I have a question about hard stops in Modelica.Mechanics.Translational.Components.MassWithStopAndFriction.
As I can understand mass should not move outside interval (smin, smax)
But it actually does in my example that I include here:
model ActuatorMechanics
Modelica.Mechanics.Translational.Sources.Force force;
Modelica.Mechanics.Translational.Components.MassWithStopAndFriction mass(m=1,F_prop=0,F_Coulomb=10, smax=0.1, smin=0, L=0.01);
Modelica.Mechanics.Translational.Components.Spring spring(c=1000);
Modelica.Mechanics.Translational.Components.Fixed fixed;
Modelica.Mechanics.Translational.Sensors.PositionSensor sens_pos;
equation
connect(force.flange, mass.flange_a);
connect(mass.flange_b, spring.flange_a);
connect(spring.flange_b, fixed.flange);
connect(sens_pos.flange, mass.flange_a);
force.f = 100;
end ActuatorMechanics;
simulate(ActuatorMechanics)
plot(mass.flange_a.s)
Am I doing something wrong?
This was a bug in OpenModelica. It's working since r11060 and a regression test has been added.
Well, this is really a question for the OpenModelica developers so hopefully one of them will pop in here and answer.
Just to give you a little bit of background on what is going on in the model, when the mass hits a stop, it switches into a different state where it constrains the mass to a zero acceleration (not velocity) and computes the reaction force necessary to hold that constraint.
This complexity is due mostly to the fact that OpenModelica (and most, if not all, other Modelica tools) have difficulty handling variable index DAEs. The trick here is to detect the point at which the mass reaches its mechanical limit, use reinit to set the velocity to zero and then enforce the no acceleration constraint I mentioned above.
This all depends on the velocity being a state. It is possible to formulate a system using this component where the velocity will be prescribed on the system (and therefore not a state). In this case, the reinit has no effect. But I would suspect that you would get a singular system of equations at that point (since you'd have essentially two equations for the acceleration of the mass once the mechanical limit is reached). So I am surprised that you are seeing movement beyond those limits.
Another possibility could be that the mass starts its motion outside the mechanical limits and somehow the switch to the alternative (constrained) equations is not made.
Again, it is really a question for the OpenModelica developers. I'm just trying to provide some insights as to things that could go wrong with such a model. Although I admit such insights are not particularly useful in answering your question.
I would suggest you also contact the OpenModelica developers (in addition to posting here on StackOverflow) since they may not see this question.
Good luck.
Related
I am hoping somebody here will be able to help me out with my small issue with one of the Simulink/Matlab code. It is quite similar to the problem that I’ve discussed earlier, but a little bit more complicated and now it is more a Simulink issue, rather than a Matlab one.
So I have a turbine which speed is controlled by the gate’s opening, hence the control voltage. By controlling the gate opening I am accelerating the turbine and at some point in time, I need to introduce a saturation effect (since I am testing the code now, it will be done an external signal). This effect won’t change the control voltage, but it affects other components of the system, hence at the same control voltage, the turbine’s speed will go up. But at the same time, I need to keep the speed at the same value as it was before the saturation effect (let’s say it was 320 rpm). To do so I need to decrease the control voltage and should keep doing it until I reach the speed as it was before. There is no need to do it instantly (this approach will be later introduced in hardware), but it will be a nice thing to check the algorithm in these synthetic tests.
In terms of the model, I was planning to use a while loop with the speed requirement “if speed > 320” again, now just to simplify things. To decrease the control voltage I was planning to subtract from the original 50 (% opening) - 0.25 (u2) at first and after that increasing this value by 0.25 until I decrease the speed below 320. I can’t know the exact opening when this requirement will be satisfied, hence I need some kind of algorithm to “track” this voltage.
So it should be something like this:
u2 = 0;
While speed > 320
u2 = u2+0.25
End
u2 is initially zero since we have a predefined initial control voltage. And obviously, when we reach the motor’s speed below 320, I need to keep the latest value of the u2 (and control voltage).
