In its Optimus-Gen2 video released on December 12, 2023, Tesla showcased significant advancements in the maneuverability of its humanoid robots. By precisely sensing and controlling force, force sensors enable the robots to perform more complex and delicate tasks, improving their adaptability and reliability in real-world applications. Below are some key points highlighted in the video and the role of force sensors in these processes:
Grasping and placing eggs: The robot's ability to easily and accurately grasp and place eggs demonstrates its fine maneuverability and precise control over force. Force sensors play a crucial role here, helping the robot sense the magnitude of the grip force to ensure that the fragile object is not damaged.
Left-hand to right-hand: A robot's ability to smoothly transfer objects from one hand to the other requires precise force control and coordination. Force sensors help the robot sense and adjust the force applied when transferring objects between the two hands, ensuring a smooth transition.
90-degree squat: The robot can perform a uniform 90-degree squat, a movement that requires coordination and balance across multiple joints throughout the body. Force sensors help the robot perceive the forces acting on different parts of its body during this process to maintain balance and stability.
Changes in walking speed: Changes in the robot's walking speed indicate that it can adjust its pace as needed. This also requires force sensors to sense the contact force between the feet and the ground, as well as the torque when adjusting the pace.
Foot and leg morphology: The robot's feet and legs are designed to mimic the human form, which helps it walk on different terrains. Force sensors help the robot sense the hardness and unevenness of the ground, thereby adjusting its gait and balance.
Optimus-Gen2 grabbing an egg (left image). 90-degree squat (right image). Source: The Paper
01. What is a force sensor?
A force/torque/force sensor is a device that detects and measures force/torque. It converts the force acting on it into an electrical signal or other form of signal to facilitate information collection, transmission, processing, analysis, and display. This article refers to force or torque sensors, or force sensors in general, as force sensors. Force sensors have wide applications in various fields such as industrial automation, robotics, medical equipment, and transportation.
The core of embodied intelligent robots lies in the perception and cognition layers. (Source: Embodied Intelligent Robots Official Account)
A force sensor mainly consists of two parts: the sensor body unit and the strain/deformation detection system. It comprises specific components such as force-sensitive elements, conversion elements, and signal processing units. It converts the detected information into an electrical signal output according to a certain rule to meet the requirements of information transmission, processing, storage, display, recording, and control. When a force is applied, it is applied to the sensor body unit, causing strain or deformation. The detection system (strain gauge or optical system) senses the strain or deformation of the body unit, converts it into a corresponding voltage through a circuit, and then measures the voltage value to characterize the magnitude of the force. This voltage value is then converted into a usable output signal, ultimately achieving force measurement.
In a force sensor, the force-sensitive element refers to the part that directly senses or responds to the measured quantity, while the conversion element refers to the part that converts the measured quantity sensed or responded to by the force-sensitive element into an electrical signal suitable for transmission or measurement. For example, the force-sensitive element of a strain gauge force sensor is an elastic sensing element (elastic body). Common elastic body materials include aluminum alloy, alloy steel, and stainless steel. The conversion element is a strain gauge attached to the elastic body, such as a resistance strain gauge. The signal processing unit is generally a circuit, including enameled wire, PCB board, etc., and the strain gauge acts as a resistor in the circuit.
02. Two Classification Methods of Force Sensing
1. Force sensors can be classified into one-dimensional to six-dimensional force sensors according to the different measurement dimensions. Each type of sensor is suitable for different application scenarios depending on the direction and point of application of the force it measures:
One-dimensional force sensors: These sensors can only measure force along a specific direction. They are suitable for situations where the direction and point of application of the force are fixed, such as simple weighing or pressure measurement. Representative products include load cells and pressure sensors.
Two-dimensional force sensors: These sensors can measure forces in two orthogonal directions, typically horizontal and vertical. They are suitable for applications requiring simultaneous measurement of forces in two directions, such as certain types of thrust measurement.
