The latest episode of UbiTennis’ Tennis Data Series is dedicated to the exploration of the products found in the current tennis sensor market, taking into consideration both those applicable on the racket and those that can be worn by athletes. The original article is available at the following link.
Before looking specifically at IMU (Inertial Measurement Unit) sensors, it should be clarified that a sensor is an electronic device used in conjunction with other electronic devices designed to detect changes in an environment, such as light, sound or motion. The sensor then transmits this data to electronic devices to which it is connected. An IMU sensor is a specific type of sensor that measures angular velocity, force, and sometimes magnetic field. The components of the IMU sensors are a 3-axis accelerometer, a 3-axis gyroscope and in this case, we are talking about a 6-axis IMU. An additional 3-axis magnetometer can also be included, so that the IMU sensor configuration is considered to be 9-axis. Technically, the term “IMU” refers precisely to the sensor only, but in reality IMU sensors are often associated with fusion software that combines data from multiple sensors to provide orientation and direction measurements. Therefore, in common usage, the term “IMU” can be used to refer to the combination of the sensor and sensor data fusion software; this combination is also called AHRS (Attitude Heading Reference System).
An IMU sensor can register 2 to 6 degrees of freedom (DOF), meaning degree of freedom the number of different ways an object is able to move in 3D space. The maximum possible is 6 degrees of freedom (DOF), which would include 3 degrees of translational motion (flat) in a straight plane, along each axis (front / rear, right / left, up / down) and 3 degrees of rotational motion through x, y and the z axes / around each axis (See figure above). The raw data collected by an IMU sensor gives an idea of the world around it, but that information can also be further processed to derive other useful insights. Sensor fusion is the mathematical art of combining data from each sensor into an IMU sensor to create a more complete picture of a device’s orientation and direction. For example, while observing gyro sensor information related to rotational motion, other acceleration (speed change) and gravity information can be incorporated from accelerometers to create a frame of reference. The representation of information usually takes place by means of Apps or dashboards, in such a way as to be able to offer a concise but precise overview of the information collected.
In the case of tennis, IMU (Inertial Measurement Unit) sensors can be positioned on the athlete’s body or on sports equipment to record information such as speed, acceleration and angles, with the possibility of being monitored and programmed. Many skills are needed in the game of tennis. In order to track these skills for learning and development, many metrics need to be recorded. Are smart tennis sensors really the way to go? Here we will analyze the main sensors available in the market to collect tennis data.
The first commercial version of sensors applied to tennis dates back to 2013 with Babolat Play. Since then, more and more products have appeared on the market trying to measure every possible metric of a tennis player with the aim of improving his performance. Furthermore, the rise of more competitive brands such as the recent Head Tennis sensor has shown that the market is growing in popularity. It is assumed that in the decade 2020 – 2030 the market will grow further, pushed by the massive diffusion of IoT devices, by the adoption from tennis academies – coaches, by the creation of data exchange communities, implying a subsequent step forward concerning the processing and refinement of information, in such a way as to be able to suggest intelligent strategies during a game. To date, in fact, raw data are collected, at best represented, but without providing any indication at a strategic level, in the context of a real game. To do this it is necessary to process the data through algorithms, which must be interpreted to find valid indications to determine the best strategy against specific rivals.
Later an analysis related to some of these products will be carried out, focusing on three aspects:
– Sensor positioning
– Utility
– Accuracy and comfort.
SENSOR POSITIONING.
Sensors positioned at the lower end of the racket.
When tennis sensors first appeared on the market, some brands designed them to be mounted on the lower end of the racket. The two most popular sensors in this category are Sony and Zepp Labs.
The Sony tennis sensor has been designed to fit rackets with a detachable end cap. A mounting accessory is inserted inside once the end cap is removed and the sensor fits snugly into mounting and secured with a simple twist. The rackets that work with Sony’s tennis sensor are: Head, Yonex, Prince and Wilson.
For a detailed analysis, we invite you to check the test made by UBITENNIS at the time at the following link.
Zepp Labs has designed their sensors to fit any racquet using a semi-permanent rigid (or Pro Mount) attachment locked onto the end cap or a flexible rubber mount that wraps around the end cap.
There have been mixed reactions to the use of a sensor on the bottom of the racquet grip. Some people say they do not notice it, while others argue that they didn’t like it at first because they tended to have a racquet grip close to the bottom of the handle and had to adapt little by little. Then there are those who felt they were pushing the sensor and thought it might fall. This was especially true in the case of the Sony sensor.
Sensors inside the racket handle.
To solve the problem or possible annoyance of having a sensor mounted on the lower end of the racket, two leading tennis racket manufacturers have continued to design and manufacture racquets with sensors incorporated or integrated into the handle of the racket.
Babolat Play is probably the first sensor-integrated tennis racket to hit the market in 2013/14. It is available in 3 models: pure drive, pure drive lite and pure aero. The differences between them are substantially in the design and weight of the rackets and not so much in the sensor’s functionalities.
The test carried out by UBITENNIS in 2015 is available at the following link.
Head Tennis Sensor came into play a little later, exactly in 2018 with a sensor that is not quite built into the rackets. Head has designed a range of racquets with a slot inside the end cap of the grip so that the sensor fits flush, and is practically one with the racquet. From the Head website, it appears there are over 50 racket models available that can accommodate the sensor. Another interesting thing to note is that the Head Tennis sensor is based on Zepp Labs technology as it is possible to appreciate the words “Powered by Zepp”.
In general, apart from the cost of the racquets and the choices limited to some models, the people who tried them seemed quite satisfied with the performance offered.
Sensor mounted on the dampener.
