**6.3. Advanced control concepts**

Implementation of advanced control concepts requires the use of a high-performance PC-based controller. The control concept has a cascade structure and is applied for both (pneumatic and hydraulic) FFA realizations [85]. Control algorithms for pressure, position and interaction control for single FFA as well as for compliant robotic arms, using different valve types, were developed and tested.

#### *6.3.1. Pressure control*

Fast and precise pressure control in the inner loop of the cascade control scheme is necessary for effective control of FFAs. To decouple the pressure subsystem from the mechanical subsystem, a control law was derived using the feedback linearization approach, which was modified with regards to specific traits of soft actuators [85]. In order to compensate non-linearities of FFAs pressure dynamics, model (6) - (7) of actuator chamber volume is used in control law as feedback and for the determined flow rate the control voltage is obtained by inverting of experimental flow map (figure 10). The effectiveness of designed pressure controller has been confirmed through several experiments (step response), in pneumatic case both for PWM-controlled on-off valves [86] as well as for different types of servo valves [64, 137].

(a) Pressure Control Scheme

**Figure 27.** Different Control Schemes

#### *6.3.2. Position control*

26 Will-be-set-by-IN-TECH

(a) Pressure Control - Different Valves (b) Pressure Control - Double Valve Arrangement

**position raw value**

**Position Control; Load 0.8kg or 1.94 Nm; double valve arrangement**

4 5 6 7 8 9 10 11 12 13 14 15 16

(d) Position Control - Double EV09-Valve

**s**

**q qs**

**[s]**

Implementation of advanced control concepts requires the use of a high-performance PC-based controller. The control concept has a cascade structure and is applied for both (pneumatic and hydraulic) FFA realizations [85]. Control algorithms for pressure, position and interaction control for single FFA as well as for compliant robotic arms, using different

Fast and precise pressure control in the inner loop of the cascade control scheme is necessary for effective control of FFAs. To decouple the pressure subsystem from the mechanical subsystem, a control law was derived using the feedback linearization approach, which was modified with regards to specific traits of soft actuators [85]. In order to compensate non-linearities of FFAs pressure dynamics, model (6) - (7) of actuator chamber volume is used in control law as feedback and for the determined flow rate the control voltage is obtained by inverting of experimental flow map (figure 10). The effectiveness of designed pressure controller has been confirmed through several experiments (step response), in pneumatic case both for PWM-controlled on-off valves [86] as well as for different types of servo valves

**q qs**

**MC Position Control GPA3.2-10RVD Position-Sequence 6; EV09-valves**

5 10 15 20 25 30 35 40 45 50 55 60 65 70

**Figure 26.** Results of Embedded Control using Switching Valves

(c) Position Control - Single EV09-Valve

**6.3. Advanced control concepts**

valve types, were developed and tested.

*6.3.1. Pressure control*

[64, 137].

**position raw value**

For the position control in the outer loop, a sliding mode control with time delay estimation (SMCTE) was proposed and successfully tested for a single rotary joint. In the control design, the specific experimentally investigated dynamic model of pneumatic driven FFA was taken into account [85, 86]. In hydraulic case the iterative feedback tuning technique was applied [85]. For pneumatic driven planar robots with two soft fluidic actuators, the SMCTE approach was implemented for decentralized joint position control and shows better results than Fuzzy control, optimized using genetic algorithm [64]. Position control for pneumatic soft-robots

#### 28 Will-be-set-by-IN-TECH 594 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

with spatial modular kinematics based on FFA modules is considered in [127]. In addition to decentralized SMCTE position controller the active gravity compensation-based on the quasi-static robot model is used in feed-forward loop to take the weight of robot mechanics into account. Experimental investigations, conducted with different loads for soft-robots with 4 and 6 degrees of freedom (DOF), show the behavior, the quality and the limits of the decentralized control concept with and without active gravity compensation.
