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Building a TITAN

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Post time: 2016-03-10 18:09:16
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TITAN is a much larger version of my Wild Thumper robot chassis which was based on my original 6WD robot chassis. The reason for making TITAN is simple, I want a robot chassis big enough to do jobs like fire fighting, plowing snow, mowing the lawn or carrying heavy garden supplies like a few bags of fertilizer.

To help give you an idea of how much bigger TITAN is, have a look at the difference in wheel size. That's a 13 inch (330mm) wheel from a quad bike. It makes the 5 inch (125mm) Wild Thumper wheel look small. The chunky herring bone pattern of the tread should suit a wide range of off-road terrains.

Difference_in_wheel_size 800.jpg Tread 800.jpg

Although it is probably not essential for a snow plow I think suspension  is essential for an offroad robot as it allows the robot to maintain  maximum traction on uneven ground.

TITAN uses the same "Super Twist" suspension system that is used in the Wild Thumper chassis. The front and rear suspension are "floating" which allows the wheels to rotate ±30° about the pivot point freely but still support weight and absorb shock. This keeps all wheels on the ground even when the terrain is extremely rough.

Titan suspension system 800.jpg

The middle section has a standard independant suspension system. The wheels can still rotate ±30° but it requires much more force. This stabilizes the chassis and stops it leaning to one side or the other.

Titan maximum suspension travel  -30 degrees 800.jpg

Unlike my previous robot designs, TITAN uses brushless motors with planetary gearboxes. The brushless motors are essentially 3 phase stepper motors with hall effect sensors built in. The motor drivers use the feedback from the hall effect sensors to determine when to step the motor. As with stepper motor drivers, the brushless motor drivers use current limiting.

Motor and controller 800.jpg

Normally I like to design my own motor driver circuit but for this chassis we are using 6x ZM-6405E "off the shelve" motor drivers. Each ZM-6405E motor driver is rated for 18V - 40V @ 5A.

To control the speed of all six motors I am going to use a Spider controller. The Spider controller has the ATmega2560 processor. This gives me lots of I/O pins, timers, PWM outputs and analog inputs. Everything I need for controlling a big robot and accesories. If necessary I can add a servo shield for controlling high powered servos.

Red Back Spider2 (800).jpg    HD servo sheild2.jpg

The brushless motors I am using are 24V but the hall effect sensors have 0V - 5V outputs which I can use instead of encoders. This will let me measure distance and precisely control the speed of all 6 motors. As I will be monitoring 3 sensors from each of the 6 motors I will use a timer interrupt and port manipulation instead of external interrupts to read the sensors.

As the robot is quite large I do not want high speed. If it went out of control then it might hurt someone. The motors I am using have been wound especially for high torque at low speed. The motors are only rated for 1000 RPM but their torque is 0.6Nm (6.1 kg.cm).

I chose this motor because it allowed me to use a smaller gearbox. Each planetary gearbox has a 36:1 gear ratio. Top speed is only about 0.5 meters / second. Not excitingly fast but a safe speed for a robot chassis that can weigh more than 50Kg.

Titan top removed no batteries 800.jpg

Two large bays provide enough room for 6x DM12-12 SLA batteries. These 12V, 12AH batteries are commonly used in electric bikes. As the motors are 24V, this gives me a maximum capacity of 36AH. The motors are 60W each (2.5A @ 24V). With all motors under full load the current draw is 15A so the 6 batteries will give a minimum run time of about 2 hours.

The DM12-12 batteries typically weigh about 3.6Kg so the full load of 6 SLA batteries would weigh about 22Kg however they are a common size and you can get lithium versions which are lighter but typically 5-6 times more expensive.

One LiFePO4 DM12-12 battery I looked at was half the weight, same capacity but claims 5x the service life. LiFePO4 batteries do not have the best energy density compared to other lithium battery chemistries but are considered much safer.

Prototype 1 800.jpg    Prototype 2 800.jpg

The prototype was tested using a hobby RC transmitter and receiver. As you can see in the photos the suspension keeps all wheels on the ground for maximum traction on rough terrain.









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