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.