Why These Three Parameters Matter
Humanoid robots and collaborative robots are rapidly moving from labs to production lines. As the core component of joint actuation, the correct selection of the motor directly determines the robot's load capacity, motion precision, and endurance. Among the many motor types, the Frameless Torque Motor has become the mainstream choice due to its compact structure and ability to be directly embedded into joint modules – all 28 joint actuators of Tesla Optimus use Frameless Torque Motors as their core drive units.
However, when faced with a wide array of product datasheets, looking only at traditional specifications like "rated power" or "rated speed" is far from sufficient. The three deeper parameters that truly determine whether a Frameless Torque Motor can handle robotic joint operating conditions are: Torque Density, Torque Ripple, and Motor Constant (Km) . They answer three core questions: "Is it strong enough?", "Is it stable enough?", and "Can it sustain performance?". This article breaks down each parameter to help engineers and tech enthusiasts understand the true meaning behind the datasheet numbers.
I. First, Understand: What is a Frameless Torque Motor?
To understand the parameters, you first need to know what this "main component" looks like.
A Frameless Torque Motor is a motor "stripped of its housing" – it consists only of two core electromagnetic components: the stator and rotor. It has no housing, no bearings, and no output shaft. This means it cannot work independently like a conventional motor; instead, it must be directly embedded into the robot's joint structure – the stator is fixed to the joint housing, and the rotor is directly connected to the load shaft.
This "frameless" design offers three key advantages: torque density per unit volume is approximately 30% higher than traditional motors, backlashes in the drivetrain are eliminated resulting in approximately 50% higher rigidity, and the hollow structure accommodates internal robot wiring requirements. For these reasons, it has become the core power component for collaborative and humanoid robot joint modules.
II. Torque Density – How "Powerful" the Motor Is
What is Torque Density?
Torque density, simply put, is how much torque the motor can output per unit volume or unit weight. It is typically expressed in two ways: volumetric torque density (Nm/L) and gravimetric torque density (Nm/kg).
Robot joint space is extremely limited. You cannot endlessly increase the motor diameter to get higher torque – that would make the joint bulky and difficult to integrate. Therefore, torque density essentially measures the "compactness" of the electromagnetic design: in a given space, the motor with a stronger magnetic field and higher current efficiency can output more torque.
How to Evaluate This Parameter?
When selecting a motor, you should base your decision on the peak torque demand under the worst-case operating conditions, and reserve a safety margin of 10%-20%. For humanoid robot joints, peak torque demand can be as high as 5-10 times the rated torque. For example, during a gait cycle when a single leg supports the entire body weight, the hip joint motor needs to output several times the torque required for walking at a constant speed instantaneously.
Also note that torque density is closely related to cooling conditions. Because the frameless motor relies on the mechanical structure it is embedded in for heat dissipation, the actual continuously available torque inside a sealed joint may be only 50%-70% of the nameplate value. Therefore, when evaluating torque density specifications, be sure to consult the derating curve provided in the product datasheet.
Currently, the torque density level of domestically manufactured motors in China is improving rapidly. For example, a company's U-series Frameless Torque Motors cover outer diameters from 16 to 200 mm and rated torques from 0.01 to 65 Nm, meeting diverse requirements from micro-joints to heavy-duty joints.
III. Torque Ripple – How "Stable" the Motor Is
What is Torque Ripple?
Even if you feed the motor an ideal constant current, its output torque will not be a perfectly smooth straight line; there will be small periodic fluctuations – this is torque ripple, typically expressed as the percentage of the ripple amplitude relative to the rated torque.
There are two main sources of torque ripple:
Cogging Torque: Fluctuations caused by changes in magnetic attraction between the stator teeth/slots and the rotor permanent magnets. It is the main contributor to torque ripple and an inherent characteristic of permanent magnet motors.
Harmonic Torque: Electromagnetic harmonic components caused by factors such as winding distribution not following a sinusoidal pattern and magnetic circuit saturation.
For robotic applications, the practical impact of torque ripple is crucial. Excessive torque ripple leads to "cogging," manifested as jitter and discontinuity during low-speed joint operation, directly affecting performance in applications like precision assembly and medical surgery.
How to Evaluate This Parameter?
Industry-leading levels typically require torque ripple below 1%. For precision operations like dexterous hands, torque ripple may even need to be controlled within 2%.
