CaseStudy | RecurDyn

Creating an accurate digital twin of a human user for realistic modeling and simulation

- Multibody system simulation using biomechanical human body models in RecurDyn

Abstract
Recent applications of digital twins have been used either in a purely technological context (for machines or vehicles), or in purely medical contexts (e.g. in understanding cardiovascular disease).

One school of thought is convinced it is necessary to bring these two parts together by including a digital twin of the user, driver or operator in the technological or medical CAE analysis.

This technical article describes a human body model (HBM) wizard developed for RecurDyn and discusses what is already possible and what is in the development pipeline for the near future. Biomotion Solutions provides software to quickly build HBMs in industrial-grade simulation packages.

Biomechanical models of the human body

Human dummies have successfully been used for automotive crash testing since active movements by the hardware dummy are neither necessary nor technically feasible. Likewise, in computer simulation of automotive crashes, digital versions of the passive hardware dummy are used as a state-of-the-art solution. Crash simulation allows the influence of the external forces and accelerations acting on the human driver to be analyzed. Since the mass of the human body and the forces it applies to the car are small in comparison to the car, the man-machine interaction is neglected in most cases. On the other hand, however, it has surprisingly been shown that the results of a physical coupling between an F-16 pilot and his fighter plane are too strong to be ignored. So-called “roll ratcheting” is caused by the amplification of the fighter pilot’s steering commands to the aircraft which is fly-by-wire controlled. Pilot-induced oscillations have been described in the literature[2] and it has been shown that the dynamics of a 17-ton jet plane can sometimes only be understood if the humanvehicle interaction is taken into account in the simulation process.

Biomechanical models of a human operator are applicable in a broad variety of fields. It is often more appropriate to use (re)active human body models that include the interaction between the human operator/user and the technical system. For instance, to predict key factors like safety, performance or comfort, realistic user models have to be incorporated in the CAE process.

Some simulation packages available on the market provide complex human musculo-skeletal models, but these packages lack the specific technical elements necessary for engineers to build complex technical simulation models that include bearings and technical gear, for example. On the other hand, industrial grade multibody systems (MBS) simulation allows for complex mechanical models of products (e.g. vehicles), but often neglects the human operators or only models them in low detail because the manual modeling and parametrization of individualized biomechanical human body models (HBM) is a complex and error-prone task.

Biomotion Solutions specializes in the simulation of human body models inside industrial grade multibody simulation software. Using VariBody, the HBM wizard for RecurDyn (part of Biomotion Workbench), body weight, stature and gender input will automatically generate HBM models that can then be imported as sub-models in an MBS model.

Model generation

The mechanical principles of human motion have long been the subject of scientific research. For instance, in his book De motu animalium [3], Giovanni Alfonso Borelli, who lived in Italy from 1608 to 1678, described mechanically as systems of lever arms, deflection pulleys and ropes, the interplay between muscles and bones to generate the motions that can be observed in animals and nature. Since then, much research has been done in the field of biomechanics as it became known. With the development of computer simulation techniques, biomechanical models of humans have been developed to better analyze human motion.
For MBS simulations, the human body can be modelled as a marionette consisting of 17 rigid bodies. For the MBS model, it is necessary to provide the mass and inertia tensor for each of these 17 segments. Along with these, some geometric properties have to be specified to define the location of the joints. There are two principal ways to parametrize these body properties for each segment: determine the parameters like the mass, inertia and geometric properties for one specific subject for individual computer-aided simulation, such as for surgical planning or to analyze an athlete’s performance; or, which is more common, build human body models on statistical bases using some established data sets (see examples in the references [4,5,6]).
To better understand this statistical concept, Da Vinci’s famous image of the Vitruvian man very nicely illustrates how the human body features some basic symmetries or proportions that make it possible to deduce the different segment lengths based on the subject’s height. The use of statistical data and regression equations represents one approach to modeling the human body. Another is to calculate the mass properties of the segments based on the calculated volumes (cylinders, truncated cones and ellipsoids) and the mean densities. One such prominent HBM is the Hanavan model [7], but this method requires many segment lengths and radii to be measured and therefore more effort during the modeling process.
Since these two main methods to determine the parameters differ – one results in a more individual model but requires higher effort, and the other is quite easy but describes a “mean” body, we decided to use the method which is easier to deploy in engineering contexts for the model wizard. Consequently, the VariBody model wizard for RecurDyn builds a complete HBM using just three input parameters: body weight, body height and gender to calculate the segment parameters by means of regression equations.

