AN #138 - RPM meter and rotational speed sensor KMI15/1 with AT90S2313 (ATtiny2313)

This AN show how to adapt or remake old analog RPM gauge in car to another sensor (engine :) ) or so with some theory about.

First some technical info about used sensor and schematics :

Sensor KMI15/1




The KMI15/1 sensor detects rotational speed of ferrous gear wheels and reference marks.The sensor consists of a magnetoresistive sensor element, a signal conditioning integrated circuit in bipolar technology and a magnetized ferrite magnet. The frequency of the digital current output signal is proportional to the rotational speed of a gear wheel.





Sensor have only two pins, one for power supply and second for current output. Output is current and have two states, 7mA for logic 0 and 14mA for logic 1. Power supply is typically 12V (used in cars). Working frequency are up to 25.000Hz.
For using with MCU this output must be converted to classic TTL logic. Conversion provide circuit below.
I1 is current source 7/14mA and is used only for testing, in real operation I1 is replaced with sensor output. R1/C1 convert current to voltage : 7mA = 0.7V and 14mA = 1.4V.
TLC 339 (LM339) is classic voltage comparator what compare input voltage 0.7V/1.4V and make on output of comparator 0 or 5V. Comparator is set to switch voltage on input above cca. 1V to logic 1 and below 1V to logic 0.
Output of comparator goes to circuit with 555. This circuit works as schmitt-trigger and is used to ,,clean,, pulses from comparator.



Schematic : convertor from current output of sensor (7/14mA) to TTL logic (0/5V)



Schematic with MCU is simple and I think not need any comments. RS232 line is used only to debug/setup program but in testing I recommend it.



Note : PWM output from MCU cannot be used to drive any gauges directly ! Between PWM out and your gauge you must use amplifier to get more current and convert impedance. I use NE5532 but can be used any of simple OPAMP like LM138 or so connected as ,,voltage follower,,. (schematic above)



Finished electronics (without follower, follower is placed directly in gauge)

Program for MCU can be divide to some stages :
  • get information about pulses via PulseIN in BASCOM (Figure 1)
  • convert PulseIN value to frequency (Hz)
  • recalculate frequency value to PWM value (Figure 2)
Table in Figure1 show various combination of XTAL and Genus value used in PulseIN value. Genus value you can found in mcs.lib (search for : [_PULSE_IN] ), you can see it here as : @genus(9) ;THIS FOR 10 US UNITS but approx. depends on XTAL.. Genus(9) is default value present in mcs.lib. If you want change it, simple rewrite it in .lib and compile .lib before compiling your program.

Figure 1 - Hz vs PulseIn value


Hz 4Mhz
(genus 9)		 10Mhz
(genus 9)		 4MHz
(genus 2)		 4Mhz
(genus 1)		 4Mhz
(genus 0)		 4Mhz
(genus 20)
10 5128 6172 11844 15489 20135
 2796
50 1025 1242 2368 3097 4026 559
100 512 617 1184 1548 1954 279
150 341 414 789 1032 1342 186
200 256 310 592 774 1006 139
1000 51 61 118 154 195 27
1500 34 41 78 103 133 18




Best combination for my application I found with genus(0) and 4MHz XTAL (10MHz maybe better but in test stage I not has any 10Mhz crystal free :) )
Example of program is here, this program get pulse from PINB.2 and recalculate it to Hz. 

Dim W As Word
Dim Pinhz As Word
Dim Temp As Single
Dim Rpm As Word

Pinhz = 20135

Pulsein W , Pinb , 2 , 1

Temp = Pinhz / W
Temp = Temp * 10
Rpm = Temp / 16.5
Temp = Rpm * 60


When you look in line ,, Rpm = Temp / 16.5 ,, you see constant 16.5, this constat show how many pulses you has for one RPM and need to be changed to yours. Look at table below.
Next important line is ,, Pinhz = 20135 ,, , it constatnt of PulseIN value at 10Hz (table up).

Figure 2 - RPM vs PWM


<
RPM Hz (2 pulses/RPM) Hz (2,5pulses/RPM) Hz big wheel (16,5 pulses/RPM)
100 3,33 4,2 27,5
800 26,67 33,3 220
1000 33,33 41,7 275
1500 50,00 62,5 412,5
2000 66,67 83,3 550
2500 83,33 104,2 687,5
3000 100,00 125,0 825
3500 116,67 145,8 962,5
4000 133,33 166,7 1100
4500 150,00 187,5 1237,5
5000 166,67 208,3 1375
5500 183,33 229,2 1512,5

This table show various RPM values with various numbers of pulses per revolution of motor.
Frequency for your own sensor you can calculate simple : RPM * [pulses per 1 revolution] / 60 , in my case frequency for 100 RPM : 100 * 16,5 / 60 = 27,5 Hz for 100 RPM

