Add translational library #85

kianmolani · 2022-06-21T19:50:47Z

This PR is linked to issue #84

YingboMa · 2022-06-21T19:54:39Z

.vscode/launch.json

@@ -0,0 +1,17 @@
+{


You can add .vscode and .DS_Store in global git ignore setting. Check out https://sebastiandedeyne.com/setting-up-a-global-gitignore-file/

YingboMa · 2022-06-21T19:56:03Z

test/Mechanical/translational.jl

@@ -0,0 +1,36 @@
+import ModelingToolkitStandardLibrary.Blocks
+include("/Users/kianmolani/Documents/Project Files/Codebases/ModelingToolkitStandardLibrary.jl/src/Mechanical/Translational/Translational.jl")


This line should be removed.

How can I add the Translational module to the package and be able to import it using use ModelingToolkitStandardLibrary.Mechanical.Translational? Do I have to first merge this PR? I have tried some possible solutions but without success.

YingboMa · 2022-06-21T19:57:47Z

test/Mechanical/translational.jl

+import ModelingToolkitStandardLibrary.Blocks
+include("/Users/kianmolani/Documents/Project Files/Codebases/ModelingToolkitStandardLibrary.jl/src/Mechanical/Translational/Translational.jl")
+using ModelingToolkit, OrdinaryDiffEq, Test
+using Plots


Don't import Plots in test. CI cannot actually see plots.

baggepinnen · 2022-06-21T20:03:50Z

src/Mechanical/Translational/Translational.jl

+export Flange
+include("utils.jl")
+
+export Fixed, Inertia, Spring, Damper, IdealGear


Suggested change

export Fixed, Inertia, Spring, Damper, IdealGear

export Fixed, Mass, Spring, Damper, IdealGear

ValentinKaisermayer · 2022-06-22T10:16:21Z

src/Mechanical/Translational/Translational.jl

+module Translational
+
+using ModelingToolkit, Symbolics, IfElse, OrdinaryDiffEq
+using OffsetArrays


Remove not needed packages

ValentinKaisermayer · 2022-06-22T10:19:21Z

Doc strings are missing and actual tests

baggepinnen · 2022-06-22T10:36:07Z

Have a look at https://github.com/baggepinnen/AutomaticDocstrings.jl to quickly generate docstring stubs, helps a lot when you have a lot of docstrings to generate

kianmolani

I will delete the output folder before the PR is merged - I've just included the figures here for your reference (though lmk if there's a better way to share output results and plots). Also, I don't know whether we want to run tests against Modelica simulation data. I certainly think it's useful as an initial sanity check, but don't know if it's appropriate to include as part of continuous testing. If we do decide to continue testing against Modelica simulation data, I also think that the tests themselves can be better implemented than what's been done here.

ValentinKaisermayer

Can you add the connectors in the doc strings as well? And name the states s(t) to be consistent with the rest of the library.

If you want to add images, simply drag them into a comment or an issue.

ValentinKaisermayer · 2022-06-24T16:29:07Z

src/Mechanical/Translational/utils.jl

+Base.@doc """
+    Flange(;name)
+
+1-dim. translational flange of a shaft.


Suggested change

1-dim. translational flange of a shaft.

1-dim. translational flange.

ValentinKaisermayer · 2022-06-24T16:29:45Z

src/Mechanical/Translational/utils.jl

+
+# States:
+- `s`: [m] Absolute position of flange
+- `f`: [N] Cut force in the flange


Suggested change

- `f`: [N] Cut force in the flange

- `f`: [N] Cut force into the flange

ValentinKaisermayer · 2022-06-24T16:31:04Z

src/Mechanical/Translational/utils.jl

+Base.@doc """
+    Support(;name)
+
+Support/housing of a 1-dim. translational shaft


Suggested change

Support/housing of a 1-dim. translational shaft

Support/housing 1-dim. translational flange.

