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

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)	10 Mhz (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).

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 geting 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 powerfull 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 (schemtic 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.

Figure 2 - RPM vs PWM