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    MTM - Mobility / Syspension Design

Red ZoneDrive Assembly
Located within the Red Zone, the Drive Assembly is responsible for supplying drive from the Motors to the wheels. The Drive Assembly is a series of gears and linkages located within the chassis. Stresses introduced through the gear train are dispersed through the chassis avoiding exploding gears and the dreaded ‘clicking’ of slipping gears. While extended gear trains tend to introduces ‘flex’ and ‘slop’ the relative compact volume of the red zone allows for tight and complex gearing configurations to be built.

When considering what type of Drive Assembly to build we need to conceder the following 2 questions

1   What steering implementation we want to use?

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.

Exclusive Drive Assembly
‘Skid Steer’ Drive Assembly for the M.O.P Chassis. Independent motors drive each wheel with an Angle sensor mounted on a single drive axel to monitor rotations.

    The main disadvantage of this type of drive and control implementation in LEGO is that not all motors are the same. Each motor turns at a slightly different rate depending on wear, loading, and available power). Any variation between the two drive motors results in the Rover changing orientation. Accumulative error compounds this occurrence and in prolonged operation the Rover can become horribly disorientated. Methods to compensate this involve mechanically linking motors, Rigging up rotation sensors around drive shafts to measure axle revolutions, and fancy control software to constantly monitor motor output – All somewhat compromising the simplicity of the initial idea.

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.


    Exclusive Skid Steering
    Differential Drive Gearing
Are there any alternatives to independently driving each wheel? How do we get around varying motor performance? Well lucky for us the problem is not that great if we change the relationships between the motors and wheels.

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)

‘Skid Steer’ Drive Assembly for the M.O.P Chassis. Mechanically linked motors drive an Adder / Subtractor gear assembly. Angle sensors are mounted off each drive axel to monitor both axle rotations.


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.


    Differential Skid Steering

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


Both Drive Gearing configurations within the M.O.P (Exclusive and Differential)
use Rotation Sensors to obtain axle rotation information relative to wheel rotations.

The fist column illustrates the drive gearing to the wheels, while the second column shows the gearing to the Rotation Sensor (this is where Rotation Density comes into play). The top row illustrates this relationship to an Exclusive skid steering implementation, and the bottom row to a Differential skid steering implementation.

Coloured gears indicate a change in gear ratio within the drive assembly.


9V Motor

Martyn Boogaarts was asked by LEGO (Netherlands) to a design a Rover for demonstration at the inaugural Dutch FLL challenge.

Over four hours Martyn designed the Modular-All-Terrain-Rover. He says, ” I wanted to show that it is possible to create a robot that can compete in all the tasks and still stays fully within the limitations of the game”

The All-Terrain-Dozer has a common chassis with ‘plug-n-play’ attachments which are specifically suited to each of the mission objectives within the FLL competition.

More images and information about Martyn’s All – Terrain Dozer can be found here.

FLL ‘Mission Mars’ contest information can be found here.


Adder, Subtractor, what the difference?. No, its not a calculation machine as you know it, but it’s more familiar than you would think.

A differential, or Adder / Subtractor gear train gets its name form the fact that it can ‘add’ or ‘subtract’ the output of each drive shaft. Depending on the configuration you are able to negate the product of one output with that of another producing a difference.

Many different Adder / Subtractor and differential gearing combos have been developed and LUGNET Technic’s discussion group has a fabulous resource on the subject found here.

The LEGO Rotation (Angle) Sensor as the name suggests detects rotations. Its body has a hole that easily fits a LEGO axle. When connected to the RCX, this sensor counts a single unit for every 16th of a turn the axle makes, (16 units per full rotation). This gives us an accuracy of 22.5 degrees per unit (16 units making 360 degrees).

By counting rotations, and combining them with Drive Ratio, wheel diameter and track you are able to calculate orientation, drive direction and total distance traveled. In fact all of the telemetry needed for basic navigation can come from a single Rotation sensor.

However the Rotation sensor can become troublesome when used in some applications.

Philippe Hurbain pulled apart his Rotation Sensor to find out why. And Steve Baker determined the best operational speeds at which to poll the sensor,

Unless you want to modify your sensor like Philo, try to stick to axle rotation rates around 50 to 300rpm and the Rotation sensor will not drop any counts. You can drive the sensor directly from the LEGO 9V Geared Technic motor - and you can run it 5x slower than that and still not lose counts. However, don't run 25x slower or 5x faster than the motor or you will lose counts. Rotation axle speeds less than 12 rpm or greater than 1400 rpm are definitely bad news if you need accurate readings


To find the Revolutions per Minute (RPM) of the Rovers wheels (r) we need to know the Following things.

Motor RPM: The RPM of a typical LEGO Geared 9V Technic motor under normal operating load sits at about 220 RPM.

Drive Ratio: The ratio between motor RPM and Drive axle RPM. We can figure this out by calculating the Gearing ratio of the Drive train.

A Geared 9V Technic Motor with an 8t gear is driving a 24t gear (3:1) which is linked (1:1) to a Worm gear which is intern driving the final output axle through a 24t gear (24:1). The gear ratio looks like this:

3:1 x 1:1 x24:1 or 72:1

For every 72 turns of the motor, the drive axle turns once.

Motor Speed = 220 rpm
Drive Ratio = 72

Wheel RPM = 220/72 or 3.05


To calculate the rovers operating speed we only need multiply the Wheels RPM with its diameter.

If our Rover Wheel has a diameter of 82mm then we can work out our Rover’s operating speed.

We can use the following equations:

Gearing Ratio = 72:1
d = 82mm
r = 3.05
PI = 3.142

To find Kilometers per hour
kph = (PIdr)60/1,000,000
kph = (3.142 x 82 x 3.05)60/1,000,000
= 0.047 kilometers per hour

To find meters per minute
mpm = (PIdr) /1,000
mpm = (3.142 x 82 x 3.05)/1,000
= 0.788 meters a minute

To find centimeters per second
cps = (PIdr) /600
cps = (3.142 x 82 x 3.05)/600
= 1.31 centimeters a second


Mechanical accuracy is a measure of the drive train’s ability to supply a sufficient axle rotation ‘density’ to control sensors. Rotation density is the ratio between Rotation Sensor Axle rotation and Wheel Axle Rotation.

To find the Rotation Density of a specific axle within a drive Gearing assembly we need to identify the ratio between Wheel RPM and the RPM of the axle on which the rotation sensor is mounted.

The two illustrated examples (bottom of the page) have the following Rotation Density ratios:

Exclusive Drive Gearing
(Sensor rotations per wheel revolution)
Motor = 72
Rotation Sensor = 216
Wheel = 1

Rotation Density = 216:1
3456 units (216x16) per wheel revolution

Differential Drive Gearing
(Sensor rotations per wheel revolution)
Motor = 288
Rotation Sensor = 864
Wheel = 1

Rotation Density = 864:1
13824 units (864 x 16) per wheel revolution

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