CS452 - Real-Time Programming - Spring 2015

Lecture 18 - Trains

Public Service Annoucements

  1. Kernel 4 due in class on 18 June.
  2. Exam schedule is out; 19.30, 5 August is the date the registrar gave us. Please bring your exam schedule to class next Monday, 15 June.


Train Properties

Where is a train?

For you project you choose landmarks

You then know when the train is at a given landmark, and find a way -- most likely by integrating velocity -- to know how far it is past the landmark at any given time. If so, you need to know each train's velocity.


1. Calibrating Stopping Distance

The simplest objective:

Sequence of events

  1. Train triggers sensor at t
  2. Application receives report at t 1 = t + Δ 1
  3. You give command at t 2 = t + Δ 1 + Δ 2
  4. Train receives and executes command at t 3 = t + Δ 1 + Δ 2 + Δ 3
  5. Train slows and stops at t 4 = t + Δ 1 + Δ 2 + Δ 3 + Δ 4

Questions you need to answer

Comments

  1. The sequence of events above has a whole lot of small delays that get added together
  2. Knowing where you stop is very important when running the train on routes that require reversing
  3. Clearly, knowing when you stop is equally important.

This is very time-consuming!

Now make a table

Sensor 1 Sensor 2 ...
Speed 6
Speed 8
...

There are enough measurements in each cell of the table that you can estimate the random error. (Check with other groups to make certain that your error is not too big.)

Based on calibrations I have seen in previous terms you will find substantial variation with speed setting and train, little variation with sensor.

Group across cells that have the `same' value. Maybe all have the same value.

Hint. Interacting with other groups is useful to confirm that you are on track. Of course, simply using another group's calibration, with or without saying so, is `academic dishonesty'.

Measuring the time to stop

In addition to the stopping distance you will want to know the time it takes to stop. A simple way to do so is

  1. Start a stopwatch when you give the stop command.
  2. Stop the stopwatch when you see that the train is stopped.

This might not be accurate enough for you. When you have calibrated the velocity and can stop anywhere on the track there's a better way.

  1. Give the stop command so that the train will stop with its pickup on a sensor, recording the time you when you give the command.
  2. When the sensor triggers, check the time.


2. Calibrating Constant Velocity

At this point there are a few places on the track where you can stop with a precision of a train length or better. However, suppose you want to stop not sitting on a switch.

To do this successfully you have to be able to give the stop command anywhere on the track.

Knowing the Current Velocity

An implicit assumption you are making is that the future will closely resemble the past.

  1. You measure the time interval between two adjacent sensor reports.
  2. Knowing the distance between the sensors you calculate the velocity of the train

    Note that on average the lag mentioned above -- waiting for sensor read, time in train controller, time in your system before time stamp -- is unimportant.

  3. After many measurements you build a table

Using Resources Effectively

The most scarce resources

The most plentiful resource

Any time you can use a plentiful resource to eliminate use of a scarce one you have a win. For example

Practical Problems You Have to Solve

  1. The table is too big.
  2. The values you measure vary randomly.

The values you measure vary systematically

3. Calibrating Acceleration and Deceleration

How Long does it Take to Stop?

Try the following exercise.

  1. Choose a sensor.
  2. Put the train on a course that will cross the sensor.
  3. Run the train up to a constant speed.
  4. Give the speed zero command at a location that stops the train with its contact on the sensor
  5. Calculate the time between when you gave the command and when the sensor triggered.
  6. Look for regularities.

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