Force Position Control for a Pneumatic Anthropomorphic Hand Alexander Bierbaum, Julian Schill, Tamim Asfour and R¨ udiger Dillmann Institute for Anthropomatics University of Karlsruhe (TH) Karlsruhe, Germany {bierbaum,schill,asfour,dillmann}@ira.uka.de Abstract—Robot hands based on fluidic actuators are a promis- ing technology for humanoid robots due to their compact size and excellent power-weight-ratio. Yet, such actuators are difficult to control due to the inherent nonlinearities of pneumatic systems. In this paper we present a control approach based on a simplified model of the fluidic actuator providing force and position control and further fingertip contact detection. We have implemented the method on the microcontroller of the human hand sized FRH-4 robot hand with 8 DoF and present results of several experiments, including system response and force controlled operation. I. I NTRODUCTION An important application for a humanoid robot is interaction with an environment designed for humans. Therefore, the robot should be able to manipulate and grasp objects. The desired manipulator system and especially the robot hand should provide sufficient power for grasping heavy objects, preciseness for dexterous manipulation and compliance for careful handling. For applications and experiments in hu- manoid robotics a lot of robot hands have been developed providing different characteristics in these categories. Among them are the four finger DLR-Hand II [1] which is driven by brushless DC motors situated in the hand, providing 3 DoFs per finger and position feedback. The NASA Robonaut hand [2] with 14 DoFs has been developed for space applications. Both hands are significantly larger in size than a human hand. For their advantageous power-weight-ratio the development of fluidic hands has been proposed. Famous representatives are the MIT-Utah hand [3] and later the Shadow hand using antagonistic air muscles as actuators [4]. Due to size and weight of the actuators these robot hands were not suitable for integration in a humanoid robot system. The pneumatic FRH-4 Hand which is also of the size of a human hand, has been developed at the Karlsruhe Research Center [5] as a prosthetic hand and development has been continued as a humanoid robot hand. The flexible fluidic actuators have been integrated directly to the finger joints and are operated using air as pneumatic medium. The actuators expand upon inflation causing a rotary motion of the joint. They may be op- erated pneumatically or with hydraulic media. With pneumatic operation closed circulation of the medium is not required, as the air may be released to the environment. Therefore pneumatic systems are easier to build. The air compressibility leads to a highly nonlinear control problem for position and force control, but provides the chance to design an inherently compliant system. To control inflation and deflation of a pneumatic actuator elec- tronically controllable valves are required which are divided into the categories of pressure regulators and switching valves. Pressure regulators allow precise pressure and flow control, as has been demonstrated e.g. in [6]. In [7] force and position control of a pneumatic robot arm has been reported which also uses pressure regulators. Unfortunately this valve type is currently too large to be integrated in a human sized hand. In [8] the position control system for a pneumatic robot arm using solenoid switching valves was presented. The authors used a PWM based valve control scheme [9]. A joint trajectory generator and pressure controller for a bipedal walking robot driven by artificial pneumatic muscles was presented in [10]. A bang-bang controller was used for controlling pressure and to actuate the switching valves. A model-based torque and position control scheme for the flexible fluidic actuator type that is used in the FRH-4 hand has been presented in [11]. The method computes a target pressure as manipulated variable, the implementation of the underlying pressure control loop for switching valves has not been discussed. Results for integration of the controller with a single actuator were shown. In this paper we present a force position control scheme for the pneumatic actuators of the FRH-4 robot hand. We introduce a simplified plant model based on the zero-load actuator charac- teristics to linearize major non-linearities of the system. Using this model we have developed a joint angle controller with torque limitation and a method for detecting fingertip contact from pressure and position sensor data. The algorithm has been implemented on a microcontroller system for controlling a robot hand with 8 DoFs in real-time. Results are given in terms of system responses from different experiments. An accompanying video shows demonstrations of the capabilities of the controlled hand. This paper is organized as follows. In the next section the relevant details of the actuator and sensor system of the FRH- 4 hand are given. In Sec. III we present the simplified plant model with control scheme and implementation. Further, we present experimental results in Sec. IV. Finally, we give a conclusion and an outlook on our future work in Sec. V.