ValentinKaisermayer · 2022-06-24T16:32:14Z

src/Mechanical/Translational/utils.jl

+
+# States:
+- `s`: [m] Absolute position of the support/housing
+- `f`: [N] Reaction force in the support/housing


Suggested change

- `f`: [N] Reaction force in the support/housing

- `f`: [N] Cut force into the flange

ValentinKaisermayer · 2022-06-24T16:33:14Z

src/Mechanical/Translational/utils.jl

+"""
+    PartialCompliant(;name, s_rel_start=0.0, f_start=0.0)
+
+Partial model for the compliant connection of two translational 1-dim. shaft flanges.


Suggested change

Partial model for the compliant connection of two translational 1-dim. shaft flanges.

Partial model for the compliant connection of two translational 1-dim. flanges.

ValentinKaisermayer · 2022-06-24T16:35:41Z

src/Mechanical/Translational/utils.jl

+"""
+    PartialCompliantWithRelativeStates(;name, s_rel_start=0.0, v_rel_start=0.0, a_rel_start=0.0, f_start=0.0)
+
+Partial model for the compliant connection of two translational 1-dim. shaft flanges where the relative position and speed are used as preferred states


Suggested change

Partial model for the compliant connection of two translational 1-dim. shaft flanges where the relative position and speed are used as preferred states

Partial model for the compliant connection of two translational 1-dim. flanges.

ValentinKaisermayer · 2022-06-24T16:40:13Z

test/Mechanical/translational.jl

@@ -0,0 +1,58 @@
+import ModelingToolkitStandardLibrary.Blocks
+include(pwd() * "/src/Mechanical/Translational/Translational.jl")


Suggested change

include(pwd() * "/src/Mechanical/Translational/Translational.jl")

using Modelingtoolkitstandardlibrary.Mechanical.Translational

As written, this line returns the error Translational not defined. Is this because the Translational module has not been added to the package using the proper procedures (that is, using JL's package manager)? Note that when creating the Translational library, I merely added a new folder to the repo and proceeded to add the requisite lines of code.

ValentinKaisermayer · 2022-06-24T16:40:47Z

test/Mechanical/translational.jl

+    @test mass_v_julia[end] ≈ 0 atol = 1e-3 # all energy has dissipated
+    @test mean(broadcast(abs, (mass_s_julia - mass_s_modelica))) ≈ 0 atol = 1e-5 # jl and modelica s vs. t results are approx equal
+    @test mean(broadcast(abs, (mass_v_julia - mass_v_modelica))) ≈ 0 atol = 1e-4 # jl and modelica v vs. t results are approx equal
+    @test mean(broadcast(abs, (mass_a_julia - mass_a_modelica))) ≈ 0 atol = 1e-2 # jl and modelica a vs. t results are approx equal


Can you make the test self-sufficient?

I can't think of a way to do this without importing the Modelica simulation results. Please let me know if you see a better way. What are your thoughts in general about running these sort of tests? Do you think they better serve as one-offs for sanity checking (i.e. not to be repeated again), or for continuous testing? Please see my original comment on this issue: #85 (review)

ValentinKaisermayer · 2022-06-24T16:42:28Z

src/Mechanical/Translational/utils.jl

+"""
+    PartialElementaryTwoFlangesAndSupport2(;name, use_support=false)
+
+Partial model for a component with two translational 1-dim. shaft flanges and a support used for textual modeling, i.e., for elementary models


Suggested change

Partial model for a component with two translational 1-dim. shaft flanges and a support used for textual modeling, i.e., for elementary models

Partial model for a component with two translational 1-dim. flanges and a support used for textual modeling, i.e., for elementary models

ValentinKaisermayer · 2022-06-24T16:43:23Z

src/Mechanical/Translational/Translational.jl

@@ -0,0 +1,21 @@
+"""
+Library to model 1-dimensional, translational mechanical systems


Suggested change

Library to model 1-dimensional, translational mechanical systems

Library to model 1-dimensional, translational mechanical components.

kianmolani · 2022-06-24T17:09:12Z

Can you add the connectors in the doc strings as well? And name the states s(t) to be consistent with the rest of the library.

