How to Set Initial Position for a Mass Block in Simscape?

I am working on a mechanical translational system in Simscape, and I need to set an initial position for the Mass block. However, the Mass block only allows setting an initial velocity and force, not position.

I tried using an Ideal Motion Sensor to specify the initial position, but it does not contribute to the problem's resolution, so it does not work for setting the initial condition.

In my version of MATLAB:

• The "Ideal Translational Motion Actuator" does not appear in the Simscape library.
• The "State Targets" option is not available in the Solver Configuration block.

What is the correct way to set an initial position for the Mass block in Simscape when these options are unavailable?

Would using a high-stiffness spring temporarily be a valid workaround, or is there another recommended approach?

[Yifeng Tang]

Hi @José,

I would be interested in learning more about your statement that you "need to set an initial position for the Mass block". Would you be able to provide more details and why you say that? Maybe we can find another way to approach the problem where the initial position of the mass isn't necessary. I suspect it's actually the position of some other component that you really needed.

[José]

Thank you for you r response!

I understand your point, but my concern comes from the fact that this system can be modeled using differential equations, where an initial position can be explicitly included as an initial condition.

For example, if we consider a simple mass-spring-damper system, the differential equation is:

m \ddot{x} + c \dot{x} + kx = F

where x represents the position. In classical differential equation solvers (e.g., dsolve in MATLAB), we can specify initial conditions such as:

x(0) = x_0, \quad \dot{x}(0) = v_0

This is a standard way to fully define the problem. However, in Simscape, while I can set an initial velocity in the Mass block, there is no direct option to set an initial position.

Since Simscape is a physical modeling tool, I assume there must be a way to impose an initial position, just like in traditional mathematical modeling. That's why I am looking for the correct way to do this in Simscape.

Would you recommend a specific approach to include an initial position in a Simscape mechanical system?

Answer 1

[Yifeng Tang]

Hi @José,

In the mechanical translational domain, the domain variables are the velocity and the force. In general, the displacement/position is calculated as . And you are right, the initial position is required to get the position at any given time.

That said, the governing equation of certain components doesn't care about the position. For example, the mass, whose governing equation is . Position is not explicitly involved here. And that's why there isn't an initial condition for position in this block.

In some other components, like a spring, position and its initial value do matter, since , and you will have the option to specify the initial position, or more precisely the initial deformation of the spring.

As such, when you have a mass-spring-damper system, you will need an initial deformation of the spring and an initial velocity of the mass (which is the same as the initial velocity at one end of the spring and/or damper) to define the full set of the initial conditions. The "position" of the mass block, I would argue, isn't a realy thing until you measure it with respect to a point that you call "zero". In this case, it's natural to define the position as the distance between the current position and the neutral position of the spring, so we are really talking about the deformation of the spring.

I know this sounds a bit "philosophical", but what this means for Simscape modeling is simple: set the initial position in the spring deformation. If you do want to measure the position of the mass where zero is defined at the neutral position of the spring, attach a translational motion sensor to the mass and set the initial position to be the same as the initial deformation of the spring.

Hope this makes sense.