Thursday, March 20, 2014

Study of the Solution

Study of the solution
There are three kinds of odometry solutions to solve this problem, common odometry with wheel's sensors, visual odometry and inertial sensors, has said previously. 
Visual odometry and inertial sensors have great advantages mostly because this sensors are small, compact and can be mounted in a non-invasive way. But this sensors have too much downsides, visual odometry lacks the necessary resolution for high velocity, the robustness to be used in every type of road floor and every luminosity conditions. Inertial sensors also fail by the lack of resolution and accuracy and also by the high cost. As one of the most important goals is to obtain high accuracy this two systems have to be disregarded.
The optical solution, like the ones provided by Kistler [1], is a great solution to measure the velocity, is easy to setup, non invasive and compact. However, there is still the need of an additional device to monitor the steering angle which is a very important information.
The solution to be focused in this project will be a odometry system with wheel's sensors, this is the best solution because the accuracy can be easily increased with higher quality sensors than the existing ones and the cost can still be low. The main downside is the complexity of the apparatus that need to be developed to support the sensors with a non invasive and easy setup.

For wheel velocity sensors there are two main options, the optical sensors and the inductive sensors. In the optical sensors group there are the incremental optical encoders and absolute optical encoders. All of the above are based in pulse counting in which its frequency indicates the velocity of the vehicle, except the resolvers which are inductive analog sensors.

  • Incremental optical encoders:
The main components of the optical encoder's is a disc made of glass or plastic with transparent and opaque areas, a light source and photo detector array that reads the pulses generated by the optical pattern from the disc's position. Increasing the number of pulses increases the resolution. These type of devices are relatively inexpensive and well suited for velocity feedback low to high speed systems. Some encoders have two channels displaced one from the other and by determining which one is the leading channel it is possible to calculate the direction of rotation. The addiction of a channel also has the benefit of increasing the resolution (Figure 1). There are some downsides associated to this sensors, in the event of a power interruption all relative position information is lost, this sensors are also more sensitive to damage by external agents. [2]

Figure 1 - The phase relationship between channel A and B can be used to determine the direction of rotation. With the unique states S1 to S4 it is possible to increase the resolution with a multiplication factor of 4. [3]

  • Absolute optical encoders:
The application of this type of sensor is mostly associated to slower rotational applications where the loss of the position information cannot be tolerated. Absolute encoders produce a unique digital code for each distinct angle of the shaft (figure 2). Each track of the disk codes a bit, increasing the number of tracks increases the resolution and also increases the diameter of the disk and consequently the decrease in shock and vibration tolerance. Absolute encoders are best suited for slow rotating systems such as direction angle. The main downside of this type of sensor is the increasing fragility and cost as the resolution increases. [2]

The line of light passes throgh the coded pattern of the rotot and that corresponds to a unique code that specifies the absolute angular position.

  • Resolvers:
Resolvers are inductive sensors, its stator houses three windings, an exciter winding and two two-phase windings. The rotor has a coil which is the secondary winding, exciting the two two-phase windings on the stator. Because the resolver is an analog device the theoretical resolution is infinite, however, there is an inaccuracy due to variations in the transformation of the voltage. Resolvers have the benefits of being very robust devices widely used in industrial applications, the main drawback of the resolver is the high cost compared to encoders. [4]

Odometry solution
There are several ways and combinations to solve this problem, after some research and thinking, the next topics are the chosen solutions to be discussed. The two two-phase windings generate a sine and a cosine wave. With this information it is possible to calculate the angular displacement of the shaft.
  • One encoder in each of the rear wheels:
with this configuration it is possible to calculate the velocity and orientation with the difference in velocity on the two wheels. This is a simple configuration, with low setup complexity and low cost. Although we can calculate the vehicle orientation angle, this system doesn't provide the front wheel orientation which is a very important information.
  • Encoder on one of the wheels and angle monitoring of the steering wheel:
This solution implies the existence of some device on the inside of the vehicle cabin, it is very hard to install a device on the steering wheel without it being a obstacle to the driver.

  • Encoder and angle monitoring in a single front wheel: 
This is a practical solution because with one single device we can monitor all the wanted magnitudes, this makes it a good solution in terms of compactness and easy to setup. The major downside is the complexity of the apparatus needed to support the sensors to keep with all the front wheel degrees of freedom.

  • Encoder in one of the rear wheels and angle monitoring of one of the front wheels: 
With this configuration we can monitor both wheel's velocity and steering angle. There is the downside of the need of two separate devices, one for the front and one for the rear wheel. However this is a low complexity system to produce and setup.

[1] -
[2] - "Where am I? Sensors and Methods for Mobile Robot Positioning" by J. Borenstein, H. R. Everett and L. Feng
[3] - "Sensors for Mobile Robots: Theory and Application" by H. R. Everett
[4] "The Measurement, Instrumentation and Sensors Handbook" by John G. Webster

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