System specificaties

1
System specificaties
2
transfer function
3
sensor specification

• specifications specify errors, shortcomings
• specifying sensor performance is NOT just
• accuracy 0.1
• or
• non linearity 0.5
• or
• response time 2 ms
• always mention test conditions
• be suspicious!

4
error types

• additive errors
• multiplicative errors
• deterministic (systematic) errors
• stochastic (random) errors
• each type should be considered and tackled
  separately because they are of different nature,
  and have different origin

5
system specifications 4 groups

• static characteristics
• describing the performance with respect to very
  slow changes
• dynamic characteristics
• specifying the sensor response to variations in
  time and in the measurand
• environmental characteristics
• relating the sensor performance after or during
  exposure to specified external conditions
  (pressure, temperature, vibration, radiation)
• reliability characteristics
• describing the sensor life expectancy

6
Major sensor specifications

• sensitivity (S dy/dx linearity assumed)
• non-linearity (linearity error specified
  straight line)
• offset (zero error)
• drift (in offset, in sensitivity)
• temperature (temp.coefficient) supply voltage
  aging
• resolution (smallest detectable input change ?
  accuracy!)
• noise (W/Hz V/?Hz A/?Hz) (for white noise)
• response time (specified criteria graphical)
• frequency response (bandwidth graphical)
• accuracy (should include all conditions!)

7
non-linearity (1)

• End-point nonlinearity ?xmax / xmax

8
non-linearity (2)

• Smallest quadratic error
• best-fit nonlinearity

9
CMRR (common mode rejection ratio)

• Verschil-meting
• xv x1 - x2
• Probleem
• xv x1 ? ?? x1? - x2 ??? x2?
• x1- x2 ? ?? x1? ??? x2?
• als x1 ? x2 grote relatieve fout

10
verschil- en somsignalen

• Oplossing verschilversterker
• verschilsignaal vv v1 - v2
• gemeenschappelijk (som) signaal
• vg (v1 v2) /2

11
verschil (differentiaal)versterker

• verschil differential mode (DM)
• gemeenschappelijk common mode (CM)

ideale differentiaalversterker vo A.vd
12
Common mode rejectie

• niet-ideale verschilversterker

met
de common mode rejection ratio (CMRR)
13
Zero error, offset
14
resolutie

• Resolutie (oplossend vermogen)
• kleinst waarneembare verandering
• voorbeelden
• signaal-kwantisatie
• AD-omzetting
• sensor-constructie
• draadgewonden potentiometer
• CCD

15
Resolutie - voorbeeld 1
draadgewonden potentiometer (als positiesensor)
16
Resolutie - voorbeeld 2
spatiele resolutie (CCD-camera)
17
Einde system specificaties