Smart Seating Solution for the Elderly Individuals
Intellichair is a smart posture and fall-detection chair designed to improve safety, ergonomics, and rehabilitation monitoring.
It uses four load cells (mounted at the chair legs) to continuously monitor weight distribution and balance. By computing both Symmetry Index (SI) and Center of Pressure (CoP) metrics — widely used in biomechanics — Intellichair can:
- Detect bad posture (leaning left, right, forward, or backward).
- Alert when imbalance or tilt exceeds safe limits.
- Recognize chair tip-over or fall events and send distress alerts through IoT/Smart Home systems.
Intellichair combines low-cost hardware (Arduino + load cells) with clinical-grade concepts from biomechanics research, making it useful for:
- 🧓 Elderly fall-prevention
- 💺 Office ergonomics & productivity
- 🏥 Rehabilitation & physical therapy feedback
The chair is equipped with four load cells (Front-Right, Front-Left, Back-Right, Back-Left) mounted at the legs.
These sensors continuously measure weight distribution to monitor posture, balance, tilt, and sitting behavior.
If the chair falls or tilts beyond a safe threshold, the system automatically triggers a distress message through the connected smart home network.
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Arduino Nano 33 BLE Rev2 (microcontroller)
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4 × Full Bridge Load Cells (weight sensors)
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4 × HX711 Amplifier Modules (to read load cell signals)
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Buzzer Module (posture alert system)
The sensor module consists of a load cell paired with an HX711 amplifier.
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Load Cell: A strain gauge-based sensor that measures the force (weight) applied to it. Each corner of the chair has one load cell, allowing the system to calculate weight distribution.
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HX711 Amplifier: A precision 24-bit ADC (Analog-to-Digital Converter) specifically designed for weight scales. It converts the tiny electrical signals from the load cells into readable digital values for the Arduino.
- Can be powered via USB or external protoboard supply.
- Must provide stable 5V to VIN (or regulated 3.3V to 3V3 pin).
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Weight Measurement
- Each load cell outputs a small voltage proportional to applied force.
- HX711 modules amplify and convert this to digital values.
- Arduino reads the data using calibrated scale factors.
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Calibration
- Each load cell is assigned a calibration constant.
- This converts raw readings into grams/kilograms.
- Taring ensures the chair reads 0 when empty.
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Posture Detection
- Total Load = sum of all four load cells.
- Symmetry Index (SI) is computed.
- Shifts in SI indicate leaning forward, backward, or sideways.
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Tilt/Fall Detection:
- If one or more legs suddenly read zero or near-zero load,
- OR if SI shifts beyond a defined safety threshold,
- The system concludes the chair is tipping or has fallen.
- Upon detecting a fall event, Arduino sends a BLE/IoT signal.
- The smart home system immediately relays a distress alert (e.g., SMS, app notification, or emergency call).
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Output
- Data printed to Serial Monitor (or transmitted via BLE).
- Can be used to:
- Detect bad posture.
- Alert user when leaning too much.
- Track sitting patterns over time.
- Elderly users → monitor sitting posture to prevent falls.
- Office workers → track ergonomics and prevent back pain.
- Rehabilitation → provide feedback during physical therapy.
There are two pathways of quantification - symmetry based metrics and Center of Pressure (CoP) based metric.