Overall, it is a small code and should be done in Simulink (don’t want to introduce any other Fcn function into the model). I’ve never used while and if blocks in Simulink, but so far I came up with this system. It’s a simplified version of my model, but the control principle is the same.
We are getting the motor speed of 350, compared with 320 (the speed before “saturation), and if our speed after saturation is higher, we need to reduce the control voltage. To trigger the while loop block I’ve decided to use a simple switch. The while block meanwhile is:
Definitely not the best implementation but I was trying a lot of different combinations and without any real success. I am always getting the same error:
Was trying to use a step signal instead of the constant “7” – to model acceleration of the motor, and was getting the same error at the moment of acceleration above 320 threshold. So looks like the approach is almost right but mathematically it fails to find the most suitable solution. I’ve tried to implement a transport delay in the memory part of the while subsystem but was getting errors during compilation all the time.
Are there any obvious (and not so) mistakes? Or maybe from the beginning, I should have chosen another approach… I really hope that somebody will be able to help. Thank you in advance and have a great day.
I do not think that you have used While block correctly.
This is what I have done, I used a "Matlab function" block instead of "While" block as follows,
The function in Matlab function is
function u2=fcn(speed,u2d)
if speed>320
u2=u2d+0.25;
else
u2=u2d;
end
And the results I have got, Scope 1
Scope
Edit
As you prefer a function free model, the following may do the same.
Developing with various game physics engines over the years, I've noticed that on the same machine I observe widely different results in physics simulations between runs. Most recently, the Unity engine does this, even though physics are calculated at set intervals of time (FixedUpdate) -- as far as I can determine it should be completely independent of frame-rate.
I've asked this question on game forums before, and was told it was due to chaotic motion: see double pendulum. But, even the double pendulum is deterministic if the starting conditions are exactly controlled, right? On the same machine, shouldn't floating point math behave the same way?
I understand that there are problems with floating point math accuracy, but I understand those problems (as outlined here) to not be problems on the same hardware -- isn't floating point inaccuracy still deterministic? What am I missing?
tl;dr: If running a simulation on the same machine, using the same floating point math(?), shouldn't the simulation be deterministic?
Thank you very much for your time.
Yes, you are correct. A program executed on the same machine will give identical results each time (at least ideally---there might be cosmic rays or other external things that affect memory and what not, but I would say these are not of our concern). All calculations on a computer are deterministic, and so all algorithms of a computer will necessarily be deterministic (which is the reason it's so hard to make random number generators)!
Most likely the randomness you see is implemented in the program with some random number generator, and the seed for the random numbers varies from run to run. Should you start the simulation with the same seed, you will see the same result.
Edit: I'm not familiar with Unity, but doing some more research seems to indicate that the FixedUpdate routine might be the problem.
Except for the audio thread, everything in unity is done in the main thread if you don't explicitly start a thread of your own. FixedUpdate runs on the very same thread, at the same interval as Update, except it makes up for lost time and simulates a fixed time step.
source
If this is the case, and the function itself looks somewhat like:
void physicsUpdate(double currentTime, double lastTime)
{
double deltaT = currentTime - lastTime;
// do physics using `deltaT`
}
Here we will naturally get different behaviour due to deltaT not being same from two different runs. This is determined from what other processes are running in the background, as they could delay the main thread. This function would be called irregularly and you would observe different results from runs. Note that these irregularities will mostly not be due to floating point inprecision, but due to inaccuracies when doing integration. (E.g. velocity is often calculated by v = a*deltaT, which assumes a constant acceleration since last update. This is in general not true).
However, if the function would look like this:
void physicsUpdate(double deltaT)
{
// do physics using `deltaT`
}
Every time you do simulations using this you will always get the exact same result.
I've not got much experience with Unity or its physics simulations, but I've found the following forum post which also links to an article which seems to indicate it's down to precision with the floating point calculations.
As you've mentioned, a lot of people seem to keep rehashing this question!
The forum also links to this blog post which may shed some light on the issue.
I'm currently studying dynamic programming solutions to Markov Decision Processes. I feel like I've got a decent grip on VI and PI and the motivation for PI is pretty clear to me (converging on the correct state utilities seems like unnecessary work when all we need is the correct policy). However, none of my experiments show PI in a favourable light in terms of runtime. It seems to consistently take longer regardless of the size of the state space and discount factor.
