The controller for engagement and disengagement of the hip spring is based on the probabilistic model of the human motion that classifies whether the user requires the support of the exoskeleton or the exoskeleton should remain disengaged to allow free motion [11]. The control architecture has two main interconnected building blocks: safety and high-level control. The block diagram in Fig. 5 shows the connections between building blocks of the passive spinal exoskeleton framework.
Safety, as the primary concern of wearable robots, is ensured by monitoring both, the feedback signals and the control signals. The monitoring of both signals is based on an invariance control that supervise the nominal controller, and correct the control outputs if the system states are about to leave the admissible state-space region. Switching between the base and the corrective controllers ensure that the safety features are invariant. For the passive spinal exoskeleton, the safety is additionally ensured by also monitoring the feedback signal. If the feedback signal is within the permissible regions that can be achieved by the user, then the controller output u is enabled. The safety condition is governed by
$$\begin{aligned} u={\left\{ \begin{array}{ll} u, &{} \text {if }-20^{\circ }<q_t<110.\\ 0, &{} \text {otherwise}. \end{array}\right. } \end{aligned}$$
(1)
where \(q_t\) is the upper body angle relative to the thigh. Note that this ensures that the feedback states are in the admissible set. Due to the mechanical characteristics of the passive spinal exoskeleton mechanism, the high-level control for the passive spinal exoskeleton can only account for uniformly reducing spinal load. The control to achieve this is given by
$$\begin{aligned} M_{exo} = uKM_h, \end{aligned}$$
(2)
where \(M_{exo}\) is the moment component of the passive exoskeleton, u is the binary control signal that activates the clutches of the passive exoskeleton, K is a constant describing mechanical properties of the passive exoskeleton, \(M_h\) is the moment component of a human. The high-level control consists of two modes of operation: Manual and Automatic. In a manual mode user can freely chose to either engage or disengage the clutch. While in automatic mode, the control system automatically engages or disengages the clutch when needed. For example, when user is bending forwards the system will automatically engage the clutch at the beginning of the bend to provide support and reduce spinal loading. Similarly, when a user is walking, the system keeps the clutch disengaged to allow unconstrained motion.