## 3 Measurements
### Campaign of Measurements  
A couple of keys on the musical keyboard are actuated manually in classical music (Figure 1). In the implemented organ instrument, the keys are actuated using a pneumatic actuator (Figures 5 and 8a,b). The pressure measurements are obtained as readable data using the piezoresistive force sensor FleciForce Sensor located on the key of the musical keyboard and connected to the oscilloscope as shown in Figure 8b. Pressing hard, the sensor's resistance is lower such that the resistance changes from infinite to \(\sim\)300 k. The pressure on the key, where the piezoresistive force sensor is located, is carried out by a pneumatic linear actuator (Figure 9).  
A pneumatic linear actuator is powered by compressed gas and designed to convert compressed air into a linear motion. The amount of force that a pneumatic linear actuator is able to produce is related to the size of the piston and the pressure of the compressed gas. This means that by increasing the inlet air compression or the width of the piston, the effective actuator force will increase. Pressure can often be adjusted while in use, allowing the proper amount of linear force to be created. In our case, the valve system is used to control the air pressure. The key motion was determined by applying the force to the keys of the keyboard with a given key reaction speed by the actuator system in order to simulate the same reaction of an organist. In a data acquisition system, the force and velocity of the key reaction are measured in a mechanical control system. The sound produced by the mechanical actuator in the organ pipe is acquired and amplified by a microphone and collected by an oscilloscope (Figure 10).  
Figure 8: (a) Experimental setup of organ instrument and measurement apparatus; (b) laser displacement system, pneumatic actuator, flexible pressure sensor on musical keyboard.  
The sampling frequency is 300 kHz for a good description of the harmonics of sound waves generated by the pipes. Generally, humans can detect sounds in a frequency range from about 20 Hz to 20 kHz. So, it is preferred to extend the sampling frequency at 300 kHz. The sound signal in Figure 10 emitted by the pipes exhibits a noise phenomenon that can be due to mechanical noises and air leakage. Unfortunately, there was no possibility to physically isolate the noise from the recording; however, the signal was amplified and filtered. The miniature-sized strain gauge load cell (FlexiForce Pressure Sensor-SparkFun), which is mounted on a musical keyboard key, measures the contact force between the probe and the key. The load capacity of the cell is 1 kgf (9.81 N). When the force is applied, the relative change in resistance is measured by the sensor. The load cell signal is converted into a numeric value expressed in voltage. The load cell measurement read in the oscilloscope is expressed in volts and can be converted to force in \(kg_{f}\) (9.81 N) or in \(g_{f}\). When the force is applied, the sensor measures the relative change in resistance. The load cell signal is converted into a numerical value expressed in voltage V[mV]. The load cell measurement read in the oscilloscope is expressed in volts read by the oscilloscope, taking into account that 14 mV is equivalent to 1 g force \(g_{f}\), and the force can also be expressed in newtons (1 g = 0.0098 N). The force is obtained as stress on the key of the organ pipe (the key is stressed by the force of a finger in general) and by making the movement at a certain speed. The force expressed in \(g_{f}\) in function of time is reported in Figure 11.  
Figure 10: Sound signal achieved during the test measurement.  
Figure 9: Pressure signal achieved during the test measurement.  
The key displacement for C key is expressed in mm with maximum value of 11 mm (Figure 12).