In order to conduct the high-precision rotation control of electric motors having high rotation and high output, an angle sensor is necessary for real-time detection of the rotation angle of the motor rotor. The drive system of a hybrid vehicle adopts a gasoline engine combined with an electric motor, and therefore its angle sensor is exposed to a severe operating environment of high temperature around 150°C and high vibration, while being completely immersed in transmission fluid.
Tamagawa Seiki Co., Ltd. has developed an absolute angle sensor, the “VR-type Resolver” (known as ‘Singlsyn’) that performs with high reliability even under such extreme operating environments. The company has also developed the “Smart Coder” RD converter that accurately converts output signals into digital signals. These devices are being mounted not only in the world’s first hybrid car—the Toyota “Prius”—but also in Ford’s “Escape Hybrid” and Honda’s “Accord Hybrid.”
Tamagawa Seiki Co., Ltd. http://www.tamagawa-seiki.co.jp/japan/index.html
In the 68 years since being established in 1938, Tamagawa Seiki has been engaged in the development and manufacture of devices for measuring “angels.” During the pre-war period, they manufactured oil gauges for military aircraft, and the company’s technology is still being utilized for the angle sensors installed in equipment used for self-defense purposes. Moreover, Japan’s National Astronomical Observatory has adopted the company’s angle sensors at the Nobeyama Radio Observatory and “Subaru Telescope” in Hawaii. In 1997, the company delivered the “Singlsyn” VR-type Resolver for Toyota’s “Prius,” thus demonstrating their high technological capabilities both inside and outside Japan.
The following shows the "VR-type Resolver" angle sensor for motors mounted in hybrid cars.
The VR-type Resolver demonstrates high reliability under a severe operating environment of high temperature, vibration, and immersion in transmission fluid.
A typical electric motor features a structure in which a rotor provided with slits comes in contact with two brushes (electrodes) thereby reversing the polarity of the electromagnets according to rotor movement to generate rotational torque. For an internal combustion engine operating at high rotation and high output as used in motorized vehicles, however, the brushes contacting with the rotor can get burned up due to severe friction, thus requiring the adoption of a brush-less mechanism. A resolver is an angle sensor that detects the rotational angle of a rotor instead of using brushes. Backed by a history of 68 years, Tamagawa Seiki has manufactured resolvers for high-precision angle detection ever since the pre-war years. Yet, it was not until recently that they began doing business with automakers.
“It all began around 1993 when we first received orders from Toyota for resolvers to be used in the motors of an electric vehicle (EV) named RAV4EV, as well as orders received from the Japan Railway Technical Research Institute for a wheel-in-motor (motor integrated with a wheel) system for train cars.”
Mr. Kitazawa was put in charge of developing these new products. EV resolvers were to be subjected to a severe operating environment of over 150°C and high vibration. Conventional resolvers surely could not withstand under such conditions.
First, the development team devised a VR-type resolver that monitors variations in output voltage by using a coil wrapped around the periphery of an oval metal plate attached to the rotor. At this stage, the resolvers for train cars were still only prototypes for experimental purposes and those for the EV were delivered monthly in units of dozens, mostly manufactured by hand. There was an idyllic, relaxed atmosphere surrounding both the development and production teams.
Against this background, one event changed the entire situation. “In 1996, (then) President Okuda of Toyota unexpectedly announced almost out of the blue that, ‘We will begin selling hybrid vehicles one year from now.’ This meant that we had to establish a system for manufacturing two thousand motor resolver units monthly for these hybrid vehicles.”
There is usually a two-year period scheduled for the development and mass production of automobile parts, but the company was given only half the time. The level of performance required was also very high. Since the drive system of hybrid vehicles integrates a gasoline engine and an electric motor, the resolver installed therein is also immersed in transmission fluid. High operational reliability was therefore necessary under such a severe operating environment, not to mention low production cost since the device was to be mass-produced.
The VR-type resolver developed for the EV could possibly be used as a basic design. The problem was providing handcrafted quality for a mass-produced device. In fact, the company had previously only produced small lots of high precision devices, so this “mass production” project was their first experience.
“We had to come up with the specifications in a few months and place an order for molding. One mold order costs 20 million yen, so we couldn’t afford to fail but, at the same time, had to move forward without fear of failure. It was a period of relentless tension.”
The development progressed more smoothly than expected, and the production line began turning out products a few months prior to the planned delivery time. The project would only have a happy ending, however, if the products passed the durability test.
Unfortunately, a major problem was discovered at that stage. The products coming off of the line did not meet the required specifications.
“To identify the cause of this problem, we spent long hours engaged in examining, discussing, and reevaluating things, and sometimes even slept at the factory. We didn’t have enough time left before the deadline, so we were almost ready for the worst possible scenario: apologizing to Toyota and asking for a postponement.”
The angle sensor is positioned as a core part of the drive system. Should delivery be postponed, the release of “Prius” would also have to be postponed, thus posing a serious situation indeed.
“At the last possible minute, we had a breakthrough regarding the problem, and mass production was achieved just in time. Looking back now, we were fortunate in detecting the problem at that stage, since it would have been disastrous if we did so after delivery.”
Initially, overseas manufacturers supplied the RD converter that converts signals from the resolver into angle information, but now the company has already spent several years developing more reliable RD converters in-house. The award was given this time for the combined system of both devices.
At present, in addition to Toyota’s vehicles, this VR-type resolver is mounted in the hybrid cars of Honda and Ford, as well as the electric car developed by Keio University, the “Eliica,” which holds the world record as the fastest electric car. The market for this device is rapidly expanding through its application in electrically driven power steering systems. Electric vehicles or fuel-cell vehicles now being developed for environmental reasons adopt an electric motor as their drive system. This significant development made possible by Mr. Kitazawa and his team members will surely be utilized as a core technology in future manufacturing.
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