Here I want little explain how is measuring RPM in cars.
,,Normal gas,, cars with 4 cylinders and classic ignition generate 4 pulses for 2 revolutions (pulses are taking from mechanical contacts from ignition ,,divider,,). This pulses usually has states ,,grounded,, and ,,free,, because over this contacts is switching ignition coil (free state not must be really free because is switching inductive load !).
In newest cars is usually used other systems to getting RPM (sensor getting pulses from flywheel between engine and gear), this must be discovered yourself :)

My car is 5 cylinder (AUDI 100) and is normal situation I can get pulses from ignition and I can have 5pulses per 2 revolutions of motor - 2.5pulse/1 revolution.
But I am not in normal situation :) My car is DIESEL and as you knows, DIESEL engines not has ignition :) What now ? :)
Normaly DIESEL engines with RPM gauge get pulses from alternator what has special pin for RPM meter, but as I say I am not in normal situation :) and I has bigger and more powerful alternator from gas engine.

Again, what next ? I look over engine and only one reasonable place for getting pulses for RPM meter is on engine head, on camshaft and his metal wheel what is used to drive diesel-pump. This wheel has 33 tooths (ideal for this type of sensor) and camshaft : engine has ratio 1:2 (this means -> engine make 2 revolution and camshaft 1).
From this with simple calculation I get 16.5 pulse for 1 engine revolution. Simple, not ? :)

This stage of AN can be simple adapt to any impulse source(sensor) and recalculate RPM or get only Hz value, but calculating RPM to PWM is a bit harder, ok, move to next figure.

As I say in previous, recalculate frequency to PWM value is not simple, but your Excel can help a lot.
For check parameters of original RPM gauge in car I use MCU (schematic from top + voltage follower) with test program and RS232 line. Simple program in MCU get value for serial line and this value wrote to PWM registers. Program may look like this :

Config Timer1=Pwm,Prescale=64,Compare A Pwm=Clear Down,Pwm=10

Dim A as word

Do

Input "PWM=" , A
Pwm1a = A

Loop

You can wrote random values in to terminal as you want until you found value of PWM what is associated to specific RPM on gauge and wrote it in to Excel table what can look as table below.
After, you can generate your graph with value what you measure. In my graph measured values are showed by red line and you can see ,,linearity,, of your gauge or PWM. After this, use Excel function named ,,Trend line,, what generate ,,ideal,, values based on your data, this line is showed on graph by blue line. Trend line function is nice because after generate ,,ideal,, value it can show mathematic formula (in graph y = PWM value while x = RPM).
Program for calculate PWM from RPM look like this :

Dim Temp As Single
Dim Rpm As Word

Temp = 258.38 * Log(temp)
Temp = Temp - 1437.2

Rpm = Temp
Pwm1a = Rpm


Input value Temp has your actual RPM and on output, Temp has calculated PWM by math formula generated by Trend line.
Important line in program is ,, RPM=Temp,, , RPM is Word while Temp is Single. This line make only conversion from Single to Word because PWM register has problem with Single values, without this line your program can work but some calculated values is not acceptable for PWM register (some values works, some not).
As you can see on graph , some calculated values are little different as measured but error in calculation is very acceptable because we use Analog gauge and error is not critical.

RPM PWM Trend Line
600 230 215,64
800 290 289,97
1000 330 347,63
1200 370 394,73
1400 420 434,56
1600 460 469,07
1800 490 499,50
2000 530 526,72
2200 550 551,35
2400 580 573,83
2600 610 594,51
2800 630 613,66
3000 660 631,49
3200 670 648,16
3400 685 663,82
3600 695 678,59
3800 705 692,56
4000 715 705,82
4200 720 718,42
4400 730 730,44
4600 735 741,93
4800 740 752,92
5000 750 763,47
5200 755 773,61
5400 765 783,36
5600 775 792,75

Complete program what I used in MCU (2313) is here (simple, no ? :) ) :

Note : this program calculate with recompiled mcs.lib with Genus(0) !

Config Timer1=Pwm,Prescale=64,Compare A Pwm=Clear Down,Pwm=10

Dim W As Word
Dim Pinhz As Word
Dim Temp As Single
Dim Rpm As Word

Pinhz = 20135
Pwm1a = 0

Do

Pulsein W , Pinb , 2 , 1

Temp = Pinhz / W
Temp = Temp * 10
Rpm = Temp / 16.5
Temp = Rpm * 60

Temp = 258.38 * Log(temp)
Temp = Temp - 1437.2

Rpm = Temp
Pwm1a = Rpm

Loop

Download source code and files in an118.zip