The PHA Lightweight Harmonic Joint Module is a lightweight harmonic joint series for robot applications sensitive to weight and volume. While maintaining structural strength and output performance, it achieves systematic weight reduction and efficiency gains for the joint module. It suits applications with high load-to-weight ratio demands, such as humanoid robot upper-limb joints, collaborative arms, and mobile platforms.
| Operating Noise | <60dB | Encoder Type | Dual Absolute |
|---|---|---|---|
| Operating Temperature | -20~60℃ | Encoder Accuracy | 16-bit |
| Service Life | >10,000h | Power-off Brake | None |
| Backlash | ≤15 arcsec | Communication Protocol | CANopen |
Using new reducer materials and structural optimization, it achieves significant weight reduction in key parts while maintaining strength and load capacity. The lightweight design improves the robot's motion agility and dynamic response, drives further weight reduction of the whole system, and effectively extends battery life.
Through thermal-simulation-optimized design and a black anodized surface finish, it improves heat radiation efficiency and lowers the steady-state temperature rise by 2°C under the same conditions, protecting performance consistency and service life under sustained heavy load.
Mass power density rises by an average of 25%, delivering stronger output at similar or lighter weight. An in-house skewed-slot high-power-density motor raises the slot fill factor and reduces cogging torque, holding torque ripple below 5% for smoother, quieter operation.
The brake is removed to reduce weight. A power-off brake typically adds 15–30g on smaller models and more on larger ones, plus the brake assembly adds axial length. For applications where the joint does not need to hold position when power is cut — such as upper-limb joints on humanoid robots, lightweight arm segments, or educational robots that operate in controlled environments — the weight savings outweigh the loss of passive holding torque. Applications requiring gravity compensation or position hold at power-off should use the PHU or RHU series with optional brake instead.
The PHA uses 16-bit dual absolute encoders, providing 65,536 counts per revolution on both motor side and output side. The PHU uses 19-bit encoders with 524,288 counts per revolution. For most lightweight robotic arm and upper-limb applications, 16-bit resolution provides sufficient positioning accuracy. The 19-bit resolution on PHU is advantageous for applications demanding the highest precision, such as precision assembly or inspection tasks. Both series support power-off position memory within single-turn range.
The PHA is designed for applications where low joint weight directly improves system performance. Primary use cases include: humanoid robot upper-limb joints (shoulder, elbow, wrist) where lighter arms improve dynamic response and reduce energy consumption; lightweight collaborative robot arms where payload-to-weight ratio matters; educational and research robots where cost and weight are constrained; and mobile service robots where battery life is a priority.
All PHA models use a 101:1 reduction ratio. This simplifies inventory management and control system configuration when deploying multiple PHA joints across a robot. A 101:1 ratio provides a good balance between torque multiplication and output speed for the lightweight application scenarios PHA targets. For applications requiring different ratios (50:1 to 160:1), the PHU or RHU series offers broader ratio options.
In a multi-joint robot arm or humanoid limb, each joint actuator must move not only the payload at the end of the limb, but also every joint and link segment beyond it. A 200g increase in weight at the elbow means the shoulder actuator must work harder during every motion cycle. This cascading effect means that reducing weight at distal joints (those farthest from the base) has a disproportionately large impact on the overall system’s energy consumption, speed, and thermal load.
For a 6-DOF robot arm, a rough rule of thumb: every 100g added at the wrist joint increases the required shoulder torque by approximately 0.5–1.0 N.m depending on arm length. Over thousands of motion cycles per day, this adds up to significantly higher energy consumption and faster motor heating.
A typical integrated harmonic joint actuator consists of:
| Component | Typical Weight Share | PHA Approach |
|---|---|---|
| Motor (stator + rotor) | 30–40% | High-density frameless torque motor, optimized winding |
| Harmonic reducer | 25–35% | Steel flexspline and circular spline (no substitute for strength) |
| Encoder(s) | 3–5% | Dual 16-bit magnetic encoders (compact, no optical components) |
| Driver board | 5–10% | Integrated FOC driver, minimal board footprint |
| Housing | 15–25% | Aluminum alloy, wall thickness optimized via FEA |
| Brake | 5–15% | Eliminated entirely in PHA series |
The harmonic reducer itself (flexspline + circular spline + wave generator) is steel and cannot be meaningfully lightened without compromising gear life. So the main levers for weight reduction are: removing the brake, optimizing housing wall thickness, and using compact encoder and driver designs.
Reflected inertia — the effective inertia that the motor sees — scales with the square of the distance from the rotation axis. A component 0.3m from the joint axis contributes 9 times more reflected inertia than the same component at 0.1m. This is why proximal actuation (mounting heavy motors close to the body or base) is a common design strategy in humanoid robots and lightweight arms.
But in integrated joint actuators, the motor sits at the joint itself. This means the actuator’s own weight is the single largest contributor to reflected inertia at that joint. Reducing actuator weight — as the PHA series does by eliminating the brake and optimizing the housing — directly reduces reflected inertia and improves dynamic responsiveness.
For mobile robots (humanoids, quadrupeds, wheeled service robots), joint actuator weight has a direct relationship with battery life. Heavier joints require higher motor currents during acceleration, and the total energy budget for a motion cycle scales roughly linearly with the mass being moved. In a humanoid robot with 20+ joints performing continuous locomotion, a 10% reduction in total actuator mass can extend operating time by a measurable margin — particularly for battery-powered platforms where every watt-hour counts.
This is the core value proposition of the PHA series: by trading off the brake and using 16-bit instead of 19-bit encoders, the PHA achieves a lighter package that directly translates to longer battery life and faster dynamic response in weight-constrained applications.
Lightweight design involves trade-offs. The PHA series is not the best fit when:
Selecting the right series comes down to identifying which constraint dominates: weight (PHA), versatility (PHU), humanoid-specific wiring and force control (RHU), or high-torque impact resistance (RP/PP planetary).