If you want to add images, simply drag them into a comment or an issue.

Thank you for the suggestions, Valentin. The connectors in utils.js already have doc strings added (I mirrored the documentation from the same file in the Rotational lib). Are you referring to something else? Also, what do you mean about naming the states s(t)? The position state is already denoted by s.

ValentinKaisermayer · 2022-06-24T17:14:51Z

I meant a # Connectors: section in the doc strings, listing the connectors of the components. You can have a look at the doc strings in the other modules.

kianmolani · 2022-06-24T17:25:49Z

I meant a # Connectors: section in the doc strings, listing the connectors of the components. You can have a look at the doc strings in the other modules.

Okay, that's clear. Will do, and thanks!

…ss spring damper models

kianmolani · 2022-07-14T16:44:55Z

There appear to be issues with our simulated results for a mass-spring-damper model. Julia and Modelica simulation results do not match, and the difference is very apparent. I've attached below screenshots of Julia and Modelica simulation results of our mass position vs. time for underdamped, overdamped, and critically damped systems. For the underdamped system, the difference appears to be accounted for with an offset of about 0.5, as well as a slight phase shift. For the overdamped and critically damped systems, the solutions possess different equilibrium states.

I've ensured that both Modelica and Julia simulations are initialized in the same way. Note that simulation results between Modelica and Julia for a mass-damper model are very close (tested across a range of parameter values).

This last screenshot shows how I've defined my system connections for a mass-spring-damper model:

baggepinnen · 2022-07-14T17:47:33Z

docs/src/tutorials/mass_spring_damper.md

+@named fixed = Fixed(s0=s0)
+@named mass = Mass(m=m, s_start=s_start, v_start=v_start)
+@named damper = Damper(d=d)
+@named spring = Spring(c=c, s_rel0=s_rel0)


Suggested change

@named fixed = Fixed(s0=s0)

@named mass = Mass(m=m, s_start=s_start, v_start=v_start)

@named damper = Damper(d=d)

@named spring = Spring(c=c, s_rel0=s_rel0)

@named fixed = Fixed(; s0)

@named mass = Mass(; m, s_start, v_start)

@named damper = Damper(; d)

@named spring = Spring(; c, s_rel0)

In case you didn't know, if your variable shares the same name as a keyword argument (a common case), you do not need to repeat the name. You do need to have the semi-colon first though to indicate that keyword arguments are coming.

baggepinnen · 2022-07-15T05:26:58Z

Could the difference between julia and modelica lie in the initialization? Is there a difference already at the first point saved in the solution?

kianmolani · 2022-07-15T13:03:33Z

Could the difference between julia and modelica lie in the initialization? Is there a difference already at the first point saved in the solution?

I believe that they've been initialized in the same way. Though I will triple-check this (the solutions match for the mass-damper system, tested across a range of param values, so perhaps the culprit here is the spring). I've attached the Modelica simulation settings below, though I don't know if they'd be of use to you (the simulation interval settings are the same, and so is the integration tolerance level).

baggepinnen · 2022-07-15T14:04:24Z

Let me see if I understand this correctly, the fixed point is at s = 4.5 and the spring has resting length s_rel0 = 1, then I'd expect the steady-state position to be either 5.5 or 3.5 depending on the direction the spring is pointing in. It thus looks like the Julia solution is correct since it comes to rest at 3.5? I assume there's no gravity or other external forces acting anywhere? Is the length of the spring and all other params the same in modelica?

kianmolani · 2022-07-15T14:52:34Z

Let me see if I understand this correctly, the fixed point is at s = 4.5 and the spring has resting length s_rel0 = 1, then I'd expect the steady-state position to be either 5.5 or 3.5 depending on the direction the spring is pointing in. It thus looks like the Julia solution is correct since it comes to rest at 3.5? I assume there's no gravity or other external forces acting anywhere? Is the length of the spring and all other params the same in modelica?

