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The Behind Story of Force Sensors for SUBARU
LF series

The development story

The Project Started

The Subaru project's success depended on the accuracy of the primary reflecting mirror's actuators, which is where we at Shinko Denshi contributed our advanced force sensor technology. Our encounter with the project came in 1985, when we exhibited the world's first electronic precision balances with tuning-fork sensors in Osaka, Japan. Representatives from a certain company, which was the prime contractor for the Subaru project, expressed great interest in our tuning-fork sensor. In September 1986, the company asked us if we could develop such a sensor according to the following specifications:

Capacity
60kgf
Accuracy (linearity, repeatability)
5kgf
Resolution
1kgf
Temperature drift
1gf/°C both zero point and span
Quantity
400 units

Severe Demands

We completed the No.1 test sample in July 1987 and built it into the actuator, but problems were found in the force transfer system. The following December we supplied twelve units of the No.2 test sample, and the active support system was tested for its ability to control the mirror surface. The results were basically good, but some improvements were needed, such as reduction of temperature drift, increased calculation speed, and lightening the weight of the sensor.

In November 1990, we were suddenly challenged with the following change in the sensor's specifications:

Capacity
150kgf
Linearity and repeatability error for -5°C to 25°C
less than 10gf
Span drift
20gf/year
Sampling period
within 50msec

In September 1991, we modified the No.1 test sample to meet the new specifications, but encountered a problem with a connector. In June of the following year, we supplied five units of the No.2 test sample, a 150kgf sensor with a 16-bit CPU, but we again ran into trouble with temperature drift and repeatability. Our engineers worked round the clock to determine the cause of the trouble, eventually discovering that the sensor's installation support lacked strength.

Eight Years Later

The No.3 test sample underwent testing in March 1993 and attained good results, and in the following September five units of our final product were delivered and given a perfect rating. Starting that month, we mass produced 275 sensors, with our staff working hard day and night to carefully produce and test each unit. We completed the production and delivered the sensors in March 1994, eight years after receiving the contractor's initial inquiry.

The following are the specifications of the actuator:
Rated load
1,500N
Resolution
0.01N
Repeatability
0.01N
Non-linearity
0.01N
Temperature drift
0.1N/30°C
Span drift
0.2N/year
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