This could be due the implementation (I'm using the BURLAP library), or poor experimentation on my part. However, even the trends don't seem to show a benefit. It should be noted that the BURLAP implementation of PI is actually "modified policy iteration" which runs a limited VI variant at each iteration. My question to you is do you know of any situations, theoretical or practical, in which (modified) PI should outperform VI?
Turns out that Policy Iteration, specifically Modified Policy Iteration, can outperform Value Iteration when the discount factor (gamma) is very high.
http://www.cs.cmu.edu/afs/cs/project/jair/pub/volume4/kaelbling96a.pdf
This question is related to an issue that I encountered when I was playing with some blocks. Here's the model that I have,
As you can see, there are two kinds of connections, the inputs of the first connection(from the top to bottom) is u[1],u[2],u[3], other blocks are quite self-explanatory (all default values, except that startTime = 5 for the step input block).
From my knowledge, the first kind of connection only outputs angular velocity but not angle and angle acceleration(they are both zero), which is a bit not realistic(I'll explain why I did this). The second connection outputs an angular velocity as well.
My problem was that, in the Second connection, the clutch seems working all right(after 5 seconds the clutch is engaged(relative angular velocity w_rel = 0))
However, the first connection behaves quite differently. We can see that they are all flange connections, and angular velocities are all calculated from flange_a/b.phi, so we should expect that there is no angular velocity difference in the clutch no matter what the input (realExperssion1) is. But the interesting thing is that when I simulate the model, the left flange of the clutch is not moving, the right flange is rotating instead. Here're two plots of my results.
Connection1
Connection2
Actually, I should expect to see the flange_a.phi and flange_b.phi are all zero, and then I accidentally removed the annotation __Dymola_InlineAfterIndexReduction = true in the move block, then the model behaves as what I expected. I wound be really appreciated if anyone could help me explain what I saw. Thanks A LOT!
The documentation for the Move model clearly says
The user has to guarantee that the input signals are consistent to
each other
In your case, they are not consistent. So I'm not too surprised you get a strange answer. It was not clear to me why you even attempted to go this route. You imply in the message you would explain why, but I certainly didn't understand your motivation. I suspect that the Move model exists to allow the user to provide their own explicit functions for position, velocity and acceleration that Dymola will use during index reduction instead of generating those functions from the underlying equations. Unless you can provide consistent functions, you really shouldn't use this block at all.
You should really be using a source where you specify only one of position, velocity and acceleration. If that isn't possible, then I'm afraid you'll have to explain why so we can try to understand what you are really trying to achieve here.
I am developing an augmented reality application that (at the moment) wants to display a simple cube on top of a surface, and be able to move in space (both rotating and displacing) to look at the cube in all the different angles. The problem of calibrating the camera doesn't apply here since I ask the user to place the iPhone on the surface he wants to place the cube on and then press a button to reset the attitude.
To find out the camera rotation is very simple with the Gyroscope and Core Motion. I do it this way:
if (referenceAttitude != nil) {
[attitude multiplyByInverseOfAttitude:referenceAttitude];
}
CMRotationMatrix mat = attitude.rotationMatrix;
GLfloat rotMat[] = {
mat.m11, mat.m21, mat.m31, 0,
mat.m12, mat.m22, mat.m32, 0,
mat.m13, mat.m23, mat.m33, 0,
0, 0, 0, 1
};
glMultMatrixf(rotMat);
This works really well.
More problems arise anyway when I try to find the displacement in space during an acceleration.
The Apple Teapot example with Core Motion just adds the x, y and z values of the acceleration vector to the position vector. This (apart from having not much sense) has the result of returning the object to the original position after an acceleration. (Since the acceleration goes from positive to negative or vice versa).