No other forces and all other params confirmed to be the same.

The issue might be that Modelica takes in an additional parameter that we do not: mass length (set to 1 in this simulation). Because Modelica also has the parameter s_start (the initial value of the absolute position of the mass), and because this parameter is set to 3, it means that Modelica is tracking the center of this mass. Therefore, the equilibrium state at x = 3 makes sense for the Modelica simulation.

However, again, we do not take in a parameter for mass length, and so I guess that we're treating the mass as a point mass? In any case, initializing s_start to also equal 3 in our simulation (given that we do not have a mass length) might be problematic in that there's a discontinuity in the connections. I've drawn two schematics to illustrate this point:

For Modelica:

For Julia:

kianmolani · 2022-07-15T15:01:37Z

Indeed, I've just confirmed that setting the spring length to 1.5m in our Julia simulation in order to account for this "gap" yields matching simulation results.

kianmolani · 2022-07-15T15:02:16Z

Perhaps mass length is a property that we should include as well.

codecov · 2022-07-18T17:15:47Z

Codecov Report

Merging #85 (f7f188e) into main (db37027) will decrease coverage by 5.56%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   68.59%   63.03%   -5.57%     
==========================================
  Files          23       26       +3     
  Lines         952     1036      +84     
==========================================
  Hits          653      653              
- Misses        299      383      +84

Impacted Files	Coverage Δ
src/Mechanical/Translational/components.jl	`0.00% <0.00%> (ø)`
src/Mechanical/Translational/sources.jl	`0.00% <0.00%> (ø)`
src/Mechanical/Translational/utils.jl	`0.00% <0.00%> (ø)`

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

YingboMa · 2022-07-18T17:37:49Z

You should remove .vscode and .DS_Store, then the PR is good to go after formatting.

Delete .DS_Store Delete .DS_Store Delete .DS_Store Delete .DS_Store

kianmolani · 2022-07-26T12:50:42Z

You should remove .vscode and .DS_Store, then the PR is good to go after formatting.

All .vscode and .DS_Store files have now been deleted.

YingboMa · 2022-08-08T19:56:55Z

Format failure.

ValentinKaisermayer

Other than the docs it looks good. 👍

ValentinKaisermayer · 2022-09-10T16:11:47Z

docs/src/tutorials/mass_spring_damper.md

+
+Let's start by importing what we need.
+
+```@example


Suggested change

```@example

```@example translational

ValentinKaisermayer · 2022-09-10T16:12:12Z

docs/src/tutorials/mass_spring_damper.md

+
+Now let's initialize our system state and define our components and connections. Here, `s0` is the fixed offset position of our flange housing, `s_rel0` is our unstretched spring length, and `s_start` and `v_start` are the initial values of the absolute position and linear velocity of our sliding mass, respectively. Finally, `m`, `c`, and `d` are our mass, spring, and damping constants, respectively. All units are in SI units.
+
+```@parameters t


Suggested change

```@parameters t

```@example translational

@parameters t

ValentinKaisermayer · 2022-09-10T16:12:32Z

docs/src/tutorials/mass_spring_damper.md

+
+Before solving our ODE system, let us ensure that the condition for underdamped motion is met.
+
+```


Suggested change

```

```@example translational

ValentinKaisermayer · 2022-09-10T16:12:49Z

docs/src/tutorials/mass_spring_damper.md

+
+Now let's solve our ODE system and plot our results (note that we are plotting the position of our mass as a function of time).
+
+```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])


Suggested change

```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])

```@example translational

@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])

ValentinKaisermayer · 2022-09-10T16:13:09Z

docs/src/tutorials/mass_spring_damper.md

+
+We'll now demonstrate a mass-spring-damper system that is overdamped. For such systems, `d > sqrt(4 * m * c)`, and the zeros of our characteristic polynomial are real. Let us re-initialize our `m`, `c`, and `d` constants accordingly, and re-define our components and connections.
+
+```


