Hi,
I am working on Disk Friction Clutch in simscape library. As mentioned in documentation, the clutch basically Fundamental Friction Clutch: "The Disk Friction Clutch block is a simplified implementation of the Fundamental Friction Clutch block. The Fundamental Friction Clutch requires the kinetic and static friction limit torques as input signals. The Disk Friction Clutch does not require the input data. Instead, the block calculates the kinetic and static friction from the clutch parameters and the input pressure signal P." Using fundamental friction clutch, we may manipulate the friction torque using slip speed based look-up table. Same concept is described in Disk Friction Clutch by μ is the viscous drag coefficient as a constant in default.
The problem is that there is no way to use viscous drag approach (μ != 0) and work with Disk Friction Clutch since even with zero Pressure input the clutch is always transmits a torque in simulations (does not rely on pressure). Thus, the block cannot represent the actual states of a friction clutch which can be summarize at least as: OPEN, SLIP, CLOSED. With viscous drag, the only SLIP and CLOSED states can be represented and this is causing problems in the simulation. In other words, there is no transition from OPEN to SLIP with viscous drag. Please check notes/attachment.
My question is if is this issue known/intentional and is there a workaround/recommandation for OPEN clutch status and transition from OPEN to SLIP?
Note: Also in Fundamental Friction Clutch the same issue exist but using example of custom clutch, a workaround somehow can be introduced. However, I would like to understand and ask if there is a possibility to represent more accurate behaviour in simulations. As it can be seen from attached figure, for disk friction clutch, when I check the example in Simulink and plot the pressure against torque values with nonzero viscous drag coefficient (=0.5) there is a transferred torque even if pressure is zero.
where 
. Therefore, even if the pressure is given as zero, some torque is transmitted due to the term 
.