3D force sensor: Capable of measuring forces in three orthogonal directions, typically the X, Y, and Z axes. Suitable for scenarios requiring the measurement of forces in arbitrary directions in space, such as force control of robots.
Four-dimensional force sensor: Based on three-dimensional force sensor, it adds the measurement of torque in a specific direction. Suitable for complex applications that require knowing the direction of the force and the torque it generates.
Five-dimensional force sensor: measures three orthogonal forces and two orthogonal moments. This type of sensor has relatively few applications, but it can provide more comprehensive force and moment information.
A six-dimensional force sensor is a special type of force sensor capable of simultaneously measuring three orthogonal forces and three positive torques. This sensor is the highest-dimensional force sensor, providing the most comprehensive and accurate force information. Six-dimensional force sensors are suitable for applications requiring precise control and perception of complex interactive forces, such as robotic surgery, precision assembly, and drag-and-drop teaching.
The most common types are one-dimensional, three-dimensional, and six-dimensional force sensors, while two-dimensional, four-dimensional, and five-dimensional force sensors are less common.
Force sensors are classified according to the force measurement dimension. Source: Kunwei Technology, Lichi Sensing, Anxin Securities.
2. Based on their different measurement principles, force sensors can be classified into photoelectric, strain gauge, capacitive, and piezoelectric types. Each type of force sensor has its specific advantages and limitations. The choice of which type of sensor to use usually depends on the specific requirements of the application, including measurement range, accuracy, response time, cost, and environmental conditions. Among them, strain gauge force sensors have the widest range of applications. Force sensors based on piezoelectric, capacitive, and photoelectric measurement principles have a certain theoretical research and experimental foundation, but their downstream applications are not yet widespread.
Force sensors are classified according to their measurement principles. Sources: Instrument Information Network, Chemical Instrument Network, Sensor Expert Network, *Handbook of Modern Sensors*, Huodun Official Website, Hansode Official Website, TE Connectivity Official Website, Anxin Securities.
Strain gauge force sensors measure force by utilizing the deformation of a material under stress. Typically, a strain gauge (a sensitive element) is bonded to a substrate material. When the substrate deforms under stress, the resistance of the strain gauge changes, and the magnitude of the force is determined by measuring this change in resistance. They are widely popular due to their stability, high stiffness, high signal-to-noise ratio, high accuracy, low cost, and ease of use, and are extensively used in industrial automation, robotics, and various testing equipment.
According to the classification of strain gauges, strain gauge force sensors can be divided into metal resistance strain gauges (tool steel, stainless steel, aluminum alloy or beryllium copper, etc.) or silicon (semiconductor) strain gauge force sensors, the latter having better performance.
Semiconductor strain gauges are highly valuable in applications requiring precise measurement of minute forces, such as in precision instruments, microelectronics, and biomedical engineering, due to their high sensitivity and rapid response. Metal strain gauges, on the other hand, are better suited for force measurement in industrial environments due to their stability and durability. The choice of force sensor depends on the specific application requirements and environmental conditions. For example, metal strain gauges may be preferable for applications operating in high-temperature or harsh environments, while semiconductor strain gauges may be a better choice for measuring minute forces or achieving high-precision control.
03
Force sensor industry chain
The design and manufacture of force sensors involve multiple technical fields such as mechanics, materials science, and electronic engineering. Taking strain gauge sensors as an example, the upstream of the force sensor industry chain mainly includes sub-sectors such as materials, components and assemblies, and production or testing equipment. The midstream mainly involves the processing, manufacturing, packaging, and testing of force sensors. Downstream applications are wide-ranging, and can be applied in fields such as industry, consumer electronics, automotive electronics, robotics, medical, and aerospace.
