Challenge

Project

IIan Braganza1 Nov 2021

Aim: To carry out a system-level simulation of an all-terrain vehicle using Simulink.

Objective: To prepare a technical report on BAJA ATV explaining model properties and comment on the results.

Theory:

An all-terrain vehicle (ATV) refers to a motorized single-seated tri or quadricycle bike and buggy that is designed for off-road travel and activities. The vehicle consists of handlebars and low-pressure tires that have deep-threaded structures, which assist in driving over muddy, rocky, wetlands and dune topologies. In comparison to other utility vehicles (UTVs), ATVs offer excellent off-road capabilities and can carry heavy equipment over long distances and narrow access points. They can also be customized for high-performance riding, owing to which they find extensive applications across various industries, including sports, entertainment, agriculture, military, and defense.

CVT: A CVT or Continuously Variable Transmission is an automatic, single-speed, and gearless transmission that can shift through an infinite number of gear ratios.

A continuously variable transmission (CVT) is a transmission that operates on an ingenious pulley system that automatically varies the transmission ratio, allowing an infinite variability between the highest and lowest gears with no discrete steps or shifts. CVTs provide more useable power, better fuel economy, and a smoother driving experience than traditional transmissions that use gearboxes with fixed numbers of gear sets.

There are so many types of CVT's. A common type of CVT is the pulley type.

It consists of two pulleys, primary and secondary, which are varied for speed. One is a driven shaft and another is a driver shaft.

Model: A Simulink model for the BAJA ATV is shown below.

The following common blocks are seen in this model:

Signal Builder: The Signal Builder block allows you to create interchangeable groups of piecewise linear signal sources and use them in a model. You can quickly switch the signal groups into and out of a model to facilitate testing.

Generic Engine:

It represents a system-level model of spark-ignition and diesel engines suitable for use at the initial stages of modeling when only the basic parameters are available. It includes Optional idle speed and red line controllers.

The throttle input signal T lies between zero and one and specifies the torque demanded from the engine as a fraction of the maximum possible torque. If the engine speed falls below the Stall speed, the engine torque is blended to zero. If the engine speed exceeds the maximum speed, the simulation is stopped and an error message is issued.

The connections F and B are mechanical rotational conserving ports associated with the engine crankshaft and engine block, respectively. The connections P and FC are physical signal output ports through which engine power and fuel consumption rate are reported.

Speed vector and Torque vector are the assigned parameters in this model.

Engine Sensor:

Ideal Rotational Motion Sensor: The block represents an ideal mechanical rotational motion sensor, that is, a device that converts an across variable measured between two mechanical rotational nodes into a control signal proportional to angular velocity or angle. The sensor is ideal since it does not account for inertia, friction, delays, energy consumption, and so on.

Connections R and C are mechanical rotational conserving ports and connections W and A are physical signal output ports for velocity and angular displacement, respectively.

Simple Gear: Represents a fixed-ratio gear or gearbox. No inertia or compliance is modeled in this block. You can optionally include gear meshing and viscous bearing losses.

Connections B (base) and F (follower) are mechanical rotational conserving ports. Specify the relation between base and follower rotation directions with the Output shaft rotates parameter. Optionally include thermal effects and expose thermal conserving port H by right-clicking on the block and selecting Simscape block choices to switch between variants.

Follower (F) to base (B) teeth ratio (NF/NB): 4

CVT Subsystem:

The CVT Subsystem consists of the following blocks:

Inertia: The block represents an ideal mechanical rotational inertia.

The block has one mechanical rotational conserving port. The block's positive direction is from its port to the reference point. This means that the inertia torque is positive if the inertia is accelerated in a positive direction.

Variable Ratio Transmission: Represents a variable ratio gearbox such as implemented by mechanical belt CVT, electric, or hydraulic transmissions. The physical signal input r defines the ratio of input to output angular shaft velocities.

Connections B (base) and F (follower) are mechanical rotational conserving ports. Specify the relation between base and follower rotation directions with the Output shaft rotates parameter.

Vehicle Body Subsystem:

The Vehicle Body Subsystem consists of the following blocks:

Vehicle Body: Represents a two-axle vehicle body in longitudinal motion. The block accounts for body mass, aerodynamic drag, road incline, and weight distribution between axles due to acceleration and road profile. The vehicle can have the same or a different number of wheels on each axle. Optionally include pitch and suspension dynamics or additional variable mass and inertia. The vehicle does not move vertically relative to the ground.

