A drivetrain allows a robot to be mobile by using wheels, tank treads, or another method. A drivetrain is sometimes referred to as a drive base. Identifying which kind of drivetrain to use is one of the first considerations when designing a robot. The VEX IQ Clawbot drivetrain is fine for starting out, but additional drivetrain designs can allow the robot much more functionality, such as being able to move sideways in addition to turning and moving forward and backward. This type of movement is called omni-directional. Drivetrains may also be required to travel over obstacles. Robots which are being designed to play a game can gain a competitive advantage by selecting a drivetrain to match their game strategy.
Some Things to Consider when Selecting a Drivetrain for a Competition Robot
- Are there obstacles on the playing field which need to be driven over or climbed up on? Tank Treads or larger diameter wheels can help with going over obstacles.
- How much of an advantage is there for the drivetrain to be omni-directional?
- Is the drivetrain going to be pushing multiple/heavy game pieces, or does it need to be fast? The maximum speed or torque produced by a drivetrain can be adjusted by changing to a different gear ratio and/or by changing the diameter of the wheels.
- How high and how far out will the robot design be able to reach? Robots which reach high and/or reach out, benefit from a larger drivetrain footprint and a lower center of gravity. Small diameter wheels can help with both.
- How many motors are going to be needed for functions other than the drivetrain? Some game rules limit the number of motors on a robot.
These considerations are examples of the analysis which should be used when selecting a drivetrain for a classroom game robot or a VEX IQ Challenge robot.
Descriptions of Some Types of Drivetrains
The Standard Drivetrain is also known as a skid steer drive and is one of the most common types of drivetrains. The Standard Drivetrain can be powered by two motors and these motors can be used to directly power the drive wheels or can be part of a gear train which can have multiple drive wheels. The drivetrain can also be designed to have multiple motors and multiple wheels. These variations are sometimes called four-wheel drive, six-wheel drive, etc. This drivetrain can use a variety of the VEX plastic wheels. However, it lacks the ability to be omni-directional.
The H Drive uses three or five motors with four 200mm Travel Omni-Directional Wheels and a fifth 200mm Omni-Directional Wheel set perpendicular between the other wheels of the drivetrain. The arrangement of the wheels enables this drivetrain to be omni-directional. The fifth center wheel can become caught on an obstacle as the robot tries to roll over it.
The Holonomic Drivetrain is omni-directional. This design can be assembled with either three 200mm Travel Omni-Directional Wheels and three Smart Motors or four 200mm Omni-Directional Wheels and four Smart Motors.
The three Omni-Directional Wheels and three drive motors version is assembled with the wheels set at 120 degrees to each other. The Kiwi drive build instructions feature this type of drive.
The four Omni-Directional Wheels and four motors version can be assembled by either angling the wheels at each of the corners (sometimes called an X drive) or placing the drive wheels at the center of each side of the drive base.
These Holonomic Drivetrains require a more complex programming code for their motion than the Standard Drive. The 3 wheel drivetrain is not as stable as the 4 wheel drivetrains.
|Kiwi Drive||X Drive|
The Track Drive is another variation of the Standard Drivetrain which uses the Tank Tread instead of wheels. Tank Tread is included in the Competition Add-On Kit and available with the Tank Tread and Intake Kit. The Track Drive can easily go over obstacles. However, the Track Drive lacks the ability to be omni-directional. Traction links from the Tank Tread and Intake kit can be added to the tank treads to increase the drivetrain’s traction. The tank treads are driven by the VEX plastic Sprockets.
Comparison of Some Types of Drivetrains
|Standard Drive||H Drive||Holonomic||Track Drive|
|Minimum Motors Required||2||3||3||2|
|Wheels||Omni and/or Traction||Omni||Omni||Tank Tread|
|Programming Level||Basic to Intermediate||Intermediate||Advanced||Basic to Intermediate|
|Ability to over an obstacle||Very Good||Poor||Fair||Excellent w/Traction Links|
Slowly move wheels, sprockets, and gears to ensure there are no wires, tubing, elastic materials, or hardware which will be caught by the motion, before powering up the robot.