Suggested change

```

```@example translational

ValentinKaisermayer · 2022-09-10T16:13:30Z

docs/src/tutorials/mass_spring_damper.md

+
+Let us ensure that the condition for overdamped motion is met.
+
+```


Suggested change

```

```@example translational

ValentinKaisermayer · 2022-09-10T16:14:13Z

docs/src/tutorials/mass_spring_damper.md

+
+Let's solve our ODE system and plot our results. Observe that for overdamped systems, motion is non-oscillatory.
+
+```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])


Suggested change

```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])

```@example translational

@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])

ValentinKaisermayer · 2022-09-10T16:14:32Z

docs/src/tutorials/mass_spring_damper.md

+
+Our last demonstration will be of a mass-spring-damper system that is critically damped. The condition for critically damped motion is `d = sqrt(4 * m * c)`, where the roots of our characteristic polynomial are both equal to `d/2m`. Again, let us re-initialize and re-define our system state, components, and connections, solve our ODE system, and plot our results. Observe that for critically damped systems, motion is non-oscillatory.
+
+```


Suggested change

```

```@example translational

bradcarman · 2022-09-14T23:40:45Z

Hi, I'm reviewing the code and I noticed that the Flange connector uses absolute position for the state variable. Might I suggest changing this to velocity? The reason for this is so we have less matching parameters to set. Let me show an example...

The below code requires that each of the 3 components have a position parameter set, and s_rel0 is really just s0-s_start, so it can already be seen as redundant.

s0 = 4.5 # fixed offset position of flange housing
s_start = 3 # initial value of absolute position of sliding mass
s_rel0 = 1.5 # unstretched spring length
v_start = 10 # initial value of absolute linear velocity of sliding mass

@named fixed = Fixed(; s0)
@named mass = Mass(; m=1, s_start, v_start)
@named spring = Spring(; c=10, s_rel0)

eqs = [
    connect(mass.flange_a, spring.flange_a)
    connect(spring.flange_b, fixed.flange)
]

@named model = ODESystem(eqs, t, [], []; systems=[fixed, mass, spring])

sys = structural_simplify(model)
prob = ODEProblem(sys, [], (0, 10.0))
sol = solve(prob, ImplicitMidpoint(), dt=0.01)

Plotting sol[mass.s]...

If I remove the position parameters from Fixed and Spring, you can see we get the incorrect result...

@named fixed = Fixed()
@named mass = Mass(; m, s_start, v_start)
@named spring = Spring(; c)

Now, for comparison (using #107) we can see that only the Body position is needed, which to me makes more sense because we really only need to track absolute positions of actual moving rigid bodies. Additionally the Fixed component is defined by simply setting the port velocity to 0.

x0 = 3 # initial value of absolute position of sliding mass
v0 = 10 # initial value of absolute linear velocity of sliding mass

function Fixed(; name)
    @named port = Port()
    eqs = [port.v ~ 0]
    return compose(ODESystem(eqs, t, [], []; name = name), port)
end

@named fixed = Fixed()
@named mass = Body(; m=1, x0, v0)
@named spring = Spring(; k=10)

eqs = [
    connect(mass.port, spring.port_a)
    connect(spring.port_b, fixed.port)
]

@named model = ODESystem(eqs, t, [], []; systems=[fixed, mass, spring])

sys = structural_simplify(model)
prob = ODEProblem(sys, [], (0, 10.0))
sol = solve(prob, ImplicitMidpoint(), dt=0.01)

This is also how other acausal modeling tools I know of define their thru and across (flow) variables for translational components.

baggepinnen · 2022-09-15T05:10:46Z

Might I suggest changing this to velocity?

The intention is to follow modelica stdlib, if we're deviating from them, it's probably an oversight and should be fixed :)

ValentinKaisermayer · 2022-09-15T05:37:00Z

Although, Simscape uses velocity and angular velocity for its mechanical domains.