Connection H is the mechanical translational conserving port associated with the horizontal motion of the vehicle body. The resulting traction motion developed by tires should be connected to this port. Connections V, NF, and NR are physical signal output ports for vehicle velocity and front and rear normal wheel forces, respectively. Wheel forces are considered positive if acting downwards. Connections W and beta are physical signal input ports corresponding to headwind speed and road inclination angle, respectively. If the variable mass is modeled, the physical signal input ports CG and M are exposed. CG accepts a two-element vector representing the x and y distance offsets from vehicle CG to additional load mass CG. M represents the additional mass. If both variable mass and pitch dynamics are included, the physical signal port J accepts the inertia of the additional mass about its own CG.

Parameters are:

Mass: 250 kg

Number of wheels per axle: 2

The Horizontal distance from CG to front axle: 1 m

The Horizontal distance from CG to the rear axle: 0.35 m

CG height above ground: 0.7 m

Gravitational acceleration: 9.81 m/s^2

Tire: Represents the longitudinal behavior of a highway tire characterized by the tire Magic Formula. The block is built from Tire-Road Interaction (Magic Formula) and Simscape Foundation Library Wheel and Axle blocks. Optionally, the effects of tire inertia, stiffness, and damping can be included.

Connection A is the mechanical rotational conserving port for the wheel axle. Connection H is the mechanical translational conserving port for the wheel hub through which the thrust developed by the tire is applied to the vehicle. Connection N is a physical signal input port that applies the normal force acting on the tire. The force is considered positive if it acts downwards. Connection S is a physical signal output port that reports the tire slip. Optionally expose physical signal port M by setting Parameterize by to Physical signal Magic Formula coefficients. Physical signal port M accepts a four-element vector corresponding to the B, C, D, and E Magic Formula coefficients.

Parameters (common for all tires) are:

Parameterize by: Constant Magic Formula coefficients

Magic Formula B coefficient: 10

Magic Formula C coefficient: 1.9

Magic Formula D coefficient: 1

Magic Formula E coefficient: 0.97

Double-Shoe Brake: Represents a brake arranged as two pivoted rigid shoes symmetrically installed inside or outside a drum and operated by one actuator. The actuator force causes the shoes to exert friction torque on a shaft connected to the drum.

Connection S is a conserving rotational terminal associated with the drum shaft. Connection F is a physical signal port representing the force input to the actuator. Positive force creates friction torque that resists shaft rotation. Optionally include thermal effects and expose thermal conserving port H by setting Thermal port to Model.

Parameters are:

Drum radius: 150 mm

Actuator location radius: 100 mm

Pin location radius: 125 mm

Pin location angle: 15 deg

Shoe beginning angle: 5 deg

Shoe span angle: 120 deg

Torque vs Speed characteristics:

Input 1.1:

Brake and Throttle input:

The Brake input is constant i.e. zero throughout the time of 80 sec.

Throttle input is 0.3 for the first 20 sec, accelerates to 1 at the same and remains constant throughout the remaining time till 80 sec.

CVT Ratio:

The CVT gear ratio is 2.3 for the first 4 secs, then drops down to 1.8 till 35 secs and remains constant thereafter till 80 sec

Output 1.1:

Velocity:

CVT Shaft Speed:

Engine Speed:

1.2 With Dashboard

A Simulink model for the BAJA ATV Model including the dashboard is shown below. The rest of the parts and subsystems are common in the above part 1.1.

Dashboard: A dashboard is a control panel usually located directly ahead of a vehicle's driver, displaying instrumentation and controls for the vehicle's operation.

The initial conditions are as follows:

Brake Input = 0

Throttle Input = 0.3

Input 1.2:

It comprises a throttle and brake input through a knob which can be varied unlike the signal builder in the previous model without a Dashboard. The Engine RPM gauge and Velocity gauge displays the changes done by the respective knobs.

Output 1.2:

Velocity:

CVT Shaft Speed:

Engine Speed:

Part 2: BAJA CVT using Lookup Table

2.1 Without Dashboard

A Simulink model for the BAJA ATV Model including the Lookup table is shown below.

Lookup Table: A lookup table is an array of data that maps input values to output values, thereby approximating a mathematical function. Given a set of input values, a lookup operation retrieves the corresponding output values from the table. If the lookup table does not explicitly define the input values, Simulink can estimate an output value using interpolation, extrapolation, or rounding, where:

- Interpolation is a process for estimating values that lie between known data points.

- Extrapolation is a process for estimating values that lie beyond the range of known data points.

- Rounding is a process for approximating a value by altering its digits according to a known rule.

A lookup table block uses an array of data to map input values to output values, approximating a mathematical function. Given input values, Simulink performs a “lookup” operation to retrieve the corresponding output values from the table. If the lookup table does not define the input values, the block estimates the output values based on nearby table values.

In place of CVT ratio input, a lookup table is assigned in the models below. It needs to be loaded in the workspace having the file name as Lookuptable_data.mat