Reducing torque ripple is one of the core challenges in motor design. Common engineering methods include: optimizing the pole-slot combination, using skewed slots or skewed poles, adjusting permanent magnet width and arc coefficient, and adding auxiliary slots on tooth tips. However, note that there is often a trade-off between reducing cogging torque and increasing output torque – some designs that suppress cogging torque (such as increasing the air gap length) can reduce output torque. Furthermore, for applications with extremely stringent torque ripple requirements, manufacturers may offer slotless (air-core) Frameless Torque Motors, which eliminate cogging torque entirely at the cost of sacrificing some power density.
Therefore, when evaluating torque ripple specifications, it is not about "the lower, the better," but about finding the optimal balance between "operational smoothness" and "torque output capability."
What is the Motor Constant Km?
The motor constant Km is perhaps the "least familiar" yet "most practical" of the three parameters. Many product datasheets do not even provide this value directly, but its importance in motor selection is no less than that of torque and speed.
The definition of Km is:
Km = Kt / √R
Where Kt is the torque constant (torque produced per unit current), and R is the winding resistance. Its physical meaning is: under the condition of dissipating 1 watt of resistive loss power, how much torque can the motor output? The unit is Nm/√W.
Why is this definition important? Because when the motor operates, the winding resistance generates heat. The accumulated heat raises the temperature, ultimately limiting the motor's continuous operation capability. A higher Km value means that for the same amount of heat generated (same resistive power dissipated), the motor can output more torque. In other words, Km measures the motor's true torque output capability under thermal constraints.
To draw an analogy: If torque density measures the motor's "explosive power," then Km measures the motor's "endurance." A motor may have a very high peak torque, but if its winding resistance is also high (thin wire, many turns), it will heat up quickly during sustained high-current operation, and its continuous output capability will be limited – in this case, its Km value is often not high.
How to Evaluate This Parameter?
When comparing motors from different manufacturers or different models, Km is a fairer metric than simply looking at "rated power" or "peak torque." The reasons:
Two motors of the same volume may have similar peak torque, but if one has a significantly higher Km value, it indicates that it can maintain more stable performance during long-term operation and is less likely to derate due to heating.
Km couples torque output capability with thermal losses, providing a more realistic assessment of the motor's performance under continuous robot operation.
In practical selection, you can proceed as follows:
1. Calculate the required minimum Km: Given the load torque T and allowable resistive loss P, then Km_min = T / √P. Select a candidate motor with a Km value greater than this minimum.
2. Pay attention to test temperature: The calibration temperature for Km and Kt is typically between 20°C and 40°C. Different manufacturers may calibrate at different temperatures; the higher the temperature, the lower the Kt value. When making cross-comparisons, ensure the calibration conditions are consistent.
3. Proactively request data: As mentioned earlier, many datasheets do not directly provide the Km value. It is recommended to proactively ask the supplier for this parameter during the selection process.
V. Relationships and Collaborative Consideration of the Three Parameters
Torque density, torque ripple, and motor constant Km are not isolated indicators; they have inherent relationships and design trade-offs.
Parameter | Core Question | High Value Means | Typical Engineering Approaches |
Torque Density | Is it strong enough? | High torque output in a small volume | High-performance rare-earth magnets, optimized pole-slot combination |
Torque Ripple | Is it stable enough? | Smooth motion, precise positioning | Skewed poles/slots, optimized pole arc coefficient, slotless design |
Motor Constant (Km) | Can it sustain performance? | More torque output for the same heat generation | Lower winding resistance, optimized thermal path |
When a motor strives to improve torque density (e.g., by enhancing air-gap flux density), it may lead to increased torque ripple. Conversely, an excessive pursuit of low torque ripple (e.g., using a slotless structure) can reduce torque density. Therefore, a good Frameless Torque Motor design finds the optimal balance point among these three parameters.
Conclusion: Selection is Not a Numbers Game
Returning to the engineer's daily work scenario, it is easy to fall into the mindset of "bigger parameters are better" when selecting components. However, a truly mature selection strategy determines trade-offs based on the actual operating conditions of the robot joint:
Heavy-duty lower limb joints? Prioritize torque density to ensure load capacity and overload margin.
Precision dexterous hand or surgical robot? Prioritize torque ripple to ensure low-speed smoothness.
Industrial robots operating continuously for long periods? Prioritize Km value to ensure thermal stability and long-term reliability.
As the humanoid robot industry enters a critical phase of mass production scale-up in 2026, domestically manufactured Frameless Torque Motors in China are rapidly catching up to international levels in key parameters like torque density and torque ripple, with prices only 50%-70% of comparable overseas products. For engineers, understanding the parameters and seeing through the physical meaning behind the datasheet numbers is the key step from "making it work" to "making it work well."
