A. Consumer Survey and Findings
First, a market survey was conducted to understand the needs of potential customers and technicians, and to gain a deeper understanding of the market that our product aims to cater to, which received 108 responses from vehicle owners in India, USA, Britain, and France. The findings of the survey also helped us identify potential areas of improvement from the list of special features requested by the customer base.
The survey found that most consumers who own an SUV, a compact SUV, or a sedan, having never heard of a towbot before, are open to the idea of purchasing their own towbot. Many consumers preferred to have the bot fit in the boot of their own vehicles and agreed that it should be reasonably easy to hoist and lower it themselves. The bot is expected to tow their vehicle for up to 5km, over fully paved roads.
Figure 2. Interest Expressed In Owning The ProTow Bot
B. Design Calculations
Tractive force required is calculated by considering rolling resistance and gradient load. For a pulley of diameter 60 mm, the torque and rotational speed of the motors are calculated from the tractive force and the pre-defined speed of the bot.
Tractive force = m*g*sinθ (gradient load) + µr*m*g*cosθ (rolling resistance)[4]
Considering the mass of vehicle as 1500kg and a rolling coefficient of 0.015, the following table shows the tractive forces for various gradients.
TABLE I
FORCE REQUIRED FOR TOWING
Gradient (%)
|
Slope angle (degrees)
|
Net required tractive force (N)
|
Max tractive force that can be applied (N)
|
0.00
|
0.00
|
220.50
|
10300.50
|
10.00
|
5.71
|
1682.11
|
10249.38
|
20.00
|
11.32
|
3099.13
|
10100.47
|
30.00
|
16.71
|
4435.21
|
9866.09
|
40.00
|
21.81
|
5664.17
|
9563.77
|
50.00
|
26.58
|
6771.26
|
9213.05
|
60.00
|
30.98
|
7752.16
|
8832.61
|
70.00
|
35.01
|
8610.54
|
8438.50
|
The above plot is between the maximum/required tractive force and gradient. As per the plot, the bot will be able to travel in surfaces up to 60% gradient. Considering the static coefficient of friction between dry roads and tyres as 0.7, the torque required for each motor is calculated[4, 5].
TABLE II TORQUE CALCULATION
Gradient (%)
|
Torque required (N-m)
|
Torque per motor (Ncm)
|
0.00
|
6.62
|
165.38
|
10.00
|
50.46
|
1261.58
|
20.00
|
92.97
|
2324.34
|
30.00
|
133.06
|
3326.41
|
40.00
|
169.93
|
4248.13
|
50.00
|
203.14
|
5078.45
|
60.00
|
232.56
|
5814.12
|
70.00
|
258.32
|
6457.90
|
From above torque calculations, power and speed required for the motors are calculated. So, motors are selected as per the requirements (each should have at least 24Nm torque with Power with > 250watts).
Skid Steering
Figure 5. Forces on a contact area element at turns
In skid steering, slip must happen at the treads. The frictional forces acting on the treads during turns are[6],
For outer tread.
$${F}_{0}={F}_{w}\left[{\mu }_{r}+{\mu }_{turn}f\left(\frac{2c}{l}\right)\right]$$
1
For inner thread.
$${F}_{i}={F}_{w}\left[{\mu }_{r}-{\mu }_{turn}f\left(\frac{2c}{l}\right)\right]$$
2
Where,
$$f\left(x\right)=x*\text{ln}\left[\frac{1+\sqrt{1+{x}^{2}}}{x}\right]$$
3
Based on the above equations, the maximum torque required is 49 Nm, which could be easily provided by the motors.
Structural Design
CAD assembly and simulations were performed to optimise the bot’s design and enable accurate selection of components such as the tread system, scissor jack, main frame, motor placement and electronic chipset placement.
For small size tracked vehicles, rubber pad grousers have better traction with higher shear force mobility than metal grousers, a pulley was designed based on the tread available in the market like pitch, width, material, and tooth depth[7].
Based on the load-carrying requirements, 6 supporting wheels were chosen. Twelve SKF 60002Z bearings were chosen to be attached on either side of the supporting wheels for mounting into the frame.
