Whole body vibration and rider comfort determination of an electric two-wheeler test rig

Introduction

In India, the major mode of transportation is two-wheeled vehicles.¹ About 15 million two-wheeled vehicles were sold in India over the last 10 years on yearly basis.² Professionals like the food and goods delivery partners, low-wage employees, and post-delivery persons mainly use two-wheelers for their daily transportation.³ Two-wheeled vehicles, owing to limited size and mass are prone to vibration when compared to four-wheeled vehicles.⁴ Whole-body vibration (WBV) mainly affects these people who are in continuous exposure to noise and vibration throughout the day.^5,6 Truck drivers, drill operators, heavy machinery workers, and forklift drivers are the victims of these vibrations.⁷ High risks of lower back pain, motion sickness, and digestive system problems have been reported due to WBV when exposed for a longer period.^8,9

The whole body vibration of the vehicle is dangerous not only to the rider but also to the vehicle as well.¹⁰ In a vehicle exposed to different terrain conditions, the driving scenario is subjected to vibration, and these are transmitted to the human body through the seat, handlebar, and footrest in the case of two-wheelers.¹¹ These vibrations transferred to the human body cause different health issues in long run.^12,13 Some researchers are working on reducing these vibrations to effectively increase rider comfort.^14–17

A vehicle's comfort is influenced by many factors such as the seat design,¹⁸ driving posture¹⁹ and environmental factors,²⁰ road condition, and suspension system to name a few. In a two-wheeler, the rider’s comfort plays a very important role, as the rider has continuous exposure to these influencing factors. Both the static and dynamic condition of the vehicle are important in predicting the rider’s comfort.²¹ Different kinds of shock absorbers,²² and damping techniques²³ play a vital role in improving the rider’s comfort. Two-wheelers especially in the Indian scenario are very much subjected to vibration due to the condition of roads even in the cities.²⁴ The comfort level is greatly influenced by potholes, humps, cracks, and riding speed, and some study work has assisted in recognizing the potholes for safe driving.²⁵

The measurement of whole-body vibration in terms of human health and comfort, perception probabilities, and motion sickness occurrence is studied with the help of ISO 2631-1 standard.²⁶ It provides guidance on measurement techniques for periodic, random, and transient whole-body vibrations.²⁷ By getting the Frequency Response Function (FRF) at critical vibrational points, variable acceleration values at multiple spots were identified. The rider's comfort depends on these values of acceleration. The higher the value of acceleration lowers the rider's comfort. ISO 2631-1 standard provides different levels of comfort faced by riders depending on the acceleration values. Table 1 gives the detailed classification of the rider’s comfort level as per ISO 2631-1.

The present study involves finding the strategic locations of vibration and evaluation of rider comfort on an electric two-wheeler (E2W) test rig. Different points at the test rig are evaluated for their acceleration values in different running conditions. The points (locations on the body of test rig) at which the amplitude of vibration is higher than other locations are considered strategic points of vibration. The vibration at the strategic points is high enough to cause discomfort to the rider. The impact hammer test²⁹ is conducted using the PCB (Pico Coulomb) Piezotronics made impact hammer of sensitivity 10.1 mV/g and data acquisition by using NI LabVIEW.

Methods

Development of simulation model

The electric two-wheeler test rig is modeled as a state space model for finding its natural frequencies. Performing the impact hammer test on the setup, the natural frequencies are obtained. The details of the work carried out are discussed in this section.

Test setup

The test setup is an electric vehicle two-wheeler test rig, which uses a 1.5 kW, brushless direct current (BLDC) traction motor powered by a 25AH LiFePO₄ battery. Figure 1 shows the photograph of different parts of the Electric two-wheeler (E2W) test rig (components sourced from Artis Technologies) and Table 2 gives the nomenclature.

Figure 1. Electric two-wheeler test rig.

Table 2. E2W test rig nomenclature.

