In this chapter, existing literature on the subject of shock absorbers and shock absorber modeling and testing is studied. Section 2.1 studies the different ways of testing shock absorbers. First, the different types of shock absorber test-rigs are studied, namely electromechanical test-rigs and hydraulic test-rigs. The pros and cons of the different types of test-rigs are discussed as well. After that, different shock absorber test cycles that are used in literature and their pros and cons are studied. This section also gives some ideas for what test-cycles to use to test a shock absorber.
Section2.2studies two main shock absorber designs, namely mono- and dual-tube shock absorbers.
It describes their working and the differences between the two designs. Lastly, Section2.2studies different types of shock absorber models that can be found in literature. It studies their workings and what is needed to fit the models. From this, it can be concluded that a type of black-box (or grey-box) model is most suited for the purpose of this project, since it’s not needed to take apart the shock absorber itself or to know the dimensions of the valves and orifices inside to fit this model to measurement data.
3. TU/e shock absorber test-rig
A Zwick-Rel 1852 material tester, formerly used by the Polymer Technology section of the Mech-anical Engineering Department of Eindhoven University of Technology to perform tensile tests, was converted into a hydraulic shock absorber test-rig by Nick Feijen, [7]. He also developed the first controller and performed the first successful tests on a shock absorber. An overview picture and a more detailed picture of the TU/e shock absorber test-rig can be found in Figure3.1.
(a) Overview (b) Detailed
Figure 3.1: The TU/e shock absorber test-rig, [17]. With (1): height adjustment block, (2) force sensor, (3) upper shock absorber clamp, (4) bottom shock absorber clamp, (5) height adjustment controls, (6) rubber mounts and (7) shock absorber.
After the work done by Nick Feijen, Jamie de Blok further improved both the usability as well as the user safety of the shock absorber test-rig by adding a Graphical User Interface (GUI), [17].
He also improved the control protocol of the test-rig. In 2018, Rob Goris further improved the GUI and the control protocol of the test-rig, he updated the feedback controller and added a feed-forward controller which is used to counteract known disturbances, e.g. inertial, gravitational and friction forces, [2]. Goris also programmed a gas and friction test to measure the gas and friction forces of the shock absorber.
In this chapter, different improvements that have been added to the TU/e shock absorber test-rig will be discussed. First, in Section 3.1, the improvements that have been made to the hardware of the test-rig will be explained. Then, in Section 3.2, an improved way to measure the shock absorber velocity is will be discussed. After that, in Section 3.3the test-rig’s control protocol is discussed. Lastly, Section 3.4 gives a short summary and draws some conclusion based on this chapter.
3.1 Test-rig hardware
In order to improve the velocity measurement, as will further be explained in Section 3.2, an acceleration sensor is added to the shock absorber test-rig. Such accelerometer translates the rectilinear acceleration of the hydraulic cylinder into an electrical signal. The accelerometer used in the shock absorber test-rig is the ADXL326 [23], of which the data sheet can be found in AppendixA.
An accelerometer consists of a structure that is micro-machined and built on top of a silicon wafer stage. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is measured using a differential capacitor. Acceleration then deflects the moving mass and unbalances the differential capacitor resulting in a sensor output whose amplitude is proportional to the acceleration, [23].
The sensor that is used came of the suspension of the 2001 BMW 318i as used by Tom van der Sande in his PhD thesis, [24]. The sensor that is used is a 3-axis, ±16 g accelerometer. Since the sensor was used in the BMW, it is converted to run on a 12V DC input, therefore an external power supply and measurement amplifier are needed, these are shown in Figure 3.2. The sensor itself is placed on the lower shock absorber clamp. Since the bottom of the tested shock absorber is directly mounted to the lower shock absorber clamp, it has the same acceleration as the shock absorber itself.
Figure 3.2: Measurement amplifier (top) and external power supply (bottom) for the added accelerometer.
Since the acceleration sensor has voltage as output, it is necessary to translate this output voltage into a physical acceleration. In order to get to this physical acceleration, the sensor needs to be properly calibrated. The two parameters that have to be calibrated are the zero g bias and the 1 g tuning parameter. The values that have been found for this sensor are: 1.566869 V for the zero g bias and 0.0615 V/g for the 1 g tuning parameter. The explanation of the acceleration sensor calibration can be found in AppendixB.
Figure 3.3: Acceleration sensor placed on the bottom shock absorber clamp.
Next to the acceleration sensor, some additional hardware is added to the shock absorber test-rig in order to reduce the amount of measurement noise present in the measurement signals coming from the test-rig. This hardware mainly consists of shielding the measurement hardware and ferrite cores to filter the measurement signals. The hardware measures to reduce this measurement noise in the signals are further explained in AppendixC.
Figure3.4 gives a schematic representation of the TU/e shock absorber test-rig. As can be seen, the test-rig consists of a hydraulic cylinder on which a shock absorber clamp is placed, this clamp is called the lower shock absorber clamp. Above that, the upper shock absorber clamp is connected to the height adjustment block, which can be used to raise or lower the upper shock absorber clamp to compensate for the difference in length of different shock absorbers. The shock absorber can be placed between these two shock absorber clamps and can then be excited by the hydraulic cylinder. The LVDT (Linear Variable Differential Transformer) that measures the position of the hydraulic cylinder, and therefore the shock absorber position, is built into the hydraulic cylinder.
The acceleration sensor is mounted on the lower shock absorber clamp and shown in green in Figure3.4. The force sensor is placed between the height adjustment block and the upper shock absorber clamp and shown in red in Figure3.4.
Force sensor (red) Height adjustment block
Hydraulic cylinder with built-in LVDT Acceleration sensor (green)
Upper shock absorber clamp
Lower shock absorber clamp Shock absorber position (z)
Rubber mounts (black)
Figure 3.4: Schematic representation of the TU/e shock absorber test-rig with in red the force sensor, in green the acceleration sensor and in black the rubber mounts on which the test-rig hangs.