In biomechanics, symmetry refers to how similar the left and right sides of the body in regard to gait (i.e. during movement like walking, running etc.). Perfect symmetry means both sides behave identically while perfect asymmetry means one side is doing more/less work, which often points to injury, imbalance, or poor posture. There are several mathematical indices to quantify this symmetry [1] :
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Ratio Index (RI): The Ratio Index uses the ratio of the values for the two limbs as the index of symmetry. For a variable X, it is defined as
$RI = (1 - \frac{X_{nd}}{X_{d}}) * 100$ where$X_{nd}$ represents the non dominant limb and$X_{d}$ is the dominant one. RI = 0% indicates full symmetry, while RI > 100% indicates asymmetry. Values can exceed 100% and are not bounded. A negative value is possible if$X_{d} < X_{nd}$ . -
Gait Index (GI): The Gait Asymmetry index is simply a logarithmic transform of the ratio index. It is defined as
$GA = ln(X_{d}/X_{nd}) * 100$ . Again, GA = 0% indicates full symmetry while GA > 100% indicates full asymmetry. Values can exceed 100% and are not bounded. A negative value is possible if$X_d < X_{nd}$ . Note that the GA can only be defined when the ratio between$X_d$ and$X_{nd}$ is positive and hence is not easily used when the variable to be measured can take both positive and negative values (flexion and extension in typical movement assessments, for instance). -
Symmetry Index (SI): The Symmetry Index is the most commonly used index and is a generalization of the RI, where the denominator is taken to be the average of the absolute values for both limbs. There are alternative definitions where either limb is taken as the reference. In such cases, the SI simply reduces to the RI. The standard SI is defined as
$SI = 2 \cdot \frac{X_{d} - X_{nd}}{|X_{d}| + |X_{nd}|}\cdot 100$ . Again, SI = 0% indicates full symmetry, whereas SI > 100% indicates full asymmetry. As in the above, values can exceed 100% and are not bounded. A negative value indicates that$X_{d} < X_{nd}$ . Because of this, in some applications, the absolute value of the SI is reported rather than the SI itself. -
Symmetry Angle (SA): The Symmetry Angle is a fairly newly developed index that captures symmetry in angular data. It is defined as
$SA = \frac{45^{o} - arctan(\frac{X_{nd}}{X_d})}{90^{o}} \cdot 100$ . SA = 0% indicates full symmetry and SA > 100% indicates full asymmetry.
Another mathematical model worth mentioning is the Limb Loading Error (LLE), which provides a loading error individual to each limb [2] :
Limb Loading Error (LLE) in % =
The center of pressure (CoP) is a key concept in biomechanics and postural control studies, and it’s exactly what researchers use the Nintendo Wii Balance Board for. If the subject is sitting or standing on a surface with 4 load sensors, each sensor measures a portion of the subject's total weight. The CoP is the weighted average point of pressure where the net vertical force acts. Therefore, if the subject leans left, the CoP leans left, if the subject leans right, the CoP leans right and if the subject sits perfectly balanced, the CoP stays in the middle.
Based on the schematic image of the Wii board, the following calculations suggest the formulae used for the CoP[3]:
wherein
In terms of the symmetry indices, most of the indices and similar measures we see in biomechanics papers are developed for gait analysis (limb movement while walking/running). They assume alternating steps and cyclical motion. But the concept of symmetry (comparing left vs. right side loads/forces) is general — researchers have also applied the same idea to seated posture, balance on force plates, and pressure-mapping studies. Therefore, it is completely relevant if we adapt these metrics to our chair design.
For the CoP metric, this aligns perfectly with Intellichair because Wii board is basically the same structure we are attempting to replicate, but with a slightly different application and purpose.
If we follow the symmetry pathway, since our Intellichair is intended for old and/or disabled individuals, therefore, the LLE isn't supposed to be a proper metric of measurement, considering the ambiguity with P1. In addition, SA incorporates a complex trigonometry formula which might be unfamiliar to clinicians [2]. The limitation of the RI is its low sensitivity and relatively small asymmetry value [2]. However, although SI gives an inflated asymmetry value, utilizing it is of relevance because of its conventional nature and the popularity.
Overall, CoP is an equally important right choice, in light of the fact that Nintendo has successfully utilized this and thus we have a production ready system.
Both SI (Symmetry Index) and CoP (Center of Pressure) methods face the threshold problem, i.e. to what extend the inclination is allowed.
- SI → Ambiguous because:
- A person with scoliosis or asymmetrical muscle strength may have a "natural" asymmetry.
- What looks like imbalance for one person may be their baseline.
- CoP → Ambiguous because:
- Different body sizes → same CoP shift means different real postures.
- Seat size matters (5 cm shift on a 30 cm seat is big, but on a 60 cm seat it’s small).
- People wiggle naturally — how much deviation is actually “bad”?
Intellichair addresses this via dual metrics + thresholds + hysteresis filtering.
[1]: Queen, Robin, et al. "A novel method for measuring asymmetry in kinematic and kinetic variables: the normalized symmetry index." Journal of biomechanics 99 (2020): 109531.
[2]: Kumar, Senthil NS, et al. "Evaluation of limb load asymmetry using two new mathematical models." Global Journal of Health Science 7.2 (2014): 1.
[3]: Bartlett, Harrison L., Lena H. Ting, and Jeffrey T. Bingham. "Accuracy of force and center of pressure measures of the Wii Balance Board." Gait & posture 39.1 (2014): 224-228.
Designed and Developed by Azizbek Kayumov, 최종우 and Prithwis Das