The Data Physics SignalStar Vector control system is commonly used in the automotive industry for durability testing of engine crankshafts. The crankshaft is an engine component that converts the reciprocating motion of the pistons to rotational motion. The crankshaft uses “crank throws” to do the conversion between the two motions. Crank throws provide additional bearing surfaces whose axis is offset from the crank, to which the “big ends” of the rods from each piston attach. The red component in the diagram below, Figure 1, shows the crankshaft and the blue shows the pistons. Figure 2 is a photograph of a typical crankshaft.
Figure 1: Crankshaft and pistons (Source: Wikipedia)
Figure 2: Typical Crankshaft (Source: Wikipedia)
There are numerous tests performed to determine the durability/reliability of a crankshaft. This article focuses on the torsion test. While the engine is running, the crankshaft is exposed to repetitive forces from the pistons firing. These forces result in torsion and strain on the crankshaft.
Torsion testing is a type of high cycle fatigue test. The crankshaft is subject to cyclic torsional forces until fatigue results in cracks and causes fractures. The number of stress cycles (cycles of the sine wave) the crankshaft can endure until failure is the fatigue life of the crankshaft. This is compared to the product specification to determine if the crankshaft is acceptable.
The Data Physics SignalStar Vector vibration control system simplifies high cycle crankshaft fatigue testing. It automates the process of determining the crankshaft’s resonance frequency and then uses phase tracking to ensure the excitation frequency tracks the shifting resonance frequency as the crankshaft fatigues.
Let’s take a closer look at torsional testing of the crankshaft. Torsional Crankshaft High Cycle Fatigue testing is typically done using an electrodynamic shaker to excite the crankshaft rig. The electrodynamic shaker is attached to the rig with a stinger. This allows the force to drive the device in one direction while allowing for some cross axis motion. The rig consists of two large rectangular plates; the crankshaft is mounted between the two plates and is suspended from either cables or straps located at the four corners of the top plate. Figure 3 below is an example of a typical test setup.
The first step in the torsional high cycle fatigue testing of crankshafts is to determine the resonance frequency of the crankshaft. The Vector controller drives the shaker with a sine wave while measuring the acceleration response on the fixture. The sine frequency is swept across the resonance frequency of the crankshaft and the transfer function between the drive signal and the response accelerometer on the fixture is measured.
The Vector controller uses this data to determine the resonance frequency, phase, and quality factor (Q). The Vector controller can be set up to automatically determine resonance with user specified search parameters for frequency range, amplitude threshold, and Q. The resonance frequency and phase are used by the controller to excite the crankshaft at its resonance frequency while controlling the acceleration during the resonance dwell. To determine the acceleration reference amplitude, a manual dwell is performed at the resonant frequency. The torque is measured and the dwell level is increased until the desired torque value is reached. Once the desired torque value is reached, the required control acceleration value is determined. Now the automated Resonance dwell test can be performed using this acceleration value for the reference level.
During the resonance dwell, the phase is continuously measured between the drive signal and the accelerometer on the fixture. This phase measurement is compared with the phase of the resonance search of the swept sine data. If the measured phase deviates from the resonance phase, the dwell frequency is automatically shifted until the phases match. This “phase-tracked” technique ensures that the crankshaft is excited at its resonance frequency throughout the duration of the test.
The resonance frequency of the crankshaft will also shift as the crankshaft fatigues. A large change in resonance frequency is an indication of crankshaft failure. The Vector controller can be set to automatically stop the test when the crankshaft begins to fail by placing upper and lower tolerances around the dwell frequency. When the dwell frequency shifts beyond these user specified tolerances, the test will stop. Crankshaft manufacturers have determined that when the resonant frequency shifts by a specific amount, a failure is imminent. By stopping the test when frequency tolerances are exceeded, the Vector controller prevents catastrophe.
Torsional high cycle fatigue testing on crankshafts can also be performed by controlling to a level measured on a strain gauge on the crankshaft and using the measured current from the amplifier as a reference. The strain guage indicates the torsion and the current from the amplifier is proportional to the force input. The rest of their setup is very similar. In this test scenario it is not necessary to run an initial test to correlate torsion to acceleration.
The Data Physics Vector is the controller of choice for Resonance Search and Dwell (RSD) because it provides flexibility in control options. Specifically:
- Frequency Sensitivity – Enables the user to specify how fast the frequency correction needs to be during a dwell test if the Dwell Type is Phase Tracked. This test parameter can be used to avoid over correction when shifting the frequency to track the phase shift. This is one of two parameters that can help prevent dithering. Dithering is when the frequency increases and decreases by very small steps due to slight changes in the phase.
- Amplitude Sensitivity – This selectable parameter determines the correction rate of the excitation level during the dwell test. This will allow you to slow down the amplitude correction in order to prevent overshoot of the desired amplitude.
- Phase Shift – This parameter fixes the maximum phase shift allowed between the nominal resonance phase and the control phase measured during the dwell test. We will not shift the frequency until the phase shifts this specific amount. This can prevent dithering by specifying a minimum phase shift prior to adjusting the frequency of the tracked dwell.
- Frequency Aborts - Available options include: % Dwell of frequency, Absolute Upper and Lower Frequencies, or Off. This parameter is used in crankshaft testing to abort the test prior to catastrophic failure.
- Amplitude Aborts – This feature allows the user to define alarm and abort levels that will be compared to the values in the Dwell table. It can be setup to abort the test if there is too large of an overshoot when the amplitude is increased to full level too quickly.
The ability to add multiple lines to the dwell table for the same dwell and specify different amplitudes is critical when running a high Q value test. Allowing the Dwell to run for several cycles at small amplitude changes allows the control to settle. These normally are not linear drive changes when increasing the amplitude so making small stepped increases prevents over shoot and creates more stability in your testing.
-Thomas Villano, Senior Applications Engineer