They did it like this:
translation.x += userAcceleration.x;
translation.y += userAcceleration.y;
translation.z += userAcceleration.z;
What should I do to find out displacement from the acceleration in some istant? (with known time difference). Looking some other answers, it seems like I have to integrate twice to get velocity from acceleration and then position from velocity. But there is no example in code whatsoever, and I don't think that is really necessary. Also, there is the problem that when the iPhone is still on a plane, accelerometer values are not null (there is some noise I think). How much should I filter those values? Am I supposed to filter them at all?
Cool, there are people out there struggling with the same problem so it is worth to spent some time :-)
I agree with westsider's statement as I spent a few weeks of experimenting with different approaches and ended up with poor results. I am sure that there won't be an acceptable solution for either larger distances or slow motions lasting for more than 1 or 2 seconds. If you can live with some restrictions like small distances (< 10 cm) and a given minimum velocity for your motions, then I believe there might be the chance to find a solution - no guarantee at all. If so, it will take you a pretty hard time of research and a lot of frustration, but if you get it, it will be very very cool :-) Maybe you find these hints useful:
First of all to make things easy just look at one axis e.g x but consider both left (-x) and right (+x) to have a representable situation.
Yes you are right, you have to integrate twice to get the position as function of time. And for further processing you should store the first integration's result (== velocity), because you will need it in a later stage for optimisation. Do it very careful because every tiny bug will lead to huge errors after short period of time.
Always bear in mind that even a very small error (e.g. <0.1%) will grow rapidly after doing integration twice. Situation will become even worse after one second if you configure accelerometer with let's say 50 Hz, i.e. 50 ticks are processed and the tiny neglectable error will outrun the "true" value. I would strongly recommend to not rely on trapezoidal rule but to use at least Simpson or a higher degree Newton-Cotes formula.
If you managed this, you will have to keep an eye on setting up the right low pass filtering. I cannot give a general value but as a rule of thumb experimenting with filtering factors between 0.2 and 0.8 will be a good starting point. The right value depends on the business case you need, for instance what kind of game, how fast to react on events, ...
Now you will have a solution which is working pretty good under certain circumstances and within a short period of time. But than after a few seconds you will run into trouble because your object is drifting away. Now you will enter the difficult part of the solution which I failed to handle eventually within the given time scope :-(
One promising approach is to introduce something I call "synthectic forces" or "virtual forces". This is some strategy to react on several bad situations triggering the object to drift away although the device remains fixed (? no native speaker, I mean without moving) in your hands. The most troubling one is a velocity greater than 0 without any acceleration. This is an unavoidable result of error propagation and can be handled by slowing down artificially that means introducing a virtual deceleration even if there is no real counterpart. A very simplified example:
if (vX > 0 && lastAccelerationXTimeStamp > 0.3sec) {
vX *= 0.9;
}
`
You will need a combination of such conditions to tame the beast. A lot of try and error is required to get a feeling for the right way to go and this will be the hard part of the problem.
If you ever managed to crack the code, pleeeease let me know, I am very curious to see if it is possible in general or not :-)
Cheers Kay
When the iPhone 4 was very new, I spent many, many hours trying to get an accurate displacement using accelerometers and gyroscope. There shouldn't have been much concern about incremental drift as device needed only move a couple of meters at most and the data collection typically ran for a few minutes at most. We tried all sorts of approaches and even had help from several Apple engineers. Ultimately, it seemed that the gyroscope wasn't up to the task. It was good for 3D orientation but that was it ... again, according to very knowledgable engineers.
I would love to hear someone contradict this - because the app never really turned out as we had hoped, etc.
I am also trying to get displacement on the iPhone. Instead of using integration I used the basic physics formula of d = .5a * t^2 assuming an initial velocity of 0 (doesn't sound like you can assume initial velocity of 0). So far it seems to work quite well.
My problem is that I'm using the deviceMotion.and the values are not correct. deviceMotion.gravity read near 0. Any ideas? - OK Fixed, apparently deviceMotion.gravity has a x, y, and z values. If you don't specify which you want you get back x (which should be near 0).
Find this question two years later, I just find a AR project on iOS 6 docset named pARk, It provide a proximate displacement capture and calculation using Gyroscope, aka CoreMotion.Framework.
I'm just starting leaning the code.
to be continued...