TABLE V
WHEEL PARAMETERS
|
Value
|
Pitch diameter
|
58mm
|
Pitch
|
9.525mm
|
Teeth
|
20
|
Width
|
40mm
|
Bore diameter
|
8mm
|
E. Structural Analysis
For analysis, a load of 700kg was assumed (based on average weight distribution of cars currently in the market. For SUV we take 1500kg total weight) and mild steel for frames and material SS304 for reinforcement. Mesh size was chosen as 3mm for accurate results. For Scissor jack Analysis, dynamic and static load condition were taken and maximum stress obtained in static stress analysis which is 488 MPa and Factor of safety 1.3. For Structure under both conditions, there was a total deformation of less than 1mm, stress up to 300 MPa, and factor of safety as 1.2.
Material for Structure is Mild steel 2062 E450 due to its high strength, high corrosion resistance and Market availability. For Reinforcement SS304 was used, which is also high corrosion resistance, provide additional strength to the structure. For pully we use Aluminium S13 for its least weight, high strength, and wear resistance.
F. Fabrication Of Prototype
Assembly of components was fabricated to maximise ventilation and assess wiring constraints. Once the CAD model was finally iterated, the required components were drafted for laser Cut (2D Draft). 2mm mild steel sheet was laser cut to frame the required structure.
G. Electronics Circuit Design And Programming
Arduino Mega was selected as the microcontroller due to the necessity of a greater number of input/output pins to accommodate motor drivers, Bluetooth module, relays and a voltage detection circuit. The power electronic driver chosen for motoring operation was the Cytron dual channel motor driver MDD20A since the maximum current at which the motor should be operated is at 20Amps. Both starting and stopping functions were defined to give as smooth an acceleration/deceleration as possible. Local variables were created to keep track of and continuously update current speed levels in both treads, to ensure smooth braking and accelerating process. HC-05 Bluetooth module was used for serial communication between the mobile application and the mobile robot due to its versatility and ease-of-programming. Motors and motor drivers were powered by two sets of 18V Lithium polymer batteries corresponding to motor specifications with a capacity of 5200mAh which would last for 7.5 minutes of peak operation, which can be extended with replacing with batteries with higher capacity[8]. To prevent over-discharging and ensure the safety of the powertrain, a voltage detection circuit was designed using a voltage divider. When a voltage below safe level was detected, a relay connected to the motor driver was triggered to cut off the power and, hence protecting the battery. Jack operations were achieved by setting the relay pins responsible for power supply to the jack HIGH or LOW using microcontroller and signals from the mobile application.
The code sketches were created to serve the following purposes.
To control the forward, backward and turning motion of the bot itself.
To hoist and lower the vehicle being towed by means of the scissor jack.
To establish contact with the user’s smartphone via Bluetooth and receive directions from the app.
To isolate the motors from the batteries upon detection of over-discharge conditions.
To achieve a change in the direction of movement of the bot, the differential turning mechanism was adopted, which requires the bot to slow to a halt and then perform the turning action when commanded by the user to do so.
After creating the required microcontroller coding sketches, testing of pin logic to establish PWM control of motors was performed. Pulse Width Modulation is a speed control mechanism that varies the duty cycle of a square wave signal to change the “average” voltage experienced by a DC motor. It is crucial to ensure that each signal cycle does not exceed a certain time limit - otherwise the motor’s speed will begin to mimic the rise and drop in voltage experienced, giving rise to unsteady acceleration and deceleration. The testing process confirmed that voltages being delivered to the motors via the drivers were indeed varied in accordance with PWM values reflecting the speeds needed at the output shafts, as expected.
Following this, the required electrical connections were made for the entire power electronics and drive circuit.
H. In-House Control App Development
Flutter has been used to develop the control interface and we made use of open-source libraries to give the needed functionality to the application. The website version of the application was also created if consumers lack the required mobile storage to support the app version. The mobile app was built keeping ease of control in mind. To resolve issues surrounding the connection of the user interface and backend (which has a varying IP address), the backend files were loaded at a particular IP address using XAAMP server.
I. Mechanical Fabrication And Assembly
After the final iteration and verification of the CAD model created, the required components were drafted for laser Cut (2D Draft). Trial assembly of components was performed to maximise ventilation and assess wiring constraints.
Figure 21. Assembled ProTow bot
Following pre-assembly, the plates were welded together using TIG weld for increased strength. The electronic circuit was retrofitted to the body, and a custom protective cover was installed over the entire electronics assembly, to minimise wire exposure to the elements as well as to prevent accidental dislodging.
Figure 20. Welding assembly