Vex Robotics Design System’s Inventor’s Guide
TABLE OF CONTENTS:
CONCEPTS TO UNDERSTAND
MOTION SUBSYSTEM INVENTORY
In the Squarebot design, the Motion Subsystem is tightly integrated with the Structure Subsystem. The two systems are built together to form the “Chassis” of the Squarebot.
Refer to the Squarebot Chassis Parts and Assembly section in the Structure chapter to see how the Motion and Structure subsystems are built in tandem.
CONCEPTS TO UNDERSTAND
Motors and Servomotors
Motors are devices that can transform electrical energy into mechanical energy. That is, they take electrical power, and create physical motion. In the VEX system, they are further divided into two main types: standard mo-tors and servomotors.
The main difference is very clear and straightforward. Standard motors spin the attached axle around and around, while servomotors turn the axle to face a specifi c direction within their range of motion (120 degrees for the Vex servo module).
Note also that the Vex motor modules and Vex servo modules rotate their shafts in opposite directions, given the same transmitter command. This minor difference is due to the internal motor designs of the two different modules. For more information on radio control operation, see the Control Subsystem section of the Inventor’s Guide.
Using Motors and Servos
While similar in appearance, motors and servomotors are suited to distinctly different types of tasks.
Regular motors should be used whenever continuous rotation is needed, such as in a robot’s main drive system.
Servomotors can only be used in cases where the boundaries of motion are well-defi ned, but have the invaluable ability to self-correct to maintain any specifi c position within those boundaries.
Speed vs. Torque
A motor can generate a set amount of power; that is, it can provide a specifi c amount of energy every second?this energy is most commonly used to make a wheel spin. Since there is only so much energy to go around, however, there is an inherent trade-off between Torque–the force with which the motor can turn the wheel–and Speed–the rate at which the motor can turn the wheel.
The exact confi guration of torque and speed is usually set using gears. By putting different combinations of gears between the motor and the wheel, the speed-torque balance will shift.
You can think of gear ratio as a “multiplier” on torque and a “divider” on speed. If you have a gear ratio of 2:1, you have twice as much torque as you would if you had a gear ratio of 1:1, but only half as much speed.
Calculating the gear ratio between a pair of gears is simple. First, identify which gear is the “driving” gear, and which is the “driven” gear. The “driving” gear is the one that is providing force to turn the other one. Often, this gear is attached directly to the motor axle. The other gear, the one that the driving gear is turning, is called the “driven” gear.
To fi nd gear ratio, you just need to count the number of teeth on the “driven” gear, and divide it by the number of teeth on the “driving” gear.
Gears can be inserted between the driving and driven gears. These are called idler gears, and they have no effect on the robot’s gear ratio because their gear ratio contributions always cancel themselves out (because they are a driven gear relative to the fi rst gear, and a driving gear relative to the last gear–you would fi rst multiply by the number of teeth on the idler gear and then divide by the same number, which always cancels out).
However, idler gears do reverse the direction of spin. Normally, the driving gear and the driven gear would turn in opposite directions. Adding an idler gear would make them turn in the same direction. Adding a second idler gear makes them turn in opposite directions again.
Idler gears are typically used either to reverse the direction of spin between two gears, or to transmit force from one gear to another gear far away (by using multiple idler gears to physically bridge the gap).
Compound Gear Ratio
Compound gears are formed when you have more than one gear on the same axle. Compound gears are not to be confused with idler gears, as compound gears can affect the overall gear ratio of a system!
In the compound gear system, there are multiple gear pairs. Each pair has its own gear ratio, but the pairs are con-nected to each other by a shared axle.The resulting compound gear system still has a driving gear and a driven gear, and still has a gear ratio (now called a “compound gear ratio”).The compound gear ratio between the driven and driv-ing gears is then calculated by multiplying the gear ratios of each of the individual gear pairs.Compound gears allow confi gurations with gear ratios that would not normally be achievable with the components available. In the example above, a compound gear ratio of 1:25 was achieved using only 12 and 60-tooth gears. This would give your robot the ability to turn an axle 25 times faster than normal (though it would only turn with 1/25th of the force)!
Gear ratio with non-gear systems
The real nature of gear ratios is a little more complex than just counting teeth on gears. Gear ratio is actually defi ned as the number of rotations that the driving axle needs to make in order to turn the driven axle around once. When dealing with toothed gears, you can find the number of turns needed by counting teeth, as you have already seen above (see “Gear ratio”).
With other types of systems, you can still find the “gear ra-tio” by measuring the number of rotations on the driven and driving axles. Some of these other drive types include belt-and-pulley drives and chain-and-sprocket drives.
