Vehicle Performance Analysis

Publish Date: Jun 08, 2011 | 1 Ratings | 5.00 out of 5 |  PDF

Overview

In this tutorial, we will analyze the performance of a small-scale vehicle. Students will develop a model to estimate the steady-state vehicle speed given various road grade. By adding the drive train dynamics to the model, students can simulate and analyze the vehicle response for any given motor characteristics. We will look at the performance difference between different traction model, analyze maximum climbing angle and the effect of weight distribution of the vehicle.


Basics Dynamics of Ground Vehicle


Vehicle System Dynamics and Controls Menu


Control of Vehicle Motion - Anti-Lock Braking System (ABS) in LabVIEW


Table of Contents

  1. General Vehicle Model on Incline
  2. Equations of Motion
  3. Frictional Force
  4. Motor Characteristics
  5. Performance Comparison
  6. Maximum Incline
  7. Weight Distribution
  8. Conclusion

1. General Vehicle Model on Incline

Recall from the Basics Dynamics of Ground Vehicle, the free-body diagram and all the forces acting on the body is shown in Figure 1.

Fig 1. Two-Axle Longitudinal Model

Table 1. Vehicle Parameters

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2. Equations of Motion

Let us assume that for a passenger car, ha ≈ hd  ≈ h and the vehicle does not leave the ground with only motion along the longitudinal axis.

The governing equations for a general planar vehicle model on an inline are

with the following assumptions

We can deduce our states equations after doing some algebra

where

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3. Frictional Force

Up until now, we have considered a constant friction coefficient between the tire and the road. In reality, this friction coefficient is a function of slip. Using a lookup table, we can formulate a mu-slip curve for this particular simulation. It's important to note that slip has a range from -1 (skidding) to 1 (slipping). Trying to keep tires operating within a desired range of slip is the basis of anti-lock braking systems (ABS). We will discuss the concept of ABS in a later tutorial.

Fig 2. Mu-Slip Curve

Since slip and skid decouple the dynamics of the rotational components from the translational dynamics of the vehicle, it is common to formulate slip and skid in a single function 

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4. Motor Characteristics

For this particular vehicle model, we assume that the driveline has two permanent-magnet DC motors that teach drive a worm-gear reducer connected to the wheel shafts. For a given DC Motor, there's a specific torque/speed curve shown in Figure 3.

Fig 3. DC Motor Torque Speed Curve

In this case, the Mabuchi motor specifications can be implemented in LabVIEW as shown in Figure 4.

Fig 4. Motor Specification in LabVIEW

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5. Performance Comparison

Let us compare the performance difference between the zero traction (slipping) and the traction model. In the zero traction case, we will set slip = 1 as shown in Figure 5. We can see from Figure 6 that the traction model takes a bit longer to reach steady-state speed. You can also change the motor specifications to compare steady-state speed difference between different motors. 

Fig 5. Block Diagram of Vehicle Simulation Zero Traction

Fig 6. Steady-State Vehicle Speed Comparison

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6. Maximum Incline

In order to determine the maximum grade that the vehicle will climb, we will look at the sum of the forces in the x-direction equation, namely

At maximum grade, the vehicle cannot accelerate anymore hence

For the simulation, let's assumed a nominal coefficient of friction, µ=0.5 and motor torque T = 0.000185 by recording the steady state value of the torque constant. As shown in Figure 7, both the zero traction model and the traction model have a maximum incline of about 28.4 degrees.

Fig 7. Max. Incline Analysis: Zero Traction Model (Left) Traction Model (Right)

Fig 8. Block Diagram of Finding Maximum Incline Angle

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7. Weight Distribution

From the model, we can see that the weight distribution contributes to the traction force of the model.

and recall that the mouse has a front caster wheel to help distributing the weight. We can see that  as l1 (distance from front pivot to CG) increases, the relative traction force will also increase. This is illustrated in Figure 9. Notice that this only affects the model with traction as the zero traction model does not take distance from pivot to CG into consideration.

Fig 9. Weight Distribution Comparison at Zero Incline

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8. Conclusion

In this tutorial, we added motor dynamics to the longitudinal vehicle model and briefly discussed the concept mu-slip function. By adding additional dynamics to the model, we are able to examine different motor specifications and their effects on the performance. Utilizing both LabVIEW Control Design and Simulation Module and LabVIEW MathScript RT Module, students can implement their kinematic equations and simulate with various vehicle and environmental parameters.

You can download an evaluation copy of the modules here

LabVIEW Control Design and Simulation Module 

LabVIEW MathScript RT Module

NI LabVIEW Robotics Starter Kit

Please contact andy.chang@ni.com to request more information about this article.

Related Links

NI Automotive Applications

Hybrid Vehicle Test and Simulation using NI's Hardware-In-The-Loop (HIL) Platform

Reference

ME 390: Vehicle Dynamics and Controls (Spring 2011)

Prof. Raul. G. Longoria, ME, University of Texas, Austin

http://www.me.utexas.edu/~longoria/VSDC/

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