Easy IoT with Sushi Sensors and Load Fluctuation Sensors - Osaka Metro’s pursuit of passenger safety, security, and comfort -

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Executive Summary

Osaka Metro, the nucleus of the Osaka Metro Group, operates nine subway lines and public buses in Osaka City and the surrounding region, and uses the latest technologies to improve safety, security, and comfort for its customers and address environmental concerns. 
To contribute to regional revitalization by integrally providing transportation and social services, the company also makes innovative use of new technologies to offer new services such as an urban mobility as-a-service (MaaS) that involves the operation of on-demand buses.*

For the comfort of its passengers, Osaka Metro has installed huge air ventilators (air intakes and exhausts) to circulate fresh air through its subway stations and tunnels. To test whether Yokogawa’s wireless Sushi Sensors could be used underground in concrete structures to collect data that aids in the detection at an early stage of any abnormalities that could affect the operation of this important equipment, Osaka Metro conducted field tests of these sensors at a number of its subway stations. Based on the findings of these tests, the company plans to install Sushi Sensors on all of its subway lines’ ventilators and air conditioning equipment.

* Shared buses that run AI-determined routes based on customers' requests to pick up passengers at the appropriate location. MaaS is expected not only to improve user convenience, but also to present solutions to issues such as traffic congestion, global warming, and the need to improve transportation access for the elderly. The company started operating on-demand buses as a social experiment in March 2021 and is continuing to expand its area and improve its services.



The Challenges and the Solutions

About the Sushi Sensor

Yokogawa's Sushi Sensor is a small wireless device that measures temperature, pressure, vibration level, and other variables and transmits this data via the LoRaWAN wide-area wireless networking protocol. The Sushi Sensor lineup includes the XS770A, an integrated type wireless vibration sensor that is easily mounted anywhere using a magnet or screw and can be set up and monitored using a smartphone app. Using preset thresholds, an alert can be sent to a supervisor whenever an abnormality is detected. The Sushi Sensor's data can be stored in the cloud or on an on-premises server, and can be monitored using a dashboard screen. By analyzing this data, it is possible to detect and correct equipment anomalies early on.


To learn more about these activities, we interviewed Shigeru Kai, an employee in Osaka Metro’s Transportation Division who has been an advocate for this company’s introduction of the Sushi Sensors. This interview was conducted in April 2022.

-- Did you really first hear about the Sushi Sensor on a TV show?

"Yes, I was watching a TV program about Yokogawa with my son, who works in the oil industry. I used to work for an industrial instrumentation company that used Yokogawa’s products, so I was pretty familiar with Yokogawa. The program included a segment that briefly introduced how the Sushi Sensor can expedite maintenance by detecting abnormal vibration at an early stage. As Osaka Metro is in the process of shifting from time-based maintenance (TBM) to condition-based maintenance (CBM), I shouted out ‘This is it!’"


Sushi Sensors in the showroom at Yokogawa headquarters


-- The part about the Sushi Sensor ran for less than a minute.

"But I was pretty sure. There are hundreds of ventilators in Osaka Metro's subway stations and tunnels. As our first subway line, the Midosuji line, is nearly 90 years old, some of this equipment is quite old and prone to failure. If a serious defect occurs, shutdowns can be quite lengthy and repair costs are very high. If we can quickly spot an impending sign of failure, we can fix the problem before the equipment fails and ensure there is no disruption in service. So my reaction when I saw the Sushi Sensor was ‘This is it!’"



Sushi Sensors mounted on an exhaust fan’s motor (left) and a bearing housing (right)


-- How often is your equipment inspected?

"Depending on the equipment, these inspections are done as often as once a month. Our people touch and listen to the equipment while it is operating and, if they have enough experience, can pick up on slight differences that may point to a problem. But not everyone can do that well, and there is also the issue of declining working population. I could see that it was not really viable to continue to rely on inspection techniques that depended on the use of the five senses. As our veterans rely on their own knowledge of the individual characteristics of each piece of equipment, I knew it would be difficult to pass on such knowledge to others, so I turned to the solution of the Sushi Sensors. By installing the Sushi Sensors, we aim to gradually eliminate the need for monthly inspections."

Sushi Sensors mounted with magnets on a huge exhaust fan

Sushi Sensors mounted with magnets on a huge exhaust fan


-- What were the results of the field tests?

"Field tests were conducted at each subway station to determine the best location for installing the gateway devices and to find out whether the devices could properly receive signals from the ventilators’ Sushi Sensors. Each test took about three days. Equipment in subway stations is often in rooms with thick walls and iron doors, and the walls tend to be constructed with reinforcing bars that interfere with the reception of wireless signals. Yokogawa’s engineers and we had to find the location in each station where reception was strongest. We eventually improvised a technique that involved walking around a station with the gateway mounted atop a stepladder (laugh). Now that we know how to do it, it usually takes no longer than half a day to find the best location in a station for a gateway device."

A gateway device on a stepladder

A gateway device on a stepladder


-- Why did you decide to introduce the Sushi Sensors?

"One of the reasons for switching to condition-based maintenance is cost reduction. So it was important for us to minimize the need for any new wiring and piping. I did a lot of research and compared different vendors’ products, and the Sushi Sensors met all our requirements. As I mentioned, I was familiar with Yokogawa’s products, having used them earlier in my career, so I knew above all that they were reliable. My son, who works at an oil refinery, is also familiar with Yokogawa’s products and has already been using the Sushi Sensors there, so I thought I could not lose out to him (laugh).
It's nice to be able to mount the Sushi Sensor anywhere with a magnet, and for it to just start taking measurements right away. It's very convenient. We did find, however, that people sometimes moved and pointed the devices in different directions, and that this affected their measurements; so we had a Yokogawa engineer make us a lot of stickers with a Tepra label printer and we put them on the sensors to show the right mounting direction."

