Project
AIM: To carry out a system-level simulation of an all-terrain vehicle.
Objective: To prepare a technical report explaining the model properties & comment on the results.
Introduction: The model provided to us is of a BAJA ATV model with Continuously Variable Transmission(CVT). BAJA is a society of automotive engineering students who compete with each other in designing and racing all terrain vehicles. The purpose of the build is to go off road and manoeuvre through different terrains.
The CVT is a continuously variable transmission which can go through a range of gear ratio’s while shifting between gears and provides better engine optimization. In normal automatic transmissions the gear ratios are fixed and cannot be changed. The CVT can maintain the same RPM of engine while switching between gears.
MODEL DESCRIPTION
In the Simscape model above we see the setup of an All-Terrain Vehicle using a CVT. The main blocks of the simscape model are the signal builder, generic engine, engine sensor, CVT with gear ratio as input, simple gear and vehicle body. A signal builder block is used to generate the throttle and brake inputs for the vehicle in this model.
From the Signal Builder we can see the default value of 0.5 given as throttle input and 0 given as brake input.
The Generic engine model is used as a system level model of a spark or diesel internal combustion engine. This type of model is used when only basic parameters are available for modelling. The speed vector and torque vector in engine torque are given from the workspace in Matlab and saved as an m file. The T-port is connected to the throttle input, F is port associated to the rotational crankshaft and B is responsible for the engine block, P and Fc are output ports through which we can get the engine power and fuel consumption rate.
The engine inertia is set to 0.008 Kg*m^2 and initial velocity at 1750 rpm. A limit of 100 RPM is provided as the speed threshold. No fuel consumption is going to be monitored for this model hence nothing is connected to that port.
The Engine Sensor block is used to measure the rpm of the engine and further give that information to the CV transmission.
The physical signal ports W and A report the angular velocity and angle, respectively, of port R relative to port C. The measured angular velocity is positive when the angular velocity at port R is greater than the angular velocity at port C. The W port is fixed to a scope block to measure the Engine RPM.
CVT (Continuously Variable Transmission)
An input shaft and output shaft which is connected to separate ideal rotational motion sensors is joined by the means of a Variable Ratio Gear block where r is the input of the gear ratio from input to output. Connections B(base) and F(follower) are mechanical rotational conserving ports. The CVT gear ratio in this model has been provided by means of a signal builder. The CR represents the gear ratio in this subsystem.
The Simple gear is fixed between the CVT and vehicle body just as a fixed gear ratio or box. By default, the gear ratio in the simple gear block has been set as 10.
Vehicle Body
The vehicle body is a basic setup of the ATV and is made to work with two inputs. The first input is given as brake from port 1 and the second input is given from the CVT box. The input port A gets the output from the cvt which drives all four wheels as seen in the model below. A double shoe drum brake is used for braking. The four wheels are then connected to a Vehicle body block which accounts for body mass, aerodynamic drag, road incline and weight distribution between axles due to acceleration and road profile.
Results:
The results can be seen from the plots window and engine torque-speed curve window. The Vspd and dis are outputs from the vehicle body which show us the vehicle speed and distance covered by the ATV. These are given as go to blocks in the vehicle body. The other two from blocks connected to the output scope are the Throttle and brake that are given at the start of the model in the signal builder.
After running the model for 200 seconds, as the input for brake was given as 0 the value in the output remains 0 throughout the run time and the input for throttle in given as 0.5 throughout the simulation. By the values given to the generic engine block as speed and torque vector from the workspace we can see the graphs generated for the distance covered and velocity generated by the All Terrain Vehicle.
We can see the distance moved by the vehicle from 0 reaches 2850m and the velocity of the vehicle increases from 0 to 53km/hr at 50 seconds and remains at the same speed till the end of the simulation time.
The CVT shaft speed can be seen from the graphs above. The silver line represents the shaft speed at the primary rotational motion sensor and the pink line shows the speed at the secondary rotational motion sensor in RPM. The secondary shaft is rotating at higher RPM because that is the final drive that is supplied to the Simple Gear block.
The Engine torque Vs speed curve is generated from the inputs given as torque and speed in the Matlab workspace window. This plot will be common for all the models until the input in the workspace are replaced with new values.
Conclusion:
From the BAJA ATV model provided to us we can see different outputs that are generated for the velocity of the vehicle and the distance covered. We can change the values given to this model to see how it performs under various different conditions and situations.