Can I use an inequality in a custom Simscape function? / How can I make a block that limits the speed in SimMechanics?

Question

I have a SimMechanics Revolute joint block with a torque as input and a rotational speed as output. I would like to limit the speed of the joint by imposing a maximum speed.

• I do not find an appropriate block in Simulink Library Browser - Simscape - SimMechanics - Second Generation - Constraints.
• Therefore, I want to make a custom block to limit the rotational speed. Is it possible to include inequalities in a custom Simscape block? Until now, I only encountered equations with == that represented energy balances, see the Electromagnetic converter for instance:

equations
    v   == p.v - n.v;
    mmf == N.mmf - S.mmf;
    mmf == Nw*i;
    phi.der == -v/Nw; % Faraday's law
end

Answer 1

I would use the Rotational Hard Stop block from the Simscape library to limit the angular motion of the joint:

From the documentation:

The Rotational Hard Stop block represents a double-sided mechanical rotational hard stop that restricts motion of a body between upper and lower bounds. Both ports of the block are of mechanical rotational type. The impact interaction between the slider and the stops is assumed to be elastic. The stop is implemented as a spring that comes into contact with the slider as the gap is cleared. The spring opposes slider penetration into the stop with the force linearly proportional to this penetration. To account for energy dissipation and nonelastic effects, the damping is introduced as a block parameter, thus making it possible to account for energy loss.

Answer 2

One way of limiting the velocity is to add torque to the revolute joint proportional to how much it exceeds the limit. You can set the GAIN very high which will result in almost no violation of the maximum velocity. It seems to me you were looking for the 'compare to constant' block, or maybe the 'relational operator' block?