Lucien,

While I agree with Jan completely, you also have two more alternatives:

1. you can use the built in transmission blocks.
2. you can also make a Modelica block that behaves like the scissors mechanism (since you have the equations for it).

As you state -- you only need one dimension, the scissors block is merely a transmission ratio between the input and the output. You can successfully model it as a lumped transmission using built-in components. For example, look at idealGear and idealGearR2T. You can model the scissors mechanism as a transmission.

Secondly, you can make your own translation block that has the behavior you need. It is a simple model (probably two equations). The advantage of using the 3D mechanics library is you get 3D animations. The disadvantage is you need to traverse the relatively steep learning curve and get your dimensions right in 3D.

Personally I would start by getting something running in 10-20 minutes by using the built in transmission components (especially since you say it is massless so the changing inertias will not need to be modeled). I would show you a simple example you do not indicate the inputs/outputs. For example: Are you using a motor? or. linear actuator? or Are you just specifying a force or torque or a motion profile on the input? What is the geometry? etc.

Regards,

Neil

Lucien,

Again, the modeling depends on what you are trying to capture with the model. For example, I grabbed a picture of a single scissors mechanism from this technical paper:

If your input is the distance, h, then the mechanism is a simple, linear transmission with a 1:2 gear ratio between input, h and output, H. This is easily modeled with two IdealGearR2T blocks -- one with a 1:1 ratio to move from translation to rotation with no ratio and a second one to go from rotation back to translation with a 1:2 ratio (output is twice the input). The two blocks together form a linear transmission (I do not believe there is a single linear to linear gear block built into WSM). You can then add springs, masses, and input sources to provide the forces to move the mechanism.

If, however, your input is S, the mechanism is highly nonlinear because (using the Pythagorean theorem for triangles)

H == Sqrt[l^2-S^2]

In this situation you would have to write your own block to translate the motion in S to motion in H using the expression above and its derivatives (see the der() function in modelica). To construct a two port modelica translation block you need an equation for position and an equation for force. (use a simple translation block such as "spring" as a starting example). If your input is S, the relationship between the position and the force is not linear.

Regards,

Neil