How fast and responsive do we want our Rover to be?

Steering Implementation - Exclusive vs. Differential
While we will cover steering method on the next page, the configuration of the Drive Assembly allows for two fundamentally different steering implemetations of the same steering method.

Typically we need two motors for controlled movement. In controlling our Rover we want to be able to move forward, reverse, left and right. Ideally we also need to keep track of wheel revolutions for distance, and orientation calculations. We can implement this in two ways

Exclusive Drive Gearing
The simplest form of drive control is to have a single motor independently connected to each wheel. When both motors are moving forward – The Rover moves forward. When the motors operate in opposite directions the Rover turns. Gearing relationships within the drive train determine speed and accuracy of these movements.

This type of Steering is often implemented on tracked or heavy vehicles (such as earth moving, construction and military vehicles) and is commonly referred to as ‘Skid Steering’. When the vehicle turns, one side ‘skids’ while the other moves. Many real life Rovers, Robots and human operated machines use this steering method. While not very elegant, it is an extremely simple concept which is easy to implement and control.

It must be noted that in real life we are able to control actuators a lot more accurately than we are able to do so with LEGO motors, but non the less varying motor performance is an issue.

In the Skid Steer example mentioned previously, the output of the two drive motors were independent, or exclusive of each other (Motor output 1 has no effect on Motor output 2). If we change this relationship to being inclusive (both motor outputs linked) we get a much more usable product. Motors linked via an ‘differential’ gear linkage can implement ‘skid steering’ with much more control and reliability.

What this means in practical terms is that rather than have one motor drive one wheel independently of the other (forward / reverse movement is achieved by both motors running in parallel) , we use one motor to drive both wheels to produce forward or reverse movement, and the other motor drive both wheels to produce left / right movement. (Motors can be thought of as acting in series with each other)

While making the gear train more complex it negates any issues of motor variance and produces very reliable and consistent performance. Again specific gearing on each motor can control the speed, or accuracy of either forward/Reverse, or left/right movement. Rotation sensors built into the Drive Gearing allow us to monitor either motor or drive axle rotations.

Speed and Mechanical Accuracy
As we know the M.O.P chassis is capable of supporting many different gearing configurations within its drive assembly. The three physical attributes of the M.O.P which determine operational speed are, Drive Gearing ratio, motor speed (RPM), and wheel size (Diameter). By modifying any, or all three of these has an effect on the Rover’s overall operational speed. The real art is knowing which attribute to change, and what impact this will have on the other two. The M.O.P chassis allows you to experiment with all three without having to re-engineer the entire Rover.

Operating a slow moving Rover is a piece of cake. It takes a while to get where it is going allowing you (the operator, or it’s own electronic brain) time to adjust, and compensate to keep it on track.. However as the Rover’s operating speed increases your available reaction time decreases. You have to make decisions and adjustments quicker, and more accurately because your Rover is moving so much faster. Operational tolerances have to be drastically reduced as speed increases.

In order to control Rover movement and orientation remotely we need the Rover’s drive assembly to deliver us information about motor and wheel rotation and position – This is often referred to as the drive assembly’s ‘Mechanical Accuracy’ or ‘Rotation density’. The greater the operational speed of the Rover, the greater Mechanical Accuracy required of it Drive assembly. This allows for more accurate (and predictable) navigation algorithms to be implemented while minimizing accumulative error associated with this type of navigation processing.

(We are talking mechanical control characteristics as apposed to software controlled characteristics. How the drive assembly physically integrates with control and navigation sensors).

The key to determining how much Mechanical accuracy your drive assembly needs is to look at the relationship between Motor, drive gearing, Wheel diameter, and sensor revolutions.

Consider a motor with a 24:1 gear ratio driving an 82mm diameter wheel. The motor will turn 24 times for each wheel revolution, moving the Rover 82mm. If you attach a Rotation sensor to the wheel axle you have a rotation density of 16:1 (16x1) units per wheel revolution. Physically this translates to 5.125mm of movement to 1 rotation unit. However if you attach the Rotation sensor to the Motor’s axle (before the gearing reduction) you get a 2400% increase in rotation density to 384:1 (16x24) per wheel revolution. In real terms this yields an increase in control accuracy from 5.125mm (82/16) to 0.213mm per rotation unit.

The point to remember here is that by altering the gear ratio between the motor and Rotation sensor we can dramatically increase the relative accuracy of the Rotation sensors output – or increase the Mechanical Accuracy of the drive assembly without affecting the gearing relationship between the motor and wheels.

Both Drive Gearing configurations within the M.O.P (Exclusive and Differential) use LEGO Rotation Sensors to obtain axle rotation information relative to wheel rotations. Both Drive assembly configurations have a high Mechanical Accuracy. In both instances the Rotation sensor is geared 3:1 in relation to the Motor. However the Exclusive Drive configuration has a Sensor to Wheel revolution ratio of 216:1 and in the case of the Differential Drive configuration a whopping 864:1 !!! Talk about splitting hairs