Measuring duty cycle of short duration pulses with long period

[wjsmarine]

Hi All,

I want to measure the Duty Cycle of piezo automotive (in particular diesel) injectors where typical idle on-time is 500us and full load is 1000us. Results will be used in a feedback loop to control other parameters. The clincher is shortest period will be around 20ms or 50hz frequency at maximum revs so the pulses are far between noting only one cylinder's injector needs to be monitored.

With gasoline solenoid injectors this is easy as their on-time is measured in ms so DC math follows the traditional (on-time*100)/off-time.

I can probably rig up some analog stuff to integrate the pulses and then scale the output voltage in A2D measurement but the varying frequency component will have major effects and is totally devoid of digital precision in the acquisition section.

I can't think of any way to measure/calculate DC without first defining what 100% DC is in microseconds - can this be done otherwise in real time? PIC of choice is 16F88 but can be changed if required.

All suggestions appreciated.

Regards,
Bill

[HenrikOlsson]

What sort of resolution do you need and what is the longest period (minimum rpm) you're expecting?

On rising edge of signal, start timer.
On falling edge of signal, read timer, this is the "on-time".
On next rising edge of signal, stop timer, this is the period time. While waiting here you need to keep track of the number of overflows that occurs and add up the total time.

/Henrik.

[wjsmarine]

Hi Henrik,

Thanks for the reply - I knew I could count on you for some helpful advice.

I understand the concept you've mentioned and have this working with good results - if I state 100% DC is 1000us. I'm using a 12F683 with 2 pots to emulate the engine, one for variable frequency (5-55hz = 600-6600rpm) the other for variable pulse width (0-1000us) and a switch for pulse polarity choice. Resolution needs to be 10us (equivalent to 1% DC).

How do I track the overflows? I've not delved into timers before this.

Thanks for your help as always,
Bill

[HenrikOlsson]

Hi Bill,
Your definition of dutycycle differs from mine but I think I understand what you want.
The period (on-time + off-time) of the signal is 18-200ms and the on-time is 500-1000us. You want to measure the dutycycle, ie the ratio between on-time and period.

If the period is 125ms and the on-time is 565us the dutycycle is 4.52%.
If the period is 45ms and the on-time is 795us the dutycycle is 17.66%.

Is this the number you're after?

Let's see if we can make this easy....
The longest time you want to measure is 200ms. Using a 16bit timer with a 4us "tick" you'll measure up to 262ms with 4us resolution without having to worry about overflows. Then the overflow can be used to signal invalid mesasurment (motor not running).

Are we on the right track?

/Henrik.

[HenrikOlsson]

795us over 45ms is 1.76%, not 17.6% as I wrote earlier.

Anyway, here's a piece of code that may do what you want. It uses TMR1 and CCP1 to measure the input signal, calculates the dutycycle and displays it with .1% resolution. It compiles for a 16F88 but I have not tested it.

' Program expects 4MHz oscillator.
' CCP1 inputpin is either RB0 or RB3 depending on the CCPMX bit in CONFIG.

CapRise CON %00000101		' Value to configure CCP1 for capturing every rising edge
CapFall CON %00000100		' Value to configure CCP1 for capturing every falling edge

CCP1IF 	VAR PIR1.2		' Alias to CCP1 Interrupt flag

Period 	VAR WORD
Width 	VAR WORD
Duty 	VAR WORD
Dummy	VAR WORD

T1 	VAR WORD
T2 	VAR WORD
T3 	VAR WORD

T1CON = %00100001		' TMR1 clock from FOsc/4, Prescaler 1:4

Main:
    ' Capture the the time where the pulse starts
    CCP1CON = CapRise
    CCP1IF = 0
    WHILE !CCP1IF : WEND
    T1.HIGHBYTE = CCPR1H : T1.LOWBYTE = CCPR1L

    ' Capture the time where the pulse ends
    CCP1CON = CapFall
    CCP1IF = 0
    WHILE !CCP1IF : WEND
    T2.HIGHBYTE = CCPR1H : T2.LOWBYTE = CCPR1L

    ' Capture the time where the period ends (which is when the next pulse starts) 
    CCP1CON = CapRise
    CCP1IF = 0
    WHILE !CCP1IF : WEND
    T3.HIGHBYTE = CCPR1H : T3.LOWBYTE = CCPR1L


    Width = T2 - T1
    Period = T3 - T1
    Dummy = Width * 1000
    Duty = DIV32 Period

    ' Duty is now in units of .1% (111 = 11.1%)

    LCDOUT $FE, 1, "W:", DEC Width, " P:", DEC Period
    LCDOUT $FE, $C0, "Duty:", DEC Duty/10, ".", DEC Duty//10, "%"

Goto Main

/Henrik.

[NickMu]

I’m not sure how much this will change things but shouldn’t maximum period expected be 20 ms not 200 ms?
And if it is so, should the timer setup be changed or it will still work?

Nick