Force sensor industry chain, source: KELI Sensing prospectus
04. Core Parameters and Key Technologies of Six-Dimensional Force Sensor
Six-dimensional force sensors mainly include capacitive, piezoelectric, photoelectric, and strain gauge types. Currently, most six-dimensional force sensors used in the market are based on strain gauge measurement. Designing and implementing a six-dimensional force sensor is far more complex than simply combining three one-dimensional force sensors and three torque sensors. Its nonlinear mechanical characteristics are significant, and it requires consideration of temperature drift, creep, crosstalk, and real-time data processing of multi-channel signals. Adding to this the complexity of six-dimensional joint loading calibration, the technical difficulty of a six-dimensional force sensor is arguably six times that of a one-dimensional force sensor.
A six-dimensional force sensor provides the most comprehensive force information and improves sensing accuracy, which cannot be replaced by multiple low-dimensional force sensors. A six-dimensional force sensor can simultaneously measure forces (Fx, Fy, Fz) in three spatial directions and torques (Mx, My, Mz) about these three directions, providing a complete vector description of force and torque. Because force is a vector quantity with magnitude and direction, a six-dimensional force sensor can accurately measure the direction of the force, unaffected by changes in the point of application. When using multiple low-dimensional force sensors in combination, the measurement errors of each sensor may accumulate, and coupling errors may exist between them. A six-dimensional force sensor, as a whole, can reduce these error sources.
Core parameters
The main performance parameters of a six-dimensional force sensor include: measuring range, overload capacity, resolution, repeatability, crosstalk, and accuracy. When selecting a six-dimensional force sensor, it is necessary to comprehensively consider these performance parameters and the specific requirements of the application scenario to ensure that the sensor can meet the required performance standards.
Measurement range (scale): This is the range of maximum forces and torques that the sensor can measure. When selecting a sensor, it is necessary to ensure that its range covers the expected maximum forces and torques.
Overload capacity: This parameter indicates the maximum force and torque the sensor can withstand beyond its rated range without causing damage or performance degradation. Overload capacity is crucial for ensuring the sensor's durability under unexpected or extreme conditions.
Resolution: Resolution is the smallest change in force or torque that a sensor can detect. High-resolution sensors are necessary for applications requiring precise force control.
Repeatability (Accuracy): This parameter reflects the consistency of a sensor when measuring the same value multiple times. High repeatability means that the sensor provides more stable and reliable results in continuous measurements.
Crosstalk: Measured by the coupling interference between forces in different directions of a sensor, it is one of the core indicators reflecting the manufacturing and calibration level of a six-dimensional force sensor. Low crosstalk means that the sensor experiences less interference between directions during measurement, which is crucial for accurate measurement.
Accuracy: Accuracy is a comprehensive reflection of the difference between the sensor's measured value and the actual value, including error factors such as hysteresis, linearity, and creep. A high-accuracy sensor can provide measurement results that are closer to the true value and is a key indicator for evaluating sensor performance.
Key issues
The key technologies of six-dimensional force sensors mainly focus on technical implementation, performance optimization, and application integration, specifically including:
Dynamic response capability: Six-dimensional force sensors need to respond quickly and accurately to changes in dynamic force, such as the contact force during robot polishing or the weighing of objects during high-speed movement.
Comprehensive mechanical overload protection: To prevent damage to the sensor when subjected to excessive force, an effective overload protection structure needs to be designed, which is technically challenging.
Temperature drift and stability control: The sensor must have a good temperature compensation mechanism to reduce the impact of temperature changes on the measurement results and maintain long-term stability.
Multi-channel signal processing: Six-dimensional force sensors need to process force and torque signals from multiple directions simultaneously, which requires highly accurate signal acquisition and processing technology.
Cross-interference and coupling errors: In multi-dimensional force sensors, coupling may exist between forces and torques in different directions, requiring precise decoupling algorithms to separate these signals. Generally, there are two ways to reduce coupling interference: structural decoupling and algorithmic decoupling. Structural decoupling and algorithmic decoupling usually need to be used in combination to achieve the best decoupling effect. Structural decoupling can reduce coupling effects and simplify algorithmic decoupling; while algorithmic decoupling can further correct coupling interference that is difficult to eliminate through structural decoupling, improving measurement accuracy.
