In June 2000, Tsukasa Kuroda, a veteran load cell designer, was given a new challenge for sensor development. The challenge was to develop an extremely low cost-weighing sensor that could be used for bathroom scales or consumer scales. The specifications were simple; one part by 3000 or 50gram resolution with 150kg capacity. There was nothing outstanding in the technical specifications, if judged from all the sensors he had developed since he joined A&D over 15 years ago. The challenge this time was very clear to him, which was to develop it at a fraction of the cost of a conventional load cell. This concept was the most important part of the next generation scales to be marketed to consumer mass-market segments. No sooner had the challenge been given to him, than our sales people added one more challenging requirement. It had to be a fraction of the conventional load cell in size and as thin as possible.

Being a measurement instrument manufacturer, A&D is always concerned with measurement accuracy in absolute terms. "Good enough for such and such applications" is not acceptable to us as design criteria. In other words, A&D is a group of critical engineers who pride themselves on the performance and specifications of the products they design. They will go to extremes, if the challenge is to create a product with the highest performance or accuracy, but if they are told to develop a product with low cost being the major theme with "me too" specifications, they can be lost or less motivated. Now and then, when cost becomes the major issue, the idea of us having limitations as a consumer product supplier haunts us, because our background or tradition is so deeply embedded in our engineers’ mentality of everything being absolutely accurate or having stringent specifications. Some of the engineers may scoff and say "We can’t design a people’s car with race car engineers."

We have three types of weighing sensor technologies; capacitance type, strain gauge type and magnetic force motor type. The capacitance sensor technology is the most common in consumer bathroom scale designs, because it offers the lowest cost solution. When there are two plates facing each other, it works as a capacitor that holds an electric charge and the value as a capacitor varies as its distance between the two plates changes. If a change in the distance relates to a change in force, like having a spring between the plates, its change in capacitance relates to the change in the force. Thus it works as a scale. However, at best it offers a resolution of one part by five hundred, as the capacitance is very susceptible to environmental changes.

After numerous arguments, pro and con, for each of the three technologies, we decided not to deploy the obvious low cost capacitance sensor technology solution, as it had limitations in accuracy. Instead we chose the second best in terms of cost. Some people were critical of its choice or a bit cynical saying "Here again we cannot be a consumer products manufacturer." Our engineers chose the strain gauge technology because it offered accuracy and long term stability. They knew they would have no problems in designing a sensor with one part by 3000 or 50gram resolution for bathroom scale applications. The issue was the low cost. On top of the low cost requirement, it had to be small and thin, both of which work against accuracy and stability.

Tsukasa Kuroda had just finished a low profile load cell for industrial applications, and he intuitively knew he could apply the same idea and reduce costs substantially. Thus, he supported the strain gauge technology and ended up having this project himself.

His intuition initially turned out to be a nightmare, because he created so many samples that ended in vain. As people around him saw him get carried away with one idea, they started calling it the K Cell or Kuroda Cell. Photo A shows a conventional single point load cell of 150kg, and Photo B is the Kuroda Cell which later turned out to be the "Smart Sensor" (with patents pending). The K Cell looks very different from the conventional one, not because it works with two strain gauges, but because its unique design is thin and compact. It is one sixth in size and one 36th in weight.

Essentially a strain gauge load cell measures the resistance changes of strain gauges due to the expansion or contraction of the metal element they are glued on. However, the metal element can show the same phenomena of expansion and contraction due to the ambient temperature changes. Another difficulty Kuroda faced is that a change of resistance will not necessarily be linear, but it becomes even less linear if the size of the element gets smaller or thinner. So its technology is simple in principle, but to design one with accuracy and stability takes an expert manufacturer’s know how.

Even in the K Cell, that had to be the lowest in cost, you could find there were some things only A&D engineers would do to assure its performance and accuracy. Look at Photo C. There is a ball bearing between the foot that touches the floor and the sensor element. This structure is fairly common when one wants to design a very accurate floor scale or truck scale. If you recall high school physics about moment of force applied on a lever, a shift of the point of force applied will change the balance or equilibrium point of the system.
Thus, accurate scales must be equipped with some mechanism to make the system immune to side force or the force that tends to shift the point of force applied. This structure naturally works against the low cost requirements but Kuroda could not close his eyes to absolute accuracy or leave room for inaccurate weighing.

The K Cell's internal code disappeared, and the "Smart Sensor" was born. The first product we are bringing to the consumer market with this sensor is a personal health scale, with 150kg capacity by 50g resolution. The price is not the lowest on the market, if you ignore accuracy, but certainly the Kuroda Cell made it possible to bring it to the consumer segment at the lowest price for its performance.
Watch for the debut of the personal health scale in August 2001.

You may address any comments concerning this editorial by email to Mr. Eto