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How reliable are magnetic encoders?

The RM22, RE22, RM36, RE36 are all based on common hall sensor ASICs. The reliability of these sensors has been calculated from accelerated life tests as detailed below.

Installed on small PCBs, 40 sensors were put through a number of stress tests where the temperature, time and voltage were changed. Every chip was tested between each stress test.

Stress test:

  • number of devices = 40
  • total time at stress = 1932 hours
  • failures = 0

Steps of testing procedure:

 

Step

1

2

3

4

5

6

7

8

9

10

Temperature [°C]

125

140

150

160

170

140

150

160

170

160

Supply Voltage [V]

5

5

5

6

6

6

5

5

5

5

Time [hours]

95

21

22

22

114

168

166

164

160

1000


Reliability - statistical calculations

Assumptions:

  • Two most common degrading mechanisms:
    • degradation of oxide layer (Activation energy = 0.5 eV)
    • metalization reduction (Activation energy = 1 eV)
  • Degrading mechanism in ICs is the same during all stress cycles.

 

 

Failures / 105 years

Confidence

60%

90%

Activation Energy

0.5 eV

1 eV

0.5 eV

1 eV

Temperature at stress

85 °C

673

39

1691

100

125 °C

3381

1025

8532

2584

 

MTTF (Mean Time To Failure) years / failure

Confidence

60%

90%

Activation Energy

0.5 eV

1 eV

0.5 eV

1 eV

Temperature at stress

85 °C

149

2564

59

1000

125 °C

30

98

12

39


For example:

At worst case we can estimate with 90% confidence that with an activation energy of 1 eV and a temperature of 125 °C we will potentially get 2,584 failures in 100,000 years. The MTTF (Mean Time To Failure) for the same parameters gives us 1 potential failure in 39 years.

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