a or b are matrices, surprising results can occur! Note: this list is not exhaustive, check you text for more.

16 Making Decisions: if else Syntax: if expression, statements else statements Example: if a>b, disp('a is greater than b'); else disp('a is less than or equal to b');

17 Making Decisions: Add code to make the ball bounce. If the y position is less than 0, make the y velocity positive (i.e., take the absolute value). Once you get that to work, make it also bounce back and forth between x=0 and x=10

18 Making Decisions: if elseif else Syntax: if expression1, statements elseif expression2, statements else statements Example: if a>b, disp('a is greater than b'); elseif a<b, disp('a is less than b'); else disp('a is equal to b');