| ??? 12/01/07 07:09 Modified: 12/01/07 07:30 Read: times |
#147680 - If they vary by 10%, ... Responding to: ???'s previous message |
Kai Klaas said:
...then a self calibration hardly makes sense!
Why? Because sensors showing such a huge tolerance very often show hysteresis of same extent. Hysteresis is an error which cannot be taken into account by any calibration. Hysteresis is part of "repeatability error", which no calibration on earth can ever wipe out. I apologize for not providing complete data , here it is: 1) Linearity (Error) < ±5% Line drawn from 0 to 50% force 2) Repeatability < ±2.5% of full scale Conditioned sensor, 80% of full force applied 3)Hysteresis < 4.5 % of full scale Kai Klaas said:
...
So, don't make the mistake to use a cheap pressure sensor and to believe you can make it precise by a smart calibration scheme.If you want highest precision, then look for a pressure sensor which offers lowest repeatability error. Manufacturers selling them often provide a special calibration service by producing an "error matrix" (which is a two dimensionable error table [pressure & temperature]) and "error poylnomial" for each individual pressure sensor. This data is also delivered to the customer, to allow him to perform a "digital compensation". No I am not doing that ,infact my process need not be very accurate (fuzzy logic type of a system /actually a pneumatic Braking system) and cant bear the expances of industry sensors which are too precise and far costly for my application. Actually the pressure vs Resistance o/p curve is with us and they say , it is not for calibration purpose and sensors will vary 10% in worst case senario. Kai Klaas said:
...
Everything to calibrate then is the electronics, unless you have an electronics that is as (or even more) precise as the pressure sensor itself. In earlier days you had analog switches, which first, during a power on for instance, electronically removed the sensor from input, then connected one or several calibrated reference signals to it and finally adjusted the computing section. But, today, we try to omit it, because the analog switches introduce lots of noise and interference to the signal chain. Its clear from your post and other Erik , and steve , that every sensor has to be dealt with indepandantly , I would like to know should the curve fitting be made at multiple points or one reference source of pressure is enough, I plan to omit pots and want to do everything with software. Regards/ AP PS: Erik this time I some how manage to format the answer with quotes , which you guys were objecting every time :) |
| Topic | Author | Date |
| Self calibrating sensor | 01/01/70 00:00 | |
| calibrating without reference/standard? | 01/01/70 00:00 | |
| Ofcourse with reference signal present. | 01/01/70 00:00 | |
| What do You mind? | 01/01/70 00:00 | |
| here is what I need | 01/01/70 00:00 | |
| so go make... | 01/01/70 00:00 | |
| glue a cheap EEPROM to the loadcell | 01/01/70 00:00 | |
| glue a cheap EEPROM | 01/01/70 00:00 | |
| Max 1457 | 01/01/70 00:00 | |
| MAX1457 that makes .. | 01/01/70 00:00 | |
| EV kit | 01/01/70 00:00 | |
| EV kit for part Max1457 | 01/01/70 00:00 | |
| Link | 01/01/70 00:00 | |
Link --- this s/w works with XP | 01/01/70 00:00 | |
| FYI | 01/01/70 00:00 | |
| If they vary by 10%, ... | 01/01/70 00:00 | |
| again and again we see | 01/01/70 00:00 | |
| If they vary by 10%, ... | 01/01/70 00:00 | |
| a miserable sensor, why the requirement | 01/01/70 00:00 | |
| I dont require precision... | 01/01/70 00:00 | |
| Can anyone I dentify its manufacturer | 01/01/70 00:00 |