Number	Part name
1	Electric panel
2	Wheel and the loading area
3	BLDC motor
4	RPM sensor
5	Desktop with Sync sols EV lab Software
6	25 Ah Battery
7	Battery Modulator
8	NI Data Acquisition system

Using the laboratory's setup, the E2W test rig is modeled as state space model.³⁰ However, to build a similar model, two considerations were made, which are briefly discussed here. The first thing to consider is the cylindrical steel roller of approximately 40 mm diameter and 200 mm in length beneath the test rig's wheel is used to simulate a real-life road surface. The roller used is a hard plastic material In this case, however, unlike the road and wheel, the roller causes a small vertical displacement to the wheel. As a consequence, the vertical displacement of the roller concerning the vertical displacement of sprung and un-sprung masses is estimated to be near zero or zero.^31,32 The second subject of consideration concerns the sprung and un-sprung masses. Sprung mass is the percentage of the vehicle's overall mass that is supported by the suspension. Un-sprung mass refers to the mass of the suspension, wheels, and other components that are directly connected to them. This implies that a vehicle's sprung mass is typically the vehicle's kerb weight, the weight of the driver, and in certain cases, the weight of the engine.³³ A state space model of the E2W test rig is shown in Figure 2.³⁴ The model nomenclature is indicated in Table 3. The acceleration values are measured by the PCB Piezotronics made accelerometers of 101.1 mV/g sensitivity and data acquisition is carried out through National Instrument’s LabVIEW software. (MyOpenLab is an open source alternative that can carry out a similar function). Three trials were conducted and the average values are considered for analysis.

Figure 2. Model of the E2W test rig.

Table 3. Test rig model nomenclature.

Variables	Description
M1	Un-sprung mass (tyre mass) – (kg)
M2	Sprung mass (Mass of test rig - Tyre mass) – (kg)
X1	Vertical displacement of M1 – (m)
X2	Vertical displacement of M2 – (m)
K1	Tyre stiffness – (N/m)
K2	Spring stiffness of vehicle suspension – (N/m)
C	Damping ratio of the vehicle suspension – (Ns/m)
U	Vertical displacement of the roller – (m)

The free body and Laplace equations of the state space model derived are as indicated below in equations 1, 2 and 3, 4 respectively:

(1)

$${\mathrm{m}}_2{\ddot{x}}_2+\mathrm{c}\left({\dot{x}}_2–{\dot{x}}_1\right)+{\mathrm{k}}_2\left({\mathrm{x}}_2–{\mathrm{x}}_1\right)=0$$

(2)

$${\mathrm{m}}_1{\ddot{x}}_1+\mathrm{c}\left({\dot{x}}_1–{\dot{x}}_2\right)+{\mathrm{k}}_2\left({\mathrm{x}}_1–{\mathrm{x}}_2\right)+{\mathrm{k}}_1\left({\mathrm{x}}_1-\mathrm{x}\right)=0$$

Laplace equations

(3)

$${\mathrm{X}}_2\left(\mathrm{s}\right)\left[{\mathrm{M}}_2{\mathrm{S}}^2+\mathrm{CS}+{\mathrm{K}}_2\right]={\mathrm{X}}_1\left(\mathrm{s}\right)\left[\mathrm{CS}+{\mathrm{K}}_1\right]$$

(4)

$${\mathrm{X}}_1\left(\mathrm{s}\right)\left[{\mathrm{M}}_1{\mathrm{S}}^2+\mathrm{CS}+{\mathrm{K}}_2+{\mathrm{K}}_1\right]={\mathrm{X}}_2\left(\mathrm{s}\right)\left[\mathrm{CS}+{\mathrm{K}}_2\right]+\mathrm{X}\left(\mathrm{s}\right)\left[{\mathrm{K}}_1\right]$$

Figure 2 shows the state space model of the E2W test rig and the corresponding notations as indicated in the figure.

Figure 3 shows the magnitude vs frequency plots of the impact hammer test conducted on the E2W test rig. The plot obtained from the NI LabVIEW, 2016 ((MyOpenLab is an open source alternative that can carry out a similar function). shows the natural frequency of the test rig as obtained at two strategic points on the test rig ash shown in Figure 3. The peaks in the graphs indicated the natural frequency of the rig. The average of the natural frequency obtained is shown in Table 4.

Figure 3. Impact hammer test on E2W test rig (a,b,c,d - Loading area- left back, right back, left front, right front), (e,f,g,h – Base mount – left back, right back, left front, right front).

Table 4. Average of natural frequencies.

Frequency (Hz)	Magnitude (g)
180.225	6.338
575.36	11.37
786.942	7.03

Obtaining acceleration using LabVIEW

National Instruments’ LabVIEW 2016 (64-bit) software is used to extract the acceleration values (MyOpenLab is an open source alternative that can carry out a similar function). To obtain the rider’s comfort as per ISO 2631 standard,²⁶ the raw acceleration is converted to RMS acceleration using the spectral analysis function in the LabVIEW software. Some open-source software like MyOpenLab can be used for data acquisition and spectral analysis as well. Fast Fourier transform (FFT)³⁵ is used to obtain the root mean square acceleration values at the strategic locations.

