Simulation-Based Development of Safety Related Interlocks - Simulation and Modeling Methodologies, Technologies and Application - page 170

Fig. 9. The second interlocking solution
Fig. 10. Simulation results of the first (a) and second (b) control solutions plotting cart position,
velocity and speed request from the operator (SICC.setPoint)
the cart to the forbidden area in the positive end of the rail. After this, the speed request
is ramped to -1 in order to revert the cart away from the border of the forbidden area.
According to the simulation, the control solution works as it was intended; however,
because it takes time to stop the cart, the location of the cart reaches approximately
6.05 before stopping. Clearly, the control solution could be improved by decelerating
the cart already before reaching the limits. In case of strict limits, violating the forbidden
area (overshoot) could also be avoided by activating the brake before the strict limits;
however, this would reduce the size of the actual, allowed working area.
The second control solution is illustrated in figure 9. In this solution, the braking is
implemented similarly to the first solution and the speed request from the user is relayed
similarly to the controller. However, when the measured location of the cart is between
0 and 1 or between 5 and 6, an automatic mode is activated and another speed setpoint is
calculated by the interlock function. The setpoint is constrained so that between 0 and 1
and between 5 and 6, the maximum allowable speed setpoint towards the forbidden area
is equal to the distance left to the forbidden area. For example, if the location of the cart
is 5.5, the maximum allowed speed setpoint to the positive direction is 0.5. In order to
relay the second, calculated setpoint signal and the mode activation signal, the interlock