Zero tracking, balances

Figure 5.5 shows the Quanser [29] self-balancing seesaw apparatus. A dc-motor-driven cart moves along a track forming the top of a seesaw pivoted at a point below the track. As the pivot is below the track, the seesaw is unstable as the acceleration of the cart is associated with a reaction force opposite to the acceleration direction, the system has a right-half plane zero. The cart had a fast inner loop to control velocity (relative to the seesaw) together with a low-pass filter to implement (5.24). 

---