Using a wizard to generate the model

An HBM could be built up piece by piece in RecurDyn using the preprocessor where every anthropometric parameter like mass or inertia is calculated by means of regression equations.
However, the large number of model parameters makes building many different HBMs with different anthropometrics, each of which has to be created manually for each model, an elaborate and error-prone task. It is, therefore, clearly beneficial to use a model wizard for this task. VariBody for RecurDyn generates the HBM in just a few steps based on user input of gender, stature and body weight.
After inputting the basic parameters, the user can choose between the available scenarios (sitting in a seat, standing, generic) and define some additional switches for the model generation. The wizard uses this input to generate a complete HBM including all joints, bodies and force elements. In addition to the HBM scenario of the “seated occupant”, a multi-segment model of a car seat is also included. The GUI allows the modification of the model’s pose using sliders for each degree of freedom. Once the desired pose is achieved, the model can be exported to an intermediate data file that can be parsed by the Biomotion model input parser in RecurDyn. After the model has been imported into RecurDyn, the user can define the desired unit system and can modify or enhance the HBM using the RecurDyn preprocessor. In our “seated occupant” example, the model was extended with some contact elements between the human body and the seat.

Example: Buggy occupant

Using the model of a Buggy (courtesy of Jacob Hustad), the deployment of the HBM as the vehicle’s occupant is demonstrated in a car simulation. First, we modified the Buggy model provided by removing the simple, rigid rider structure. Then we loaded the prepared Biomotion HBM occupant subsystem into the model of the buggy. After adding some force elements (e.g. hands steering) and making a few adjustments (definition of the mother body, fixing the seat chassis to the buggy frame), the model was complete and could be integrated.

Import data from motion capture

The mechanical principles of human motion have long been the subject of scientific research. For instance, in his book De motu animalium [3], Giovanni Alfonso Borelli, who lived in Italy from 1608 to 1678, described mechanically as systems of lever arms, deflection pulleys and ropes, the interplay between muscles and bones to generate the motions that can be observed in animals and nature. Since then, much research has been done in the field of biomechanics as it became known. With the development of computer simulation techniques, biomechanical models of humans have been developed to better analyze human motion.
For MBS simulations, the human body can be modelled as a marionette consisting of 17 rigid bodies. For the MBS model, it is necessary to provide the mass and inertia tensor for each of these 17 segments. Along with these, some geometric properties have to be specified to define the location of the joints. There are two principal ways to parametrize these body properties for each segment: determine the parameters like the mass, inertia and geometric properties for one specific subject for individual computer-aided simulation, such as for surgical planning or to analyze an athlete’s performance; or, which is more common, build human body models on statistical bases using some established data sets (see examples in the references [4,5,6]).
To better understand this statistical concept, Da Vinci’s famous image of the Vitruvian man very nicely illustrates how the human body features some basic symmetries or proportions that make it possible to deduce the different segment lengths based on the subject’s height. The use of statistical data and regression equations represents one approach to modeling the human body. Another is to calculate the mass properties of the segments based on the calculated volumes (cylinders, truncated cones and ellipsoids) and the mean densities. One such prominent HBM is the Hanavan model [7], but this method requires many segment lengths and radii to be measured and therefore more effort during the modeling process.
Since these two main methods to determine the parameters differ – one results in a more individual model but requires higher effort, and the other is quite easy but describes a “mean” body, we decided to use the method which is easier to deploy in engineering contexts for the model wizard. Consequently, the VariBody model wizard for RecurDyn builds a complete HBM using just three input parameters: body weight, body height and gender to calculate the segment parameters by means of regression equations.

Active human body models

To model a realistic twin of a human interacting with products or vehicles, it is necessary to apply a simple yet realistic motion control to the HBM. This approach enables closed-loop simulation and the calculation of product variants. A communicator interface allows engineers to control the movement of the user model via the control input (for example, using a co-simulation in MATLAB/Simulink).