Belt or chain drives are often preferred over gears when the motor and the wheel are located far apart on the robot. However, both belts and chains introduce their own special maintenance and performance requirements into the system (chains require lubrication and tension, for instance), and you should carefully weigh their advantages against other design considerations.
Often, the role of the motion subsystem on a robot will be to move the robot along the ground. The last step in the drive train, after the motors and gears, is the wheels.
Like motors and gears, different properties of the wheel will affect your robot’s performance. The size of the wheels will be an important factor here, and will affect two distinct and different characteristics of the robot: its acceleration, and its top speed.
Wheel Sizes and Acceleration
The relationship between wheel size and acceleration is simple: bigger tires give you slower acceleration, while smaller tires give you faster acceleration.
This relationship is the product of the physics of converting the spinning motion of a motor into the forward motion of the vehicle.
Motors generate a “spinning” force (torque), which wheels convert into a “pushing” force at the point where they contact the ground. The larger this “pushing” force is, the faster the robot will accelerate.The relationship between torque and force is:
Force = _ Torque _ Distance from Center
to Edge of Wheel
A longer distance between the center of the wheel and the ground will produce a smaller force for the same amount of torque, hence the larger wheel (which has the longer distance) has a smaller force, and hence the slower acceleration.
Wheel Sizes and Top Speed
At top speed, robots with the same motor and gear ratio will generally travel with the motor running at the fastest speed it can spin. Robots may take some time to reach this speed, especially if they have high gear ratios (high gear ratio = low torque), but eventually, they tend to reach it, or at least come close.
When a wheel rolls along the ground, it is effectively “unrolling” its circumference onto the surface it is traveling on, every time it goes around. Larger wheels have longer circumferences, and therefore “unroll” farther per rotation.
Putting these two observations together, you can see that a robot with larger wheels will have a higher top speed. The robot with larger wheels goes farther with each turn of the wheels, and at top speed, robots with the same motor and gears will have their wheels turning the same number of times per second. Same number of turns times more distance per turn equals more distance, so the robot with larger wheels goes faster.
Friction occurs everywhere two surfaces are in contact with each other. It is most important when considering the wheels for your robot, however, because you will need to decide how much of it you want in order to maximize your robot’s performance.
Wheel friction has both positive and negative consequences for your robot. On the one hand, friction between the wheel and the ground is absolutely essential in getting the robot to accelerate. Without friction, your robot would spin its wheels without going anywhere, like a car stuck on a patch of ice. Friction between the wheels and the ground gives the robot something to “push off” of when accelerating, decelerating, or turning.
On the other hand, wheel friction is also responsible for slowing your robot down once it is moving. A robot running over a sticky surface will go slower than one running over a smooth one, because the friction dissipates some of the robot’s energy.
On more difficult challenge courses, there will often be physical obstacles that must be traversed. Both the size of a tire and the amount of friction it generates will be very important in ensuring that you can successfully navigate them. These obstacles will be numerous and complex, so you will need to plan for them, and test your solutions to make sure that they work reliably.
Every motor in the Vex Starter Kit comes with a pre-attached clutch module. The clutch module’s purpose is to prevent damage to the motor’s internal gearing by temporarily breaking the connection between the motor and its attached wheel or gear whenever there is too much resistance. This prevents the motor from entering the potentially damaging stall (motor can’t turn) or back-driving (motor is being forced backwards) conditions.
The motor clutches are removable for maintenance reasons, but should always be replaced immediately afterwards. Do not attempt to operate the motors without the clutches installed.
MOTION PART FEATURES
The small green tires in the kit are actually two tires in one. By pulling off the rub-bery green tire surface, the grey hubs can be used directly as a set of very small, low-friction tires for your robot.
Non-Axial Mounting Points
In addition to the central hole for the gear shaft, the 60-tooth gear (and 84-tooth gear, available separately) have a number of additional off-center mounting holes.These mounting points have a number of applications. For instance, a larger structure could be built on top of the gear, which would rotate as the gear turned. Alternately, the “orbiting” motion of a non-axial mount can be used to create linear motion from rotational motion.
Gear Wear and Tear
Gears are simple plastic components, but they often bear tremendous amounts of stress in a moving system. The gears inside the motors, in particular, are subjected to large amounts of wear and tear during use in robotics applications where they are frequently required to reverse direction quickly (to make the robot go the other way, for instance).
Inevitably, these gears will wear out and need replacement. The Vex Starter Kit includes replacement gears for the internal motor gears (and you can buy more), so you can perform the necessary repairs when needed.
To replace the gears in a motor or servomotor, follow these instructions.
1. (top of page) Remove the clutch and clutch post.
2. Remove the four screws in the corners of the front of the motor case.
3. Gently lift off the top cover. Try to do so without disturbing the gears inside, so you can see the proper confi guration for later reference.
