digital conversion of speed sensor analog signal

[hanif]

Hi,

I have to replace an analog display value of speed sensor with a digital one(7 seg display). the output of speed sensor is an analog sine wave signal (400 Hz freq). the amplitude of the signal is varied as the speed changes hence indicating the speed change as follows; max signal amplitude goes up to 13 Vpp.

2 Vpp --> 5m/s
3 Vpp --> 10 m/s
.
.

should i use an ADC directly + mcu based 7 seg display or there is some thing additionally needed because i need to change the display value according to the change in analog signal (because the ADC output continuously changes with the sine wave signal which is not required). I would be highly obliged if some body help me in this matter. waiting for reply

 

[karesz]

Hi hanif,

I`m an "analog type", but I beleave, that you can apply some typical one chip micro-with his own
onchip ADC(PIC, AVR...), & they aren`t expensive.

I know only the controlling of extern ADCs-you have a "Conversion Start" pin/signal, with them you can sample/readout your ADC-values if you wish it in random time...
I think, it must be possible,handling is exactly the same as with onchip ADCs too, if not; you have to apply an storage register and fill it with ADC-data only "time to time", as it is with conversion start signal in the other case.
The ADC output is after all conversion cycling refreshed, but you see the same code if the signal amplitude is stabile/unchanged!
My be you have two timings to apply; the first is your measurement cycle & the second is the readout/display cycling?
In all case you can control your display changes trough your conversionstart signal too....

For information:
Onchip-ADCs have often "special" input range_ for example 0.5--1.2V or so, that have you to drive,
I think, it is often better-easer- to work with a "normal" (external) ADC , but it depends of your specifications...
K.

 

[FvM]

As the analog display surely has, you should have a rectifier (and a voltage divider + low pass filter) in front of the ADC.

By the way. Does the signal actually have a constant frequency? Most speed sensors are simple AC generators
with a speed proportional frequency. A frequency measurement would be the obvious signal processing method in this case.

 

[hanif]

Yes frequency is constant i.e 400Hz sine wave, only the amplitude of the signal is change according to the increase/decrease in boat speed. in fact the speed sensor is an EM Log (electromagnetic Log). the output signal amplitude is proportional to boat speed (in Knots).

I have to replace the old analog display meter with the digital one (7 seg display). this requires A/D conversion , but ADC output continuously get change because of sinewave signal. i need to get the change from ADC when the signal amplitude changes due to change in boat speed.

Please any one Help me to do it. Any help will be appreciated.