In the example you show, I think you do not need to specify s_rel0 for the spring. It should be fully determined by the mass and fixed position.

bradcarman · 2022-09-15T13:38:28Z

In this example the spring starts at it's natural length (i.e. uncompressed), therefore one should not need to specify s_rel0. By using position rather than velocity, this overcomplicates the model and now one must specify both the position of the Fixed component as well as s_rel0 for the spring.

Additionally, because the Flange is defined with s defaulting to 0, we now introduce a requirement that the model must solve for it's initial states. For a small model, this is not such a big deal, but for the large models I'm dealing with, initialization is something I'd like to avoid for 1) I need my model to start immediately, waiting for initialization is not an option for real time control, 2) I'd like to guarantee my states.

If we run this simple model without initialization and simplification, it can be seen that the result is not good.

Full code below...

using ModelingToolkitStandardLibrary.Mechanical.Translational, ModelingToolkit,
      OrdinaryDiffEq, Test
using Setfield

@parameters t
D = Differential(t)

s0 = 4.5 # fixed offset position of flange housing
s_start = 3 # initial value of absolute position of sliding mass
s_rel0 = 1.5 # unstretched spring length
v_start = 10 # initial value of absolute linear velocity of sliding mass

@named fixed = Fixed(; s0)
@named mass = Mass(; m=1, s_start, v_start)
@named spring = Spring(; c=10, s_rel0)

eqs = [
    connect(mass.flange_a, spring.flange_a)
    connect(spring.flange_b, fixed.flange)
]

@named model = ODESystem(eqs, t, [], []; systems=[fixed, mass, spring])

sys = expand_connections(model)
@set! sys.eqs = [(typeof(eq.lhs) != ModelingToolkit.Connection) & !ModelingToolkit._iszero(eq.lhs) & !ModelingToolkit.isdifferential(eq.lhs) ? 0 ~ eq.rhs - eq.lhs : eq for eq in sys.eqs]

prob = ODEProblem(sys, [], (0, 2.0))
sol = solve(prob, ImplicitMidpoint(), dt=0.01, initializealg=NoInit())

using CairoMakie
begin
    f,a,=lines(sol.t, sol[mass.s], label="sol[mass.s]");    
    lines!(a, sol.t, sol[mass.flange_a.s], label="sol[mass.flange_a.s]")
    lines!(a, sol.t, sol[spring.flange_a.s], label="sol[spring.flange_a.s]", linestyle=:dash)
    lines!(a, sol.t, sol[spring.flange_b.s], label="sol[spring.flange_b.s]")
    lines!(a, sol.t, sol[fixed.flange.s], label="sol[fixed.flange.s]", linestyle=:dash)
    a.xlabel="time [s]"; a.ylabel="position [m]"; 
    Legend(f[1,2], a)
    f
end

Additionally we can see that spring.s_rel is not automatically resolved

In contrast, by using velocity and force for the translational Connection, we only need to specify the position of the Body/Mass, and no initialization is required (using #107)...

using ModelingToolkitStandardLibrary.Mechanical.Translational, ModelingToolkit,
      OrdinaryDiffEq, Test
using Setfield

@parameters t
D = Differential(t)

x0 = 3 # initial value of absolute position of sliding mass
v0 = 10 # initial value of absolute linear velocity of sliding mass

function Fixed(; name)
    @named port = Port()
    eqs = [port.v ~ 0]
    return compose(ODESystem(eqs, t, [], []; name = name), port)
end

@named fixed = Fixed()
@named mass = Body(; m=1, x0, v0)
@named spring = Spring(; k=10)

eqs = [
    connect(mass.port, spring.port_a)
    connect(spring.port_b, fixed.port)
]

