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Driven Damped Pendulum Axes Question

Asked by Calin Pastean on 16 Feb 2015
Latest activity Answered by MKG
on 17 Feb 2015
Hello, I am quite new to MATLab, this is my very first program, which is also a uni assignment. This is my program thus far:
m=2;
l=5;
g=9.81;
c=0.5;
theta=45;
theta_dot=5;
t=0;
dt=(0.1/theta_dot);
while t<50
theta_dotdot=-(g/l)*sin(theta) - (c/m*l^2)*theta_dot; %motion eqn
theta=theta+dt*theta_dot;
theta_dot=theta_dot+dt*theta_dotdot;
figure(1);
axis([0 0 -2*pi -2*pi]);
hold on;
plot(theta_dotdot, theta);
t=t+dt;
end
What I have done so far (the code above explained): 1.Giving some values (the first part of the code). 2.The equation of motion (%motion eqn). 3.Theta and Theta_dot increasing in value (recommended method/code I was given in the assignment). 4. Figure - doesn't work, and I am not quite sure how it should work. 5. Plot - no reason for it not to work, I assume it doesn't plot anything because of the figure error above. 6. t increasing. 7. Everything above is looped, with t=t+dt What I need to do: Firstly, I need it to give me a figure of the pendulum positions (don't know how to explain this better)- the figure part of the code Secondly, I need it to plot theta_dotdot, theta. I think I got this part right, but the program does not run that part yet, im assuming because of the errors in the figure part.
There are the erros im getting:
>> run Untitled2
Error using set
Bad property value found.
Object Name: axes
Property Name: 'XLim'
Values must be increasing and non-NaN.
Error in axis>LocSetLimits (line 201)
set(ax,...
Error in axis (line 93)
LocSetLimits(ax(j),cur_arg);
Error in Untitled2 (line 17)
axis([0 0 -2*pi -2*pi]);
Error in run (line 63)
evalin('caller', [script ';']);
>>
If an answer can not be provided, I could really use some help on the errors, why am I getting them, what do they mean.

  2 Comments

Star Strider
2015 年 2 月 16 日
See the documentation for ode45. I would use it rather than attempt to integrate your ODE with your own integration routine, unless for some reason you are not allowed to.
Calin Pastean 2015 年 2 月 16 日
@StarStrider: I am pretty sure they are not expecting anything too complex, since it is a first year course, first assignment. It just seems to me that there is no link between my equations and the figure 'generator'. if I run the program I also get Figure(1) without anything on it.

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2 Answers

回答者: Youssef Khmou
2015 年 2 月 16 日
編集済み: Youssef Khmou
2015 年 2 月 16 日
 採用された回答

The implementation is acceptable, the first remark is that visualization commands of results should be outside the loop, otherwise you obtain N plots where N is the number of iterations. While loop in this case can be changed into for loop but not necessary, the following version is working , but the physical interpretation still needs correction of the code, try to adjust it :
m=2;
l=5;
g=9.81;
c=0.5;
thet=45;
dtheta=5;
t=0;
dt=(0.1/dtheta);
cc=1;
for t=1:dt:50
d2theta(cc)=-(g/l)*sin(thet) - (c/m*l^2)*dtheta; %motion eqn
theta(cc)=thet+dt*dtheta;
dtheta=dtheta+dt*d2theta(cc);
cc=cc+1;
end
figure(1);
% axis([0 0 -2*pi -2*pi]);
% hold on;
plot(d2theta, theta);

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回答者: MKG
2015 年 2 月 17 日

Hello, try the following code for your x limits: axis([-2*pi 0 -2*pi 0])
Regards, Martin

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