4. Remove the middle gear and the large shaft gear together.
*Be careful when handling gears, as they are coated with a layer of lubricant that helps them turn smoothly. Wash your hands after handling the gears!
5. Remove the side gear.
6. Remove the thin bottom gear.
7. Open the packaging for the replacement gears. Take special care when handling the replacement gears, as they are very small and slippery (they come pre-greased).
Note: The large black servo motor gear will have a black plastic key underneath the gear’s metal bushing. (Not shown)
8. Install the replacement thin bottom gear.
9. Install the replacement side gear.
10. Install the replacement middle gear and the replacement large gear together, the same way you took them apart.
11. Carefully replace the top cover. Don’t disturb the gears, or the motor may not turn properly.
12. Replace the four corner screws.
13. Replace the clutch and clutch shaft.
How does the Motion Subsystem interact withâ€
â€the Structure Subsystem?
â€ The motion and structure subsystems are tightly integrated in many robots designs, including the Squarebot. The motion subsystem can’t be constructed without certain structural components (like the chassis rails) to provide support and positional reference. By the same token, the structure subsystem must be designed largely to accommodate the motion components.
â€ On Squarebot, the structure and motion subsystems are so interconnected that you cannot build them separately. Hence, they are constructed together in the Chassis building instructions (which can be found in the Structure Subsystem chapter).
â€the Power Subsystem?
â€ The Motion Subsystem’s motors and servomotors convert electrical energy into physical energy, and so they will of course need electrical energy to work with. This energy is ultimately supplied by the Power Subsystem’s batteries, but the motors do not plug into the batteries directly. Rather, the fl ow of power is directed by the Micro Controller, which decides how much power is allowed to fl ow from the Power Subsystem to the Motion components.
â€the Sensor Subsystem?
â€ Robots often have motors and other Motion components controlled by sensors (for instance, the emergency stop function stops the motors when the bumper switch sensor is pushed). However, the Sensor Subsystem does not directly control the Motion Subsystem. Instead, the Sensors provide information to the Micro Controller, which takes that information into account, and then decides what command to send to the Motion Subsystem.
â€the Control Subsystem?
â€ Unlike radio-controlled cars, the Vex robot does not directly tie the Control Subsystem into the Motion Subsystem. The commands generated by the operator using the Transmitter are sent to the RF receiver on the robot, but from there, the commands are given to the Micro Controller, which takes this and other information into account when deciding which command to give to the Motion components.
â€ the Logic Subsystem?
â€ The Motion Subsystem plugs into the Micro Controller, which is the main component in the Logic Subsystem. Though the Motion components are â€controlledâ€ to various degrees by user input (Control Subsystem) and sensor feedback (Sensor Subsystem), the fi nal decision on what command is issued, as well as the actual fl ow of electricity (from the Power Subsystem) is all controlled by the Logic Subsystem. The Logic Subsystem governs everything the Motion components do.
Gear KitThis kit contains additional gears so you will always have the right combination available to ensure you can achieve the desired balance of torque, speed, and direction. All the parts included in this kit were also included in the Starter Kit, with the exception of the 84-tooth gear, which is just like the other gears, except with more teeth, which gives you access to even higher (or lower) gear ratios.
To expand your inventing possibilities, the 60- and 84-toothgears have extra mounting holes. These mounting holes can be used for attaching metal bars or parts to these gears when using them as part of a turntable system, or other similar designs. Consult your Vex Inventor’s Guide for building tips and examples on how to use gears to change the speed/torque balance and direction of your robot’s drive train spin.
Motor and four-wheel drive (4WD) Kit
As explained in the Motion Subsystem section of the Inventor’s Guide, standard motors convert electrical energy into mechanical energy by spinning either forward or backward at a controlled speed.
Now that you have four motors, your robot no longer needs to rely on a single motor to turn both the front and rear wheels on the same side of the robot. You can now use one motor for each wheel, and have twice as much torque available for your robot to tackle increasingly difficult terrain.
The Vex Robot Micro-Controller already has the ability to support this four-wheel drive mode. All you need to do is place a jumper clip on the appropriate Analog/Digital port, and your robot will work with all four motors. See Appendix F in the original Vex Starter Kit Inventor’s Guide for details.
As explained in the Motion Subsystem section of the Inventor’s Guide, servomotors are a type of motor that can be directed to turn to face a specifi c direction, rather than just spin forward or backward.
This kit contains additional wheels that will help your robot get a better grip on any situation. All the parts included in this kit were also included in your Vex Starter Kit, so consult the Vex Inventor’s Guide for building tips and examples on how to use them.
MOTION SUBSYSTEM INVENTORY
MOTION SUBSYSTEM INVENTORY