Another location on the racket where the sensors can be found is the shock absorber or vibration damper. The main advantage of these sensors is that it is relatively discreet and it is even more so for athletes who already use a shock absorber during play or training.
Qlipp is one such tennis sensor that works also as a shock absorber. The sensor can be fixed (or Qlipped) on the strings through the patented “twist and lock” design.
Courtmatics is another similar product. The most noticeable difference from Qlipp is in the round shape and the fact that it has no special mounting mechanism as it is attached in the same way as a normal vibration damper, pulling two strings in the middle, so that you can make room for slip the sensor. Furthermore, it is worth highlighting that these sensors have been designed to withstand direct impacts from tennis balls even at high speeds.
Sensors worn by the athlete (wearables)
Athletes who like to change racquets often and do not like to move sensors from one racket to another, or just do not like having other sensors on their rackets, might consider having the sensor on themselves. Assuming the sensor is worn consistently in the same way and mounted in the same part of the body, consistent and comparable data should be obtained.
Babolat Pop is interesting to notice was produced after Babolat Play and launched in 2015. The hypothesis is that Babolat wanted to offer to athletes more tracking options, but basically, it’s a wrist band sensor, except for the sensor unit (or IMU) which is a removable device that can be “plugged” into an adjustable wrist strap. You can find the 2015 Ubitennis article at the following link.
The latest wearable is Pivot Tennis produced by TuringSense. Essentially, it is a set of 5 (or more) motion sensors (IMUs) that can be worn by an athlete while training or playing. The sensors are clipped onto the athlete’s limbs near the joints (e.g. wrist, elbow, knee, ankle) and capture all movements of the athlete. Similar to other wearables, such as Notch or Xsens or Perception Neuron, Pivot Tennis has a motion capture system. The product at the moment is not available for consumers, but only for tennis centers and universities with the scope of research and development.
Utility: what the sensors track.
What sensors can provide is valuable information that can help monitor an athlete’s performance. A series of metrics are provided through sensors and related apps, some of which are very valuable for players to improve their game. Useful metrics include power, spin, impact position (optimal spot), number of hit shots, shot types (forehand, backhand, serve, and smash), spin types (topspin, slice, flat), spin speed, ball speed and live tracking. In any case, this is raw and not very refined data. Many sensors on the market offer additional features, such as live video tracking synchronization (Qlipp) and racket stroke tracking through 3D representation (Head). The following table summarizes all their main features. Finally, it is useful to observe that not all the products examined have been approved by the International Tennis Federation (ITF), benefitting from the status of PAT (Player Analysis Technologies). To consult the list of products approved by the ITF, please click on the following link.
Courtesy from https://sportstechnologyblog.com/2019/03/23/a-review-of-smart-tennis-sensors/
Accuracy
The collection of all this information is undoubtedly phenomenal, although some doubts have arisen, in particular on the accuracy of the information itself. The sensors are accurate in providing raw data such as number of hits, type but the accuracy should be confirmed when looking at the speed of the hit. As far as the latter is concerned, the sensors are able to estimate it, but some sensors have large margins of tolerance, which also lead to larger margins of error.
Comfort
In general, the market has questioned more about the comfort, which depends on the player’s skill level. Some sensors are quite bulky, and can potentially hinder the player’s lateral movements, taking into account a weight of about 8 grams to add to the racket or player. For experienced players, this aspect causes instability and could affect their natural stroke and technique. However, at the intermediate level where a player is constantly learning, this can help improve the game. This is especially true in cases where players do not always train with their coach; the metrics and analysis made by the sensors through the Apps provide a fair feedback so that the coach could remotely monitor the training sessions.
Conclusions
Connected rackets, sensor-equipped baseball bats, smart footballs and connected shoes are just a few examples of how technology is enriching the sports experience, bringing progress and innovation to the field. These so-called “smart” objects will be able to make sense of how professional players, coaches and amateur players understand and interpret the game during training and sport events.
Sports equipment sensors will grow in popularity, as their accuracy is refined. The sensors should be non-invasive and as light as possible. It is worth noting that the metrics may end up being monitored only by the camera in the near future (possibly only on a smartphone). The Internet of Things (IoT) will further boost popularity. The ability to track a wide range of information or and therefore to be able to share it with others is another powerful “driver”, which will favor the spread of sensors for tennis, combinated with the use of playful dynamics (gamification) at the end of the physical game. Will the sensors mounted on the racket be able to communicate with the sensors mounted on the balls? Will the sensors be able to detect the weaknesses of opponents’ shots and angles?
It is believed that to date the current Apps are not able to give indications on the strategy to follow in the framework of a game, against a specific player, as the embedded software collect raw data, and are at most able to show basic statistics, focusing on the visual aspects of the execution of the shots by a single player. Furthermore, a coach normally makes the interpretation of these collected data. In fact, at the moment the existence of an algorithm capable of making for example a predictive analysis based on the combination of different data sources, such as video images, real data of the players, training and other information from external sources, is beyond the functionalities provided by these products. It is rather predictable that the sensors applied on racquets and worn by athletes will have a useful future in preventing any physical injuries by tennis players, as by recording technical and athletic gestures, they will be calibrated on a person.
It was highlighted that the design in wearable sensors plays a decisive role in seducing the potential tennis player, given that in the launch campaigns of these products a large part of the budget is devoted to finding the shapes, designs, colors and materials more seductive and fashionable.
As a recommendation, it would mean that the smart tennis sensors could satisfy beginners and especially intermediate players who wish to improve. Most of the products and features feel more like a “Nice to have” for now and not a “Must have”. However, as the sensor market continues to grow, their popularity and frequency of use may follow, and with more testing on the courts and inputs coming from coaches, future products may become more productive and accurate.
Article and translation by Andrea Canella; edited by Tommaso Villa
