Lunar Rover Design Challenges - Sensors & Communications
Sensors - Lunar Rover Design Challenges:
We have many options for sensors on our rover. Part of the Google Lunar X Prize mission objectives is to take still pictures and video from the moon. Therefore, we have a camera that is capable of taking high definition video and pictures and sending them back to Earth. Additional cameras could be useful for backup purposes and for seeing in multiple directions simultaneously.
Current sensors can be used to detect the amount of electrical power being drawn by our electronics. If the rover somehow got into a critically low power state we could turn off non-crucial systems that are drawing a lot of power.
An accelerometer could tell us when the rover is moving. As the name implies it measures the amount of acceleration that the rover is experiencing when it moves. Accelerometers are commonly used in cell phones in order to detect when the user switches between a portrait or landscape display mode.
A wheel or rotary encoder can tell us when our wheels are moving and optionally how far the wheel has turned. The odometer in a car is a type of wheel encoder used to measure how far the car has driven.
By themselves an accelerometer and wheel encoder are limited in their usefulness. However when combined they can help us to detect if we are stuck in a rut and if a wheel is slipping. If the wheel encoders say the wheels are spinning forward and the accelerometer says we have some forward acceleration, then we are most likely moving forward. If the wheel encoders say the wheels are spinning forward but the accelerometer says there is no acceleration then we are stuck in a rut and spinning in place. Conversely if the encoders report that the wheels are not moving but we have acceleration then the rover is falling.
A current sensor can also be used in this scenario in place of the wheel encoder. If we can detect a change in the electrical current flow then the wheels might be spinning. This method is not as accurate as the wheel encoder because motors will also use electrical current to “apply the brakes” to keep the rover from rolling backwards downhill. However it gives us more options to help reduce our weight and power consumption. The data from these types of sensors could be very useful if we have to turn off our cameras due to lack of power or damage to keep us from driving completely blind.
Gyroscopes can detect the rover’s orientation. A gyroscope could tell us when the rover is traveling on a dangerous slope and is in danger of tipping over. A gyroscope is what keeps a Segway from falling over by measuring and counteracting how far the driver is leaning.
Just like the wheel encoders and accelerometer could give us more information when combined than alone, accelerometers and gyroscopes combined can give us more information than when used alone. Accelerometers and gyroscopes combined together are called an Inertial Measurement Unit (commonly known as an IMU). Such a device can be used to measure the rover’s orientation, position, and velocity.
A voltage sensor measures the electrical voltage between two points. These sensors are very useful for monitoring the charge on our battery. If it starts too get low then we need to turn off some systems and recharge with the solar panels. Cell phones usually have a voltage sensor in them. This sensor is what tells you that your cell phone battery is at 12% or 1 out of 4 power bars and needs to be recharged.
This is just a sample of the different kinds of sensors we could place on the rover. Each type of sensor has its pros and cons. Each adds weight and consumes part of our electrical power. Our rover builders will have to experiment and figure out what kind and how many sensors we need in order to safely and reliably navigate on the moon.
Communications - Lunar Rover Design Challenges:
Our communications system is another very important system that affects nearly all of the others. The onboard computer will collect sensor readings and camera footage and transmit all that data back to Earth. Since we cannot buy a really long telephone line between the moon and Earth we will have to wirelessly transmit our data back to Earth. Any type of communication system (wired or wireless) can only transmit so much data at a time. Here we run into a few more engineering trade offs. We can have a large or small bandwidth size (how much data per transmission) and we can have a fast or slow data rate (speed of transmission). Each of these options requires certain types of antennas, size and weight constraints, computing requirements, and electrical power demands. We will have to balance all of these factors in order to be efficient and reliable.
If we have a slow transmission rate then we will have to move the rover very slowly otherwise we risk driving while blind. The benefit is that slow transmission systems typically have a better guarantee of actually getting the data to its destination than faster systems. If we have a fast transmission rate then we could drive the rover from Earth almost entirely like a toy RC car.
Rover Systems Integrator