@named model = ODESystem(eqs, t, [], []; systems=[fixed, mass, spring])

sys = expand_connections(model)
@set! sys.eqs = [(typeof(eq.lhs) != ModelingToolkit.Connection) & !ModelingToolkit._iszero(eq.lhs) & !ModelingToolkit.isdifferential(eq.lhs) ? 0 ~ eq.rhs - eq.lhs : eq for eq in sys.eqs]
prob = ODEProblem(sys, [], (0, 2.0))
sol = solve(prob, ImplicitMidpoint(), dt=0.01, initializealg=NoInit())

using CairoMakie
begin
    f,a,=lines(sol.t, sol[mass.x], label="sol[mass.x]");    
    a.xlabel="time [s]"; a.ylabel="position [m]"; 
    Legend(f[1,2], a)
    f
end

bradcarman · 2022-09-15T14:34:33Z

Making my point in another way, one should be able to define the Fixed component such that position is not needed, we simply specify that the change in position is zero...

function Fixed(; name)
    @named flange = Flange()
    eqs = [D(flange.s) ~ 0]
    return compose(ODESystem(eqs, t, [], []; name = name), flange)
end

But, unfortunately this will not work in the end because connect( fixed.flange, component.flange ) will give the equation...

fixed.flange.s ~ component.flange.s

Therefore, we are forced to specify the position for every single Flange in the model. But in theory, it should not be necessary to specify the position for a Fixed component, this is really overly defining the model.

Firionus · 2022-09-15T14:53:28Z

@bradcarman How would you express in your approach a spring that has non-zero length in its relaxed state?
That would certainly be a important for someone that wants to integrate with visualization or if someone wants to model something precisely the way they imagine their coordinate system.

bradcarman · 2022-09-15T15:02:00Z

One final point (then I'll shut up, ha ha), the necessity that one must specify a spring length for the Spring component is silly when one is considering models that really just deal with stiffness. For example, defining a Hard Stop component that defines the stiffness of the boundary. Should the length/thickness of the boundary really need to be defined? Mathematically this information is irrelevant. My recommend spring definition then is...

function Spring(;name, k = 1e3, delta0 = 0.0)
    pars = @parameters k=k delta0=delta0
    vars = @variables begin
        Δx(t) = delta0
        f(t) = k*delta0
    end
    
    @named port_a = Port()
    @named port_b = Port()
    
    eqs = [
        D(Δx) ~ port_a.v - port_b.v
        f ~ k*Δx
        port_a.f ~ +f
        port_b.f ~ -f
    ]
    return compose(ODESystem(eqs, t, vars, pars; name = name), port_a, port_b)
end

Then, we only need to define 2 parameters, the stiffness, and the initial compression state (delta0).

And to answer @Firionus, for visualization, if we are plotting the position states, the only states tracked by the model are actual moving bodies/masses. So if we have 2 bodies connected by a spring, then we can plot both body positions.

Firionus · 2022-09-16T17:05:06Z

And to answer @Firionus, for visualization, if we are plotting the position states, the only states tracked by the model are actual moving bodies/masses. So if we have 2 bodies connected by a spring, then we can plot both body positions.

This doesn't hold true if the spring is connected to a fixed element. Also, visualizing springs with a finite relaxed length is much closer to a real mechanical setup than springs that are infinitely small sometimes, don't you think?

One final point (then I'll shut up, ha ha), the necessity that one must specify a spring length for the Spring component is silly when one is considering models that really just deal with stiffness.

If you find it silly you can just leave it at the default of s_rel0 = 0.0 and achieve that. I realize you'll still have to define the position of fixed points but I think this is a small price to pay for gaining better modelling of real life with the absolute position approach.

Ultimately, it comes down to the choice of programming interface. Should a "mechanical component" only keep track of velocities and forces, i.e. its energy exchange, or should it also track its absolute position. The Modelica Standard Library (MSL 3.2.3) goes with the second approach:

connector Flange "One-dimensional translational flange"

  SI.Position s "Absolute position of flange";
  flow SI.Force f "Cut force directed into flange";
  annotation (...);
end Flange;

And since @baggepinnen said:

The intention is to follow modelica stdlib

This PR gets that right, since it follows the MSL:

