Update remus100.py

If positive deflection of stern plane pitches vehicle up the calculations are represented more accurately by this:

Positive deflection -> Positive Z force (Z-down)
Negative X distance and Positive Z force  -> Positive Y moment (right hand rule)
positive Y moment -> pitches vehicle up 

The small error is in the cross product of the x distance and the applied force. My = [-x, 0,0] X [0,0,z] = [0, x*z, 0] where the Y moment is positive for a positive z force and a negative x distance. 

The simulation seems to run the same with this small change.

Author: BradenMeyers

src/python_vehicle_simulator/vehicles/remus100.py

@@ -367,15 +367,15 @@ def dynamics(self, eta, nu, u_actual, u_control, sampleTime):
         Y_r = -0.5 * self.rho * U_rh**2 * self.A_r * self.CL_delta_r * delta_r
 
         # Stern-plane heave force
-        Z_s = -0.5 * self.rho * U_rv**2 * self.A_s * self.CL_delta_s * delta_s
+        Z_s = 0.5 * self.rho * U_rv**2 * self.A_s * self.CL_delta_s * delta_s
 
         # Generalized force vector
         tau = np.array([
             (1-t_prop) * X_prop + X_r + X_s, 
             Y_r, 
             Z_s,
             K_prop / 10,   # scaled down by a factor of 10 to match exp. results
-            self.x_s * Z_s,
+            -1 * self.x_s * Z_s,
             self.x_r * Y_r
             ], float)
 
@@ -502,4 +502,4 @@ def depthHeadingAutopilot(self, eta, nu, sampleTime):
 
         return u_control
 
-    
+