1. Acceleration Based Pedometer
CSE 237B Project
by Vladimir Ermakov

2. Agenda High Level Problem description
Hardware Used/Resources Available
Design
Implementation
Conclusion

3. Problem Description
Accelerometers today do not measure distance accurately.
They measure the number of steps traveled, and multiply it by some constant step length.
This approach is not good for complex exercise sets, and hiking trips where step length varies.

4. Goal of The Project
Come up with a pedometer that does a better distance approximation.

5. Two Approaches
Use instantaneous acceleration of the foot to get instantaneous velocity and calculate exact distance traveled.
Use the acceleration of the foot to figure out the mode/style of walking and provide different step lengths for different modes.

6. Why Approach 1 Failed The accelerometer I had available is too coarse.
Error gets multiplied by t(10ms) added up 100 times a second and multiplied by t again.
This makes the error multiplied times for each second
With a regular pedometer the error is only a couple of centimeters a step.

7. Can We Do Better? Yes, use modes approximation approach.
Have different modes (My implementation has 2 modes) to measure different step lengths.
Mode1 can be Walking
Mode2 can be Running
Use foot acceleration to decide which mode to use.

8. What is the Hardware Available
MMA3201D Accelerometer (+-40g)
Two Micro-controllers:
Atiny26L performs A2D on the accelerometer
AT89C5131(32 KB programmable flash) user interface and algorithms.
Connected through a serial interface(UART)
ES13BA0FR LCD display

9. Design Two parts: Sensor Processing Unit Gets mounted on shoe
Atiny26L performs A2D conversion on Accelerometer output
Sends it to the processing unit upon request
Processing Unit
User interface: LCD, Keypad
Step counting/distance calculation algorithms
Sends requests to the Sensor via UART

10. More Details Pressing GO turns on the processing unit.
After displaying the Splash Screen the system enters MAIN state.
From here one can enter the menu by pressing SELECT or start counting steps by pressing GO.
The menu exposes multiple parameters that control the sensitivity of the pedometer.

11. Parameters Step Length – distance traveled in a step
Threshold – minimum acceleration to register as a step
Step Time – minimum number of measurements to be above threshold before a step is registered
Recoil – duration of a step. A new step will not be registered during this time.

12. Illustration of Parameters

13. States of the System
Stopped
Counting
Paused
Auto-configure

14. What is Auto-configure
Asks the user to walk 10 steps and uses that as a sample of the mode of walking.
Finds average maximum acceleration of the ten steps, and uses that as a guide when deciding what mode of walking one is in.

15. Implementation No OS, just compiler supported interrupt handling
main() with a while(1) loop.
Scheduling is of tasks is performed using timers.
No preemption except for interrupt handling.

16. Timing
An interrupt handler timer0_irq() gets called every 10 milli-seconds.
Its job is to decrement counters and to read in the current state of the keyboard register.
Several important counters:
power_off_timer
sensor_timer

17. Serial Communication The protocol is simple, byte based.
First byte is a start of packet indicator.
Its followed by 6 bytes of data – X, Y, Z
Actually each data parameter is only 10 bits, so 6 bits are wasted per parameter.
Not a bottle neck.

18. pkt_processSensor() Responsible for data processing.
Sensor data has to be centered and normalized
Filter out the noise using the threshold specified by the user (High noise to data ratio)
Check step counter
After step gets registered set the recoil timer.

19. The Menu Menu is implemented as a key driven state machine
Special cases are the Auto-configure, and Counting states where a special key(KEY_NONE) had to be generated in order to fit the model.

20. Real Time Issues LCD updating is done synchronously and is very slow.
First I tried updating the screen every half a second – problems over/under counting steps. (Diagram on next slide)
Solution – to only update the display when a step occurs.

21. Timing Problems

22. Testing
Testing was performed by running/walking around the house and block
Did very well counting steps.
Problems with determining modes, actually it barely works.

23. Reasons for Error
Acceleration does not differ that much between walking and running, so a more accurate accelerometer is needed to perform the job.
Possibly the approach with taking the maximum acceleration can be replaced with taking the average acceleration instead.

24. Conclusion
Instantaneous acceleration approach is not a feasible option.
Modes of walking approach has good potential, and could be made more reliable if more time and hardware resources were available

25. The End
Questions?