@connector function Flange(; name)
    sts = @variables begin
        s(t)
        f(t), [connect = Flow]
    end
    ODESystem(Equation[], t, sts, [], name = name, defaults = Dict(s => 0.0, f => 0.0))
end

source

kianmolani added 2 commits June 21, 2022 15:25

Add translational library with a test of a fixed mass and damper system

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Merge branch 'SciML:main' into 84-translational

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Add docstrings and tests

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Update documentation

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kianmolani marked this pull request as ready for review July 11, 2022 12:48

Added tutorials for underdamped, overdamped, and critically damped ma…

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…ss spring damper models

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Modify spring length and remove key names for keyword arguments

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baggepinnen mentioned this pull request Sep 14, 2022

Start to Mechanical/Translational library #107

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AbdulrhmnGhanem mentioned this pull request Sep 16, 2023

The inverted pendulum model prerequisites AbdulrhmnGhanem/CA.jl#11

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baggepinnen closed this Oct 26, 2023

		@@ -0,0 +1,36 @@
		import ModelingToolkitStandardLibrary.Blocks
		include("/Users/kianmolani/Documents/Project Files/Codebases/ModelingToolkitStandardLibrary.jl/src/Mechanical/Translational/Translational.jl")

	export Fixed, Inertia, Spring, Damper, IdealGear
	export Fixed, Mass, Spring, Damper, IdealGear

	1-dim. translational flange of a shaft.
	1-dim. translational flange.

	- `f`: [N] Cut force in the flange
	- `f`: [N] Cut force into the flange

	Support/housing of a 1-dim. translational shaft
	Support/housing 1-dim. translational flange.

	- `f`: [N] Reaction force in the support/housing
	- `f`: [N] Cut force into the flange

	Partial model for the compliant connection of two translational 1-dim. shaft flanges.
	Partial model for the compliant connection of two translational 1-dim. flanges.

		@@ -0,0 +1,58 @@
		import ModelingToolkitStandardLibrary.Blocks
		include(pwd() * "/src/Mechanical/Translational/Translational.jl")

	include(pwd() * "/src/Mechanical/Translational/Translational.jl")
	using Modelingtoolkitstandardlibrary.Mechanical.Translational

	Partial model for a component with two translational 1-dim. shaft flanges and a support used for textual modeling, i.e., for elementary models
	Partial model for a component with two translational 1-dim. flanges and a support used for textual modeling, i.e., for elementary models

		@@ -0,0 +1,21 @@
		"""
		Library to model 1-dimensional, translational mechanical systems


		Now let's initialize our system state and define our components and connections. Here, `s0` is the fixed offset position of our flange housing, `s_rel0` is our unstretched spring length, and `s_start` and `v_start` are the initial values of the absolute position and linear velocity of our sliding mass, respectively. Finally, `m`, `c`, and `d` are our mass, spring, and damping constants, respectively. All units are in SI units.

		```@parameters t


		Before solving our ODE system, let us ensure that the condition for underdamped motion is met.

		```


		Now let's solve our ODE system and plot our results (note that we are plotting the position of our mass as a function of time).

		```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])

	```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])
	```@example translational
	@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])


		We'll now demonstrate a mass-spring-damper system that is overdamped. For such systems, `d > sqrt(4 * m * c)`, and the zeros of our characteristic polynomial are real. Let us re-initialize our `m`, `c`, and `d` constants accordingly, and re-define our components and connections.

		```


		Let us ensure that the condition for overdamped motion is met.

		```


		Let's solve our ODE system and plot our results. Observe that for overdamped systems, motion is non-oscillatory.

		```@named model = ODESystem(connections, t, systems=[fixed, mass, damper, spring])


		Our last demonstration will be of a mass-spring-damper system that is critically damped. The condition for critically damped motion is `d = sqrt(4 * m * c)`, where the roots of our characteristic polynomial are both equal to `d/2m`. Again, let us re-initialize and re-define our system state, components, and connections, solve our ODE system, and plot our results. Observe that for critically damped systems, motion is non-oscillatory.

		```

Add translational library #85

Add translational library #85

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