id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
6074	The NF should completely eliminate Evaluate = true parameters from flat model	Francesco Casella	Per Östlund	"@perost, please check out the FEMBody package from https://github.com/looms-polimi/FEMPackage. I sent you an invitation to access the repo, if it is expired when you check this, I can send you a new one. The library will eventually be made open source, but still needs some clean-up.

Please open the FemPackage3.2.mo file and flatten the model {{{ExamplesFEM.PendulumBody}}}. 

The FEMBody model which is instantiated in this example contains code such as
{{{
  transpose(Cr) = data.inv4 + sum(data.inv5[i, :, :] * q[i] for i in 1:M);
  Jbar = data.inv7 - sum((transpose(data.inv8[i, :, :]) + data.inv8[i, :, :]) * q[i] for i in 1:M) - sum(sum(data.inv9[i, j, :, :] * q[j] for j in i:M) * q[i] for i in 1:M);
  [mDcbartilde, Jbar, transpose(Cr)] * [aa_a - g_0; za_a; ddq] = h_w_theta + h_e_theta;
  [data.Ct, Cr, Me] * [aa_a - g_0; za_a; ddq] = h_w_f + h_e_f - matrix(data.Ke * q) - matrix(d / 100 * (alpha * Me + beta * data.Ke) * dq);
}}}
which comes straight from tensor equations describing the multibody FEM model of a flexible link, see [https://dx.doi.org/10.1080/13873954.2013.807433 paper].

The parameter record {{{data}}} contains multi-dimensional arrays (tensors) of structural parameters of the flexible body, which come from modal analysis of the original FEM model. Most of the elements of those arrays are zero, so for efficient and robust handling of the model equations by the backend it is absolutely essential that:
- all the elements of the data record are evaluated (see also #6067)
- all matrix operators and reduction operators are evaluated, and all product-by-zero terms are eliminated

Adding the {{{Evaluate = true}}} annotation to each individual component of the {{{FEMData.PendulumData}}} record improved the situation significantly, drastically reducing the size of many equations and making it possible to actually simulate this test case and produce the correct results. However, there is still a lot of non-evaluated stuff in the flat model that looks pretty nasty, and in fact more involved models using flexible bodies with larger numbers of FEM modes and nodes take forever to compile and fail miserably at runtime, producing completely wrong results. I am convinced that this would not be the case if everything was properly evaluated by the frontend.

Getting this model to work in OMC may have strategic value to get a new big partner (possibly more than one) into the OSMC, so I would please ask you to assess how much work fixing this issue could take and report it here. Then we can decide what to do."	defect	closed	critical	1.17.0	New Instantiation		fixed		enrico.cunietti@… andrea.cutrona@… B.Scaglioni@…