ADC

[can]

Dear all,

I'm trying to attach a LVDT that output 0-5vdc signal to the ADC port of a 888. The LVDT is going to measure distance of 80mm within 0.4s. Can 888 give an accurate reading? Or will the readings be delayed and end up LVDT going a distance of more than 80mm?

Regards
Can

[support]

Yes, the ADC function can read the ADC voltage very quickly (< 1ms) so you should be able to get the readings as long as the ADC function is running at the time when the device is in motion.

[can]

Am I right to say that if I use a 0.01sec clock to capture the ADC, I should have an accurate updated reading? Will it be accurate if I use all the ADC with 0.01sec clock?

[support]

If you need to read it very quickly you could use a non-differentiated Custom function driven with a Norm.ON contact so the ADC function is executed at every scan of the ladder logic.  

For really time-critical mission you could use an interrupt input to trigger the custom function that will measure the ADC continuously for a period of time (e.g. during motion). E.g. you could use a For next loop to measure ADC continuously for 10,000 cycles during motion which will ensure that you measure the ADC readings during the whole time when the device is moving.

[can]

Hi. There seems to be another problem. If the ADC sensor needs a power supply of 12V but the ADC supply is only 5V, what can I do about it?

Thank you

Regards

[support]

Give your sensor its own needed power supply.

As long as the sensor analog output is in the same voltage range as the PLC (0 to 5V) you only need to connect the 0V of the sensor's power supply to 0V of the PLC's power supply to tie them to the same reference voltage.

Of course, this is assuming that your sensor's power supply is electrically isolated from the PLC's power supply before connection.