## 1 Introduction  
The versatile use of pneumatic cylinders in various industrial positioning tasks requires increased efficiency and process quality, hence the demand for the realization of preset velocity profiles. The main problem results from the transient behavior of the cylinder piston rod when starting from standstill and direction reversing. The friction induced stick-slip effects occurring in the sealing contacts significantly restrict the positioning accuracy. Currently used concepts to reduce friction in pneumatic sealing contacts consist either of adapting sealing parameters, such as the materials and geometry, or of parametrizing the contact partners by lubricating or coating the contact surfaces. However, the reduction of contact-friction by applying piezoelectrically excited ultrasonic vibrations is used in many processes of ultrasonic machining, and recently made inroads into pneumatic servo systems [1, 2, 3, 4, 5]. Theoretical and experimental investigations on metal-metal coupled contacts go back to the year 1966 and focus on wire drawing processes [6]. In [7], an explanation of macroscopic friction reduction in the presence of ultrasonic vibrations was given using [PERSON]'s law for perpendicular and parallel superpositioning of macroscopic movement in sufficient agreement with experimental results. Recently, dynamic friction models were used, e.g., in [8], to explain the stick-slip transients in dry friction contacts assuming elastic-plastic and viscoelastic material properties. The impact of excitation frequency was not taken into account though. The results in [9] state that the sliding friction of metals under longitudinal ultrasonic vibrations is significantly greater than the sliding friction of transverse vibrations, which is 80% weaker. Longitudinal superposition of the piston rod is thus a promising strategy for friction reduction. In [10] a pneumatic actuator with integrated piezo-stack actuators in a cylinder was presented; it reached a 52% friction reduction under dry conditions. Bending and longitudinal vibration modes were investigated therein. In [11] a prototype of a piezoelectrically actuated piston-rod was introduced for the first time: it reduced friction in the static rod and the moving piston seal simultaneously by parallel superimposition of the macroscopic velocity. A pin-on-disc tribometer was introduced in [12] for the experimental investigation of perpendicular induced ultrasonic vibrations. They validated the results with an elastic-plastic \"cube\" model describing the stiffness variation of the contact asperities. Any friction-affected process can be subdivided into the occurrence of breakaway transients and dynamic friction behavior. While breakaway transients occur as a transition from sticking or pre-sliding to the sliding state due to the release of pre-stress between the contact opponents, dynamic friction behavior occurs while relative motion or sliding takes place at the macro-scale. When friction reduction in elastomer-metal coupled contacts is induced by ultrasonic vibrations, these two friction states are accompanied, besides material wear, with multi-frequency excited friction characteristics, such as variations of contact stiffness, variations of shear stress distribution [13] and local occurrences of glass transitions [14; 15]. Thus, the experimental investigation of ultrasonic friction reduction requires individual identifications of these effects. For this purpose, two types of control are used in the linear tribometer introduced in this contribution. The velocity control allows the investigation of dynamic friction mechanisms and the friction force control enables the investigation of breakaway transients. All transitions of the frictional contact from static to sliding friction, including hysteretic pre-sliding, can be examined and quantitatively investigated with respect to the global parameters relative to velocity of the contact partners, the ultrasonic amplitude and the sealing material.