# Sitting Posture Recognition Systems: Comprehensive Literature Review and Analysis
## 3 Sensing Technologies
### Pressure Sensor
#### 3.1.2 Sensor Arrangements and Placement  
There are mainly two main types of sensor arrangements used for detecting variation in pressure: symmetrical and asymmetrical. An example of symmetrical is sensor arrays, and the asymmetrical approach uses a discrete number of sensors as chosen by the designer.  
The discrete sensors approach is the most popular approach used for posture recognition due to its simplicity, inexpensiveness, easy installation, and flexibility in placement. Different studies have utilized varying numbers of sensors, ranging from a minimal setup with just a few sensors to more complex arrangements with numerous sensors varying from 1 to 10. Most of the researchers used either 4 [58, 60, 71, 78, 79], 6 [62, 75, 80, 81, 82], or 9 [69, 74, 83, 84] sensors. Some researchers, such as [61], used 8 sensors. Another configuration used includes 1 [85], 3 [56], 5 [58], 7 [61], 8 [86], and 13 [73].  
A sensor array is a collection of individual pressure sensors arranged in a grid or matrix pattern, designed to measure the distribution of pressure over a surface. This is often used to detect and analyze how pressure is applied across a given area, providing detailed information about force distribution and contact patterns. A sensor array, comprising multiple sensors distributed across a surface, can provide a holistic view of the sitting posture. It enables capturing three-dimensional spatial data, offering detailed insights into posture dynamics. The most popular sensor array configuration used is 44 \(\times\) 52 [55, 70, 87] and 32 \(\times\) 32. Some researchers also used other configurations, such as 11 \(\times\) 13 [65], 3 \(\times\) 3 [74, 83], and 4 \(\times\) 4 [64]. These arrays provide a detailed pressure mapping offering a comprehensive view of pressure distribution, enabling precise posture recognition and analysis. They are non-intrusive and can easily be embedded within seating surfaces; they do not interfere with user comfort or normal sitting behavior. However, installing and maintaining a sensor array is a complex process, and the cost of deployment is high.  
Sensor placement is crucial in posture recognition using pressure sensors because it directly impacts the accuracy and reliability of the system. Almost all the approaches using pressure sensor arrays use an approach where sensors are placed on the seat except the solution offered by [68], which also placed the sensor array of the same size at the back of the seat. Similarly, in the case of the asymmetrical sensor used, the majority of the researchers placed the sensor in the sensor cushion seat, whereas [61, 62, 80] also placed the sensor on the back of the seat. Ref. [56] placed an ultrasonic sensor at the back of the seat in addition to a pressure sensor array placed on the seat. Refs. [61, 80] also placed a sensor on the arm to get more information. Ref. [88] used a micro-sized LiDAR sensor consisting of an infrared laser and a CMOS image sensor to obtain a 3D point cloud in real-time with x, y, and z coordinate information. It scans the person's sitting posture and represents it in point cloud form while maintaining privacy, ensuring that no identifiable information is captured.