The test setup is tested under different loading conditions, namely. kerb load, 5 kg load, and 10 kg load. This type of loading makes a machine or a material get stiffer as the load increases.³⁶ PCB Piezotronics made accelerometers are mounted at four strategic locations of vibration; the loading area, traction motor, suspension and the base mount of the rig. RMS acceleration, at these strategic locations, is recorded using LabVIEW programming. These values are then compared with ISO 2631 standard to analyze the rider’s comfort.

Drive cycle

The drive cycle used for the study is shown in Figure 4. Different scenarios like idling, acceleration, steady speed, and deceleration are shown in the graph. The drive cycle runs each of these scenarios for a particular time duration. The cycle begins with a preparation speed-up period of 5 seconds, followed by 20 seconds of idling, 18 seconds of acceleration, and 2 seconds of steady speed. The cycle then decelerates for the next 11 seconds, a combination of acceleration and steady speed for the next 7 seconds, decelerates for the next 30 seconds, idles for the next 11 seconds, and ends with 3 seconds of halting. The values of acceleration are recorded at the strategic locations during this cycle.

Figure 4. Drive Cycle.

Results and discussion

In this study, major strategic locations of vibrations in an electric two-wheeler test rig are found. These values are then compared with the ISO 2631-1 standard to further analyze the rider comfort. In conducting the impact hammer test, initially, the natural frequency was extracted. Further, by building a state space model of the test rig and running the setup in different driving scenarios the RMS acceleration was obtained at the strategic locations. A detailed discussion of the result obtained is provided in this section.³⁷

The acceleration values obtained at the strategic locations of vibration show that the values of RMS acceleration are in the range of 0.2 g to 3.2 g when the whole setup is considered. The strategic locations obtained are, the loading area of the rig, the traction motor, the suspension, and the base mount of the rig. These locations when checked for their vibration characteristics provide the vibration intensity. The values of acceleration obtained, when compared with ISO 2631-1 guideline gives the rider comfort in the vehicle as shown in Table 1.

Figure 5, shown below indicates the different values of acceleration at the strategic locations of vibration. Observing, the RMS acceleration it is seen that, the rider comfort ranges from fairly- uncomfortable region to extremely uncomfortable region as compared to Table 1. These values indicate that there is a lot of scope for improving the rider's comfort. Comparing Figures 4 and 5 it is observed that, as the speed increases the vibration intensity increases as well.

Figure 5. RMS Acceleration at the strategic locations.

Observing the RMS acceleration at the loading area, the value of g increases as the speed increases. During idling, the no-load acceleration of 3.29g is recorded at the loading area which is higher compared to the loaded accelerations of 0.16g. The values of RMS acceleration increase with acceleration and decrease with deceleration. As the load is increased from 0 to 10 kg, the values of acceleration decrease. This is due to the un-damped fixture of the loading area.³⁷ The values obtained are extremely uncomfortable for the rider and need the researcher’s attention in damping the vibrations. The vibration at the traction motor indicates that the amplitude of vibration is in the range of 0.21g to 0.25g and this falls under the category of very uncomfortable region. The suspension and the base mount show vibrations in the range of 0.48g to 1.308g and are in the region of the extremely uncomfortable and very uncomfortable region respectively.

Conclusions

A detailed experimental analysis of finding the strategic locations of vibration is discussed in this paper. The acceleration values play an important role in deciding the rider’s comfort. Electric two-wheeler, even though a cost-effective mode of transportation, require further research in improving rider comfort. Different strategic locations of vibration are found to affect the rider’s comfort. The following conclusions are drawn from this study:

Loading area, traction motor, base mount, and suspension were found to be the strategic points of vibration in the E2W test rig. About a 19% increase in the vibration can be observed in the loading area when the loads are removed. This loading area corresponds to the seat of the driver in an actual vehicle and is prone to huge vibration and thus needs attention in damping these vibrations. The acceleration at other strategic locations is found to be higher at loading as well as no-load conditions and these vibrations induce discomfort to the rider as well.

As the speed increases, the vibration intensity increased as well. This is due to the wheels running on the roller support, which simulates an actual road scenario. Hence, it can be concluded that in the actual driving scenario of a two-wheeler the vibration increases as the speed of the vehicle is increased. Further, the condition of the road is again an influencing factor, which increases the vibration intensity.

Overall study indicates that the electric two-wheeler is subjected to vibrations is an important area to be considered for further research work and this can be reduced by using suitable damping techniques at the strategic locations of vibrations.