How to Select a Drivetrain
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. Clawbot drivetrains are 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 forwards and backwards. This type of movement is called omni-directional. Drivetrains may also need to travel over obstacles or need to resist being pushed from the side from another robot. Robots which are being designed for a competition 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? Tracks or larger diameter wheels can help with going over obstacles.
- How much defense will the drivetrain be exposed to? Some games have a barrier which separates the opponents and a defensive drivetrain which can not be easily pushed sideways easily is not as critical.
- 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, by changing the V5 Smart Motor Gear Cartridges, 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 competition rules limit the number of motors on a robot.
These considerations are some examples, but not all, of the types of analysis which should be used when selecting a drivetrain for a competition 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 VEX wheels. However, it lacks the ability to be omni-directional.
The H Drive uses three or five motors with four Omni-Directional Wheels and a fifth 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 H Drive can utilize the 2.75” Onmni-Directions Wheels, the 3.25’’ Omni-Directional Wheels or the 4” Omni-Directional Wheels. However, this type of drivetrain can be pushed sideways by another robot because of the rollers on the Omni-Directional Wheels. The fifth center wheel can also become caught on an obstacle as the robot tries to roll over it.
The Mecanum Drivetrain design utilizes Mecanum Wheels. These wheels have angled rollers which allows them to be omni-directional. When the wheels on this drivetrain rotate opposite to each other, the rollers’ orientation causes the drivetrain to move sideways. However, the angled rollers requires more torque from the motors to drive the wheels and the drivetrain requires a more complex programming code for its motion than the Standard Drive.
The Holonomic Drivetrain is omni-directional. This design can be assembled with either three Omni-Directional Wheels and three motors or four Omni-Directional Wheels and four motors. These Holonomic drivetrains can be designed with either the 2.75” Omni-Directional Wheels, the 3.25’’ Omni-Directional Wheels or the 4” Omni-Directional Wheels. The three Omni-Directional Wheels and three drive motors version is assembled with the wheels set at 120o to each other. 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 and an example is shown below) 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.
The Track Drive is another variation of the Standard Drivetrain uses the Tank Tread Kit instead of wheels. It can easily go over obstacles. However, the Tank Drive lacks the ability to be omni-directional. The standard Tank Tread Kit does not have very good traction. Including some of the Tank Tread Traction Links from the Tank Tread Upgrade Kit in the chain of treads can increase the traction. In addition to the drive sprockets which come with the Tank Tread Kit, the High Strength Sprockets can also be used as drive sprockets.
Some design mistakes to avoid when assembling drivetrains
A design mistake which can be made with the Standard Drive is to power all the wheels with the same ratio and use wheels of different diameters. Due to the circumference difference of the wheels this design error has the larger wheels trying to pull the robot forward faster than the smaller wheels can roll.
A design mistake which can be made with the H Drive is having the fifth center wheel on a different level than the other 4 wheels. This can happen if any of the drivetrain’s drive shafts are not the same distance from the ground as the others. When this design error happens, either the center wheel or the drive wheels lifts the other off the ground.
A design mistake which can be made with the Mecanum drivetrain is not placing the Mecanum Wheels in the correct orientation. When this design error happens the drivetrain will not move sideways.
A design mistake which can be made with the holonomic drivetrains is having only one point of support for the drive shafts. This design error allows the drive shaft to pivot up and down which makes it harder for the drive shaft to rotate within the bearing.
A design mistake which can be made with the track drive is to drive the tank tread with a sprocket in the center of the track. This design error will allow the drive sprocket to skip on the chain links. The drive sprockets should have at least 120 o of tank chain wrap.
Comparison of some types of drivetrains
||Standard Drive||H Drive||Mecanum||Holonomic||Track Drive|
|Minimum Motors Required||2||3||4||3||2|
|Programming Level||Basic to Intermediate||Intermediate||Advanced||Advanced||Basic to Intermediate|
|Avoids getting pushed laterally||Omni - Poor Traction - Very Good||Fair||Excellent||Fair||Very Good|
|Ability to go over an obstacle||Very Good||Poor||Good||Fair||Excellent|
| Safety Hazard:
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.
Structural metal and hardware can be purchased at https://www.vexrobotics.com/vexedr/products/structure.
Wheels and other motion hardware can be purchased at https://www.vexrobotics.com/vexedr/products/motion.