AME 3623: Project 3: Analog Processing and Interrupts

In this project, we will sense the orientation of a "weather vane" so that we can drive the robot toward a wind source. We will use a sensor called an encoder to infer the current orientation of the vane. Unlike many encoders, its state is presented as an analog signal (most encoders use a pair of digital signals).

The learning goals for this project include:

• extracting analog data from a sensor,
• performing time-sensitive control and sensor processing,
• implementing interrupt routines, and
• designing and implementing finite state machines.

Project Due Date

• Part 1 (encoder processing): Tuesday, April 11th, 5:00pm
• Part 2 (following the wind): Tuesday, April 18th, 5:00pm

Robot Task

Design Process

Part 1: Encoder Processing

The Encoder

Before reading the sensor state, it is necessary to charge up a capacitor contained within the encoder (this will provide power to the encoder for a brief amount of time while we are reading its state). To take this first step, you must configure the pin as a digital output pin and bring it to +5V. After a delay of at least 3ms, the capacitor is charged. Re-configure the pin so that it is an input pin and set its state to 0V (both steps are necessary). After a delay of 4us, you are free to read the analog value (note that you will need an ADC prescalar of 32 in order to get the timing right).

The sensor will produce analog values of approximately 2.6V, 1.7V, 3.9V, and 4.5V (we will refer to these as integer values 0, 1, 2, and 3, respectively). When the shaft is turned in one direction, you will observe this sequence of voltages; when the shaft is turned in the other direction, the voltages will pass through the opposite sequence. For each complete turn of the shaft, the encoder will pass through this sequence of 4 steps 4 different times (so we have a resolution of 22.5 degrees per step).

Since we will be using this as an orientation sensor (for which it is important to distinguish the 16 possible orientations), it is necessary to keep a count of how many times you move through the sequence of 4 voltages. In particular, you will need to increment a counter every time the sensor passes from 4.5V to 2.6V, and decrement the counter every time it passes from 2.6V to 4.5V.

Important notes to keep in mind as you are designing your FSM:

• Due to the design of the sensor, the 4.5V level may be very short (and can be easily missed). It is therefore necessary to handle the cases where there is a transition from 2.6V to 3.9V (assume that this is "through" 4.5V), and from 3.9V to 2.6V (again, assume that this is through 4.5V).

• When the voltage is read at 2.6V and 3.9V, it is not clear whether the analog value is constant or is in transition to another voltage. In these cases, it will be necessary to verify the constant voltage using two different samples.

For more details about the sensors see (these are good sources of information, but note that they are incorrect in their report of the sequence of voltages):

• Rotation sensor
• Lego sensors (in general)

Coding Steps

This part requires the following:

• Write a function that translates a value read from the A2D converter (so 0 .. 1023) into an integer value representing one of the four expected voltage levels (0, 1, 2, 3). The prototype for the function is as follows:

  inline char decode(uint16_t val)

• Write a function that generates the necessary control signals, extracts a voltage from the sensor, and then returns the value returned by decode()

  inline char read_encoder(void)

• Design the finite state machine that you will use to robustly interpret the decoded values.

• Implement a function that captures one pass through your finite state machine (it performs one encoder read and handles the necessary state transitions and counter increments/decrements).

• Call the above FSM function repeatedly; display the state of your counter using at least 3 LEDs (if you use 3 LEDs, you should cycle through the values 0...7 with exactly one rotation).

• Demonstrate your working program

Part 2: Interrupts and Control

You must complete the following steps:

1. Embed your FSM step function into an interrupt routine (function) and configure the timer so that it is called every ~4ms.

2. Write a main program that first orients the robot toward the wind source and then drives to it. The robot should stop when it observes a beacon to its left.

3. Demonstrate your working program

Hints

• You are responsible for designing and attaching your own vane. Make sure that you attach it such that it does not slip on the shaft.

• You should not need to sample your encoder value more often than once every 4ms.

• The oscilloscopes are your friends. Use them heavily when you are unsure about the timing of events (e.g., you can look at the raw sensor data, or the state of your LEDs).

• See the Atmel HOWTO for details about circuits and programming.

• LED's on extra output pins can be used by your program to communicate debugging information to you (for example, to represent the current state of your code).

What to Hand In

Project Report

• The names of the group members.

• Describe the details of your FSM design.

• A copy of your documented program (this may be handed in as a separate file). The documentation should be such that a person not familiar with your program (but familiar with the control problem) can understand what you are doing.

Personal Reports

• Your name

• An estimate of your contribution to the project in terms of percentage of effort.

• An estimate of the contribution of each of your fellow group members.

• A justification of your contribution assessment.

fagg at ou.edu

Last modified: Sat Apr 8 18:39:21 2006