TABLE 4.
Comparison between different types of sensors.
| Structure | Test speed | Sensitivity | Feature | Advantage/limitation | Geometry | Reference |
|---|---|---|---|---|---|---|
| Velocity sensors | 10∼32 m/s (in air) | 40 μV/(m/s) between ±10–20 m/s (in air) | Antiparallel cantilever pairs exhibit an axially symmetrical sensitivity, provides a sinusoidal response | Better compensates for temperature changes. Main limiting factor is the signal-to-noise ratio (SNR) | Bent cantilever (1,500 × 100 × 4 μm) | Abels et al. (2016); Abels et al. (2019) |
| Equal to 0.7∼2 m/s (in water) | 80 μV/(m/s) between ±20–32 m/s (in air) | |||||
| Velocity sensors | 0–75 mm/s (in water) | 0.8 mV/(mm/s) (in air at 35 Hz) | Cut-off frequency is 10 Hz and has a flat frequency response | Low threshold detection limit (8.2 μm/s in 35 Hz). Fabrication complexities could reduce the surface flatness | Bent cantilever (1,400 × 1,400 × 2,700 μm) | Asadnia et al. (2015) |
| 22 mV/(mm/s) (in water at 35 Hz) | ||||||
| Velocity sensors | 0–15 mm/s (in water) | 1e-6 s (in water at 1 Hz) | The sensing fusion of SNs and CNs | Ultrasensitive and highly accurate flow-sensing abilities. Sensitivity, noise, durability, stability, and sensor fusion methodology face significant challenges | Bent cantilever (5,000 × 2,000 × 86 μm) | Jiang et al. (2019a); Jiang et al. (2017) |
| Structure | Test force | Sensitivity | Feature | Advantages and disadvantages | Geometry | Reference |
| Skin sensor | 0–50 N | 2.6 kPa/N | Pneumatic drive, adjustable stiffness | Low cost, compact size, and ease of integration in soft robotic systems. Height simulation average error | Total structure (10 mm semisphere) | He et al. (2022) |
| Skin sensor | 0∼1 N | 9.56 kOhm/mm (0.5 mm diameter) | Mechanical diodes | Sensitive, thin, and conductive channels. The gel cannot be subjected to significant shear stress at the channel’s boundaries | Ink line (13.5 mm sphere surface) | Ibarra et al. (2022) |
| Skin sensor | / | / | Movements of different parts can be easily identified | High toughness, compressive modulus, ionic conductivity, and anti-swelling behavior. Excessive HCl led to a stiffening effect | Cut as needed | Ren et al. (2022) |
| Skin sensor | 0–5 N | 10 mV/N | Four flex sensors are embedded in molded silicone | Can effectively detect and estimate forces from multiple directions. The sensor may not work properly when the object is too thin or too sharp | Reference circle (diameter of 20 mm) | Subad et al. (2022) |
| Skin sensor | / | / | Discriminate both the direction and the magnitude of whisker deflection | Design and fabrication method is very simple, low-cost, quick. Stress whitening phenomenon may occur | Filament (initial length of 160 mm, diameter of 8 mm) | Gul et al. (2018) |