Software architecture

20 02 2007

After hashing out all the hardware-software intricacies, we’ve constructed a service diagram (a class diagram, essentially) for all the high-level software. MS Robotics Studio lends itself to an object-oriented design where MSRS services are the objects. Though it is guaranteed that each service will contain several classes, the “service view” creates an easily comprehendible starting point.

services1.gif

Starting from the simplest end, each sensor service at the bottom represents a sensor (obviously). These services are intended to relieve the rest of the system from the burden of low-level communication. They handle serial communications, data packet encoding/decoding, timing issues, access issues, and any other low-level tasks that might benefit the rest of the system. For example, the cameras we are using this year are not plug-n-play; they must be accessed through the manufacturer’s SDK. The CAMERA service adds an element of acquisition transparency so all the other services may grab camera data wihout having to mingle with the manufacturer’s code.

Each of the sensor services feed into a cloud called “Sensor Fusion.” In an earlier post, I explained sensor fusion as a way to intelligently combine sensor data into valuable information. The sensor fusion cloud represents the set of services that either extract information from the data of one sensor, or extract information from a combination of data from several sensors. The NavGrid must subscribe to each of these in order to receive the processed information.

NavGrid and Arbiter subscribe to each other so that 1) Arbiter can trigger an event to request specific information from NavGrid, and 2) NavGrid can broadcast general information to subscribers. The intent here is to enable Arbiter to make special information requests while still allowing NavGrid to broadcast its state to subscribers. NavGrid’s state information may contain all events on the absolute navigation grid, or maybe only obstacles and interests in its immediate area– the exact structure has not been decided. As an example of specific information, Arbiter may ask for the interest closest to SubjuGator’s current position.

Arbiter makes all the important decision regarding movement, mission planning and control, and any other task that might require human logic if SubjuGator were manually controlled. The blue square representing the Arbiter displays only a stub for the purpose of this high-level abstract. In actuality, the Arbiter will be composed of several services– I will provide further detail about this in another post.

NavActions subscribes to Arbiter in order to recieve navigation commands. Arbiter will issue navigation commands as changes in state. That is, when Arbiter changes the state of SubjuGator’s current course (40 degrees at a speed of 50, for example) to a new course directive (stopped, facing 35 degrees), the service will broadcast the new course of action to its subscribers. Only the subscribers that can handle such a message will accept the broadcast (so, NavGrid will discard this message). NavActions works with the underlying embedded thurster-driver board to move the sub.

It has not been decided which services will benefit from the Heartbeat service yet (hence the barrage of anonymous subscribers to it), but the idea stands that a heartbeat message would prove useful by acting as a relative clock for interested services and checking system status.

For debugging purposes, the NavGridGUI will be able to subscribe to the NavGrid merely as an observer. This will allow us to see what SubjuGator sees in regards to mission interests (for example, an orange pipeline). Also for debugging purposes, NavActions subscribes (indirectly) to the XBox 360 Controller, which allows the operator to manually control the movement of SubjuGator. The speed/dir mappings service, as you notice, connects NavActions and XBox 360 Controller. This service is necessary to map values fired off by the controller to actual speed and direction values accepted by the underlying embedded thruster-driver. The speed/dir mappings service could have been merged with either of the services adjacent to it, however past experience tells that speed and direction mappings (more so speed) will change often to accomodate different environments. Where in the test pool we want full throttle to be a safe speed that will not throw SubjuGator into a nearby wall, the TRANSDEC is much larger and full throttle should efficiently drive the sub from one end to the other.

 

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