KW Article Column
Hello everyone, I’m Liu Chunfeng from Kunwei Technology. In the previous issue, we mentioned two selection schemes for robot joint torque: the high-reduction ratio scheme and the quasi-direct drive scheme.These two schemes have completely different requirements for sensor usage.
Differences in Sensor Usage
Let’s start with the first one: the high-reduction ratio scheme, represented by harmonic reducers. In this scheme, torque transparency is relatively low. What does that mean? Due to the large reduction ratio, coupled with the friction and flexible deformation of the harmonic reducer itself, the current signal on the motor side can hardly accurately reflect the actual force at the end-effector.Therefore, to achieve precise force control, you must install a torque sensor at the joint output. In this scenario, there are two main design options for sensor mounting:
One is mounted at the joint end, rotating with the load, which is the mainstream usage for collaborative robotic arms today. When mounted at the end, the sensor is usually connected to the flexspline of the reducer and rotates with it; the circular spline is fixed to the housing. However, in the circular-spline output scenario of the hat-type reducer, one end of the flexspline is fixed to the housing, and the sensor is then connected to the circular spline and rotates with it.
The other is mounted inside the joint, not rotating with the load, which is applied in humanoid robot joints. In this case, the reducer outputs directly through the circular spline and connects to the load; the outer ring of the sensor is typically connected between the cross-roller bearing and the housing, and the inner ring is connected to the flexspline. The sensor acts as a stationary part here, measuring torque solely by sensing the reaction force.
But obviously, regardless of the mounting method, the conclusion is clear: a torque sensor is a must for the harmonic reducer scheme.
What about the quasi-direct drive scheme? The requirements for sensors in this scheme are completely different. Since the reduction ratio is small, external forces can directly back-drive the motor rotor. Therefore, the controller can directly read the motor current loop signal to estimate the output torque, and in theory, the joint torque sensor can be omitted.
Note that I used the word “in theory”. In practice, if you want to achieve ultra-high-precision static force control or handle complex environments, relying only on the current loop may not be sufficient. In this case, you may still need to add a torque sensor for dual verification.
So we can summarize with a simple logic:If your robot joints are high-precision industrial robotic arms, collaborative robots (requiring human-robot interaction), or the upper body of humanoid robots (i.e., shoulders, elbows and other parts needing high-precision positioning), you will most likely adopt the harmonic reducer scheme. In this case, a joint torque sensor is standard equipment. Because the torque transparency of the harmonic scheme is low, force control cannot be accurate without a sensor.
If your robot joints are the legs of quadruped robots, or the hips, knees and ankles of humanoid robots (i.e., parts requiring frequent landing impact), with high demands for dynamic response and impact resistance, the quasi-direct drive scheme should be prioritized. In this case, the sensor can be omitted, and force control can be performed via the current loop; however, if you pursue ultimate precision or require safety redundancy, the sensor still has its place.
So back to the opening question: Should you install a torque sensor on your robot joints?The answer depends on the scheme you choose, your accuracy requirements, and your cost budget.
Our role is to provide you with the most accurate and reliable torque sensing solutions during this complex selection process. Whether you need high-precision sensors for the harmonic scheme, or want to add redundant sensors for ultimate performance in the quasi-direct drive scheme, we have mature products and solutions.
Conclusión
That’s all for this video. For high-quality force measurement solutions, please follow Kunwei Technology. See you next time!
