Fix equation references

1-introduction/related-work/4-visuo-haptic-ar.tex

@@ -87,11 +87,8 @@ Adding a visual delay increased the perceived stiffness of the reference piston,
 \end{subfigs}
 
 %explained how these delays affected the integration of the visual and haptic perceptual cues of stiffness.
-The stiffness $k$ of the piston is indeed estimated by both sight and proprioception as the ratio of the exerted force $F$ and the displacement $D$ of the piston, following \eqref{stiffness}.
-But a delay $\Delta t$ modify the equation to:
-\begin{equation}
-    \label{eq:stiffness_delay}
-    k = \frac{F(t_A)}{D (t_B)}
+\begin{equation}{stiffness_delay}
+    k = \frac{F(t_H)}{D(t_V)} \quad \text{with} \quad t_H = t_V + \Delta t
 \end{equation}
 where $t_B = t_A + \Delta t$.
 Therefore, a haptic delay (positive $\Delta t$) increases the perceived stiffness $k$, while a visual delay in displacement (negative $\Delta t$) decreases perceived $k$~\cite{diluca2011effects}.