## 2 Experimental Setup  
### Topology of Linear Tribometer  
The linear tribometer shown in Figure 1 is vertically aligned and is 525 mm high. The vertical alignment is used to prevent axial moments from leading to an uneven distribution of normal force. The sealing holder is mounted on a hollow shaft which is axially guided by an air bearing. The drive is performed by a linear voice coil actuator along the vertical axis. According to the working principle, the actuator provides forces linearly proportional to the current flow and independent from the voice coil position. This enables the measurement of frictional forces in the seal without additional dissipative forces caused by structural coupling with the test environment. The sonotrode tip of a Langevin type ultrasonic transducer is placed at the level of the sealing ring. The transducer is mounted on top of the tribometer construction plate and could be exchanged. The differential force measurement is performed by piezoelectric force sensors applied on both sides of the examined contact, one between the sealing holder and the hollow shaft and one at the back of the ultrasonic transducer housing. The sealing holder designs for the piston and rod seal and the corresponding sonotrode tip designs are shown in Figure 1. According to their technical use in pneumatic cylinders, the piston and the shaft designs enable the investigation of friction effects under two different bearing conditions of the sealing ring concerning the inner and outer diameter. The drive control of the voice coil actuator is implemented through the real-time dSPACE controller board of type DS1103 and could be switched between speed control mode for the measurement of friction dynamics and force control mode for the identification of breakaway processes. Figure 2 gives an overview of the control loops implemented to the tribometer. The US-transducer (UST) is operated in resonant mode by a digital phase control unit (DPC 500/100k) [16]. When driven in resonant mode, the velocity amplitude of the ultrasonic transducer is proportional to the current amplitude [17]. This is the reason why a simultaneously operating current controller is used to keep the velocity's amplitude constant during a measurement run. The detection of amplitude and phase information between the driving signals current and voltage was done using phase sensitive demodulation (PSD) [18]. This information was further used to keep the system at resonance constantly with a PID-controller. This hardware was also used to determine the steady-state transducer's electromechanical impedance, which means the absolute ratio of tip velocity to current. These measurements were additionally verified by laser vibrometer measurements. To ensure resonant operation of the UST and thus avoid instabilities of the control loop resulting from load-dependent increases of the system's damping, the phase-zero-crossing of the mechanical admittance was used as an indicator for the resonance frequency. This required precise preliminary identification of the parallel capacitor Cp. Knowing this parameter, the phase of the mechanical admittance was calculated internally by compensation and used as the input variable of the phase control loop. The UST is driven by a wide-bandwidth, high-power audio amplifier (G). The displacement of the sealing holder, and in turn the dynamic sealing contact (DSC), is measured by a triangulation laser sensor and is used as feedback for the implemented PID position control. The reference displacement profile is obtained by integration of an in operationally-calculated velocity profile. The force signal from force sensor 1 (see Figure 1) is used as feedback to the implemented PID force controller. The classic controller was extended by feed-forward control. This required a preliminary measurement of the current-force characteristic of the voice coil motor (VC) with a static force sensor using a linearly increasing current signal covering the complete stroke of the motor.