Tepra sticker on a Sushi Sensor indicating the correct mounting direction

Tepra sticker on a Sushi Sensor indicating the correct mounting direction

System overview

System overview


-- How do you set thresholds?

"We have already set thresholds for several sensors, using a setting that is calculated by adding a certain percentage to the actual vibration level recorded for each piece of equipment. Not all our air conditioners are needed right now, but when the weather warms up we will set the thresholds for that equipment and begin monitoring trend data. We expect to dramatically increase the number of Sushi Sensors at our facilities, and so I think it will be necessary to introduce an AI solution to monitor the vibration readings.”

"As for remote monitoring, we are currently doing that at two offices. Starting this fiscal year, we are installing the Sushi Sensors on important equipment at all major stations, and we are also increasing the number of monitoring PCs. Each subway station has a different layout, so we have to find the right location at each station where the gateway device can be installed. But as I mentioned earlier, we are able to do that pretty quickly. I joked with my colleagues that I think we can see the radio waves!" (laugh)


-- What results did you note with the Sushi Sensors?

"I was impressed to be able to see vibration readings that had been taken over a long period of time. The relationship between operating condition and vibration level is clearly visualized for each piece of equipment. I was proven that I didn't know everything that I thought I understood.”

"These findings will be useful for improving maintenance and inspection activities in the future. Using the data from the Sushi Sensors, we can identify which equipment is likely to malfunction, and then plan maintenance activities such as tightening bolts and replacing parts."

Mr. Kai (Osaka Metro)

Mr. Kai (Osaka Metro)


Utilization of load fluctuation sensor that employs force and tactile sensing technology

Yokogawa has long been researching force and tactile sensing technology that is capable of quantifying the sensation of touching an object whose surface is rough, soft, etc. This technology can be used in plant processes to quantify the condition of various objects. For example, in a chemical production process, it can be used to quantify the viscosity of a chemical substance that is being agitated in a reaction tank. Yokogawa is applying this technology to equipment diagnosis.
For Osaka Metro, a load fluctuation sensor that uses this technology is attached to a ventilator motor to measure and calculate the load fluctuation and thereby determine whether the fan belt is slipping or the fan is imbalanced. Yokogawa has patented this technology.

* Currently, this product is only available in Japan.

Overview of load fluctuation sensor system

Overview of load fluctuation sensor system


-- How did the load fluctuation sensor perform in the PoC?

"If the fan belt that drives the ventilator shaft breaks, it will be a major problem. Broken fan belt would blow the metal housing off at worst. Although a Sushi Sensor can detect abnormalities in fan and motor bearings, the vibration readings alone may not provide indication of a problem such as a slipping belt. I asked Yokogawa if there was a solution that could detect a fan belt failure, and they eventually came back with a proposal to use their load fluctuation sensor. I was so interested in it that I decided to test it. Over a period of two years, we were able to confirm the relationship between the sensor data and conditions such as poorly tensioned and slipping belts, imbalanced fans, over-injection of bearing grease, and fluctuating air flow."

"This is just my opinion, but if we can use the load fluctuation sensor to determine what the proper operating conditions should be, including belt tensioning, we can reduce the number of fan belts that are used. In many cases, maintenance personnel are unsure what is needed and use more fan belts than is necessary. Reducing the number of fan belts can result in energy savings. If a pulley is misaligned, the fan belt can break easily. I believe that the quantification of equipment conditions with a load fluctuation sensor can lead to a variety of improvements."



Customer Satisfaction

-- What are your plans for the future?

"We currently have Sushi Sensors installed on the ventilators at four of our subway stations, and plan to install them on all of the ventilators and air conditioning units throughout our subway network. The ventilators perform essential functions such as exhausting smoke in the unlikely event of a fire. While air conditioning was once considered a non-essential luxury, everyone expects that level of comfort now; with the introduction of the Sushi Sensors and the load fluctuation sensors, we believe that we can now respond quickly to any problems with this important equipment.”

"I think that many other companies that operate infrastructure here in Japan face a similar situation: Like our subway lines, their facilities were built a long time ago. I believe that our efforts with Yokogawa have led to the development of an industry-leading, cutting-edge solution."

"For the Osaka Expo, which will be held in 2025, not only government bodies but also many companies are making preparations to welcome visitors from all over the world. We are working to rapidly improve our services by introducing cutting-edge technologies. Since its founding, Osaka Metro has aimed to be a top technology company and a leader in the transport field."


-- What are your expectations for Yokogawa?

"Yokogawa’s people are kind, and I feel they enjoy their work. I've always got along with them very well. I hope they will continue to work with us to propose solutions to the many difficult challenges that we face. I would also be grateful if Yokogawa could help us in the area of human resources development. It will be important for us to identify personnel who are good at using tools, using their five senses, and working with data and IT. I think that working with external partners like Yokogawa is one way to accomplish this.”

"You can tell how much I like the Sushi Sensors by the fact that I make sure to immediately install the Sushi Sensor app whenever I upgrade my smartphone.”

 Mr. Miyagawa (Yokogawa), Mr. Kai (Osaka Metro), Mr. Nishida (Yokogawa)

(From left)
Mr. Miyagawa (Yokogawa), Mr. Kai (Osaka Metro), Mr. Nishida (Yokogawa)

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