At Source Robotics, we create robots that are safe, high-performing, and reliable. A critical part of achieving this is designing advanced motor drivers for BLDC and stepper motors that power these robots. As part of this process, selecting the right motors and gear reduction systems is essential.
One of the most popular gearbox types used in collaborative robots (cobots) and robotic arms is the harmonic gearbox. A common question about these systems is: "Are harmonic gearboxes backdrivable?"
Understanding Backdrivability
Backdrivability refers to the ability of a gear system to transmit motion from the output shaft back to the input shaft.
Example:
Imagine a robotic arm with a gear system. If you push the arm (output shaft), a backdrivable system would allow the motor (input shaft) to spin freely, reflecting the force back to the motor. In contrast, a non-backdrivable gear system resists this motion, holding its position rigidly unless actively driven by the motor.
Are Harmonic Drives Backdrivable?
The short answer is both yes and no.
Backdrivability in gear reduction systems depends largely on the gear ratio. Systems with a low gear ratio tend to be more backdrivable, while those with a high gear ratio are generally less so.
For example, a 1:1 gear ratio functions almost as a direct drive, making it easily backdrivable. This setup provides benefits such as energy efficiency and compliance, and torque transparancy. On the other hand, a 10,000:1 gear ratio is virtually impossible to backdrive, but it offers significant advantages in torque multiplication and positional stability.
With such a high gear ratio, even using a large lever to gain mechanical advantage wouldn’t allow you to backdrive the system. Instead, you’d likely end up damaging the gearbox or the lever itself. This reflects the fundamental tradeoff: low gear ratios favor backdrivability and responsiveness, while high gear ratios prioritize torque, stability, and precision.
It is one big tradeoff.
Why the Mixed Response?
The mixed "yes" and "no" answers to this question often arise because backdrivability depends heavily on context. While it’s technically possible to backdrive harmonic drives—particularly if they have a low gear ratio (The nema 17 format ones you often see on the internet are usually 30:1 and highly backdriveable)
When dealing with harmonic drives with high gear ratios (e.g., 100:1) or large impulse forces (e.g., rapid accelerations), the risk of damaging critical components, such as the flexspline and circular spline, is significant. This is why many harmonic drives are equipped with brakes or clutches to protect against high-impulse inputs, such as crashes or rapid balance corrections in legged robotic systems. Harmonic drives in this format are used in most cobots, robotic dogs and humanoids you see around. Often on top of that they have a combo of of input and output shaft encoders and torque sensors to achieve compliance.
The Real Question: Why and When Do You Need Backdrivability?
In traditional industrial robotics, the prevailing philosophy is often: "the stiffer, the better." Stiffness in robotic systems brings several advantages, particularly in precision, load handling, and control stability. To achieve these benefits, industrial robots typically use: High Gear Ratio Harmonic or Cycloidal Reducers paired with Servo Motors
Why Stiffness Matters in Industrial Robots
- Trajectory Tracking: High stiffness helps robotic systems follow precise trajectories. The lack of flexibility reduces deviations caused by external forces or internal dynamics.
- Easier PID Tuning: Stiffer systems simplify the tuning of PID loops, as the mechanical system is less prone to oscillations or unwanted compliance under control feedback
- .Position Holding: Robots designed to hold positions under load benefit greatly from stiffness. High stiffness minimizes deflection or drift, even under significant forces.
- Handling Large Loads: Stiff systems are better at supporting and manipulating heavy loads without losing positional accuracy, making them ideal for industrial applications.
The Role of Reflected Inertia
The concept of reflected inertia explains why this approach works so well. High gear ratios in harmonic or cycloidal reducers multiply the inertia of the motor, making the system resist external forces better. This "stiffness" allows the robot to:
- Absorb disturbances without noticeable positional changes.
- Maintain high precision and stability under dynamic conditions.
To achieve these benefits, industrial robots typically use: High Gear Ratio Harmonic or Cycloidal Reducers paired with Servo Motors
When to Consider and Focus on Backdrivability
Backdrivability becomes more critical in applications that demand compliance, energy efficiency, or the ability to respond to external forces dynamically like:
- Collaborative robots (cobots) designed to work safely alongside humans.
- Legged robots or balance-dependent systems requiring dynamic responses.
- Force-sensitive tasks like assembly, gripping, or delicate manipulations.
Harmonic Drives in Collaborative Robots and Legged Systems
Robots equipped with low-ratio harmonic drives (e.g., 30:1) are commonly found in smaller collaborative robots (cobots) like those from Universal Robots or Mecademic. When carefully designed, these robots achieve good torque transparency through their reducers, allowing torque to be measured indirectly via motor currents.
Force Detection and Collaboration
One of the primary uses of force detection in collaborative robots is to enhance safety and interactivity. For example, if a human applies an external force to the robot (e.g., by pushing or pulling its arm), the system detects this force through changes in motor current. Because of the good transparency of low-ratio harmonic drives, the robot’s controller can respond by:
- Stopping motion.
- Reducing speed.
- Adjusting force output.
- Changing direction.
This responsiveness also makes such robots well-suited for tasks like teleoperation, mimicking human motion, or even "teaching" the robot by physically guiding it through a desired trajectory.
Challenges of Harmonic Drives in Collaboration
Despite their advantages, harmonic drives have some inherent limitations, particularly in collaborative and legged robotic systems. These include:
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Flexure Design:The flexure mechanism in harmonic drives transmits motion between the input and output while eliminating backlash. However, this design results in low rotary stiffness compared to traditional gears with tooth-to-tooth contact.
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Nonlinear Behavior:Strain-wave transmissions exhibit highly nonlinear characteristics, where the friction torque depends on the load torque. Additionally, hysteresis effects in harmonic drives arises from the flexspline’s elastic deformation during operation. This causes a lag between input and output, particularly during direction changes or varying loads, As the joint moves through different angles, this effect creates small positional errors and nonlinear behavior, complicating force detection and control.
- Deadband and Torsional Effects:If a human applies a force to the robot’s arm, it may not be immediately detected by the motor current due to the deadband and torsional compliance of the harmonic drive. This lag reduces the robot’s ability to accurately measure internal forces and creates challenges for force-based algorithms.
Implications for Collaborative Robots
While harmonic drives are commonly used in collaborative robots, their inherent limitations—such as low stiffness and nonlinear force detection—can hinder performance in force-sensitive applications. Robots with these drives may struggle with precise internal force detection, leading to reduced effectiveness in tasks requiring high force accuracy.
For applications requiring high precision in force control and detection, alternative gear reduction solutions or additional sensors may be needed to overcome these challenges.