Calibration and calibration techniques: Six-dimensional force sensors require a complex calibration and calibration process to ensure measurement accuracy, which typically involves precise testing equipment and methods.
Sensor fusion: In some applications, it is necessary to combine force sensors with other types of sensors (such as position, velocity, and acceleration sensors) to solve the technical challenges of data fusion between different sensors.
Material selection and processing technology: Selecting suitable materials and optimizing their performance is one of the technical challenges. At the same time, the production of force sensors involves precision processing technology.
Miniaturization and weight reduction: Reduce the size and weight of the sensor while maintaining performance to adapt to space-constrained applications.
Real-time data processing: Develop efficient data processing algorithms to achieve real-time or near-real-time data processing and feedback.
Long-term stability and reliability: Sensors need to maintain stable operation over a long period of time under various environmental conditions. Material aging and fatigue life are factors that need to be considered during the design process.
Cost control: While ensuring technical performance, effectively controlling the production cost of force sensors is the key to realizing the commercialization of sensors.
Accuracy and resolution: Six-dimensional force sensors need to have high measurement accuracy and resolution to meet the requirements of high-precision applications.
System integration: Integrating a six-dimensional force sensor into a robot or other automated system requires addressing issues such as mechanical design, electronic interfaces, and software compatibility.
05. Application Scenarios in Humanoid Robots
Currently, six-dimensional force sensors are widely used in the automotive industry for crash testing, wheel hub and seat testing, as well as in aerospace, biomechanics, medical fields, scientific research experiments, robotics and industrial automation, and many other technological fields.
Table: Main Application Scenarios of Six-Dimensional Force Sensors in Humanoid Robotics
In the field of humanoid robots, six-dimensional force sensors have become standard equipment for high-performance humanoid robots. They are usually installed on the wrist, ankle, sole, or hand, and play an important role in motion control planning, posture adjustment, and force perception. This improves the flexibility of hand operation and the stability of foot walking, enhancing the autonomy and adaptability of humanoid robots in complex environments. They enable stable and rapid walking on uneven surfaces such as gravel, paving stones, lawns, thick ground, and grass, through a new foot posture control algorithm and flexible adaptive multi-surface technology.
Currently, Tesla's Optimus, UBTECH's Walker X, CloudMinds' Little Purple XR-4, Honda's ASIMO, and NASA's Valkyrie, among others, have all adopted six-dimensional force sensors to improve robot performance.
The following are some key applications and installation locations of six-dimensional force sensors in humanoid robots:
Wrist area: A six-dimensional force sensor is installed in the wrist area to help the robot perform fine hand operations, such as grasping, carrying and assembling.
Ankle and foot: Installing six-dimensional force sensors on the ankle or foot helps the robot sense the ground reaction force during walking and balance control, so that the robot control system can adjust the posture of the humanoid robot arm and body to improve walking stability.
Hand contact points: Installing six-dimensional force sensors at the contact points of the robot's hand can improve the flexibility and adaptability of hand operations, especially in scenarios requiring compliant control.
Motion control planning: The data provided by the six-dimensional force sensor is crucial for the motion control planning of the robot. It can help the robot better plan its movements and avoid collisions and damage.
Attitude adjustment: During the robot's attitude adjustment process, the six-dimensional force sensor can provide necessary feedback information to help the robot maintain balance and stability.
Force perception: The six-dimensional force sensor enables the robot to sense the force when interacting with external objects, which is very important for applications that require precise force control.
Six-dimensional force sensors are suitable for use on humanoid robots, including the wrist, ankle, and dexterous hand.
In addition, humanoid robots may use three-dimensional force sensors, joint torque sensors, or one-dimensional tension sensors, while some may also use tactile sensors, electronic skin, etc.
The combined use of multi-dimensional force sensors: In some applications, in addition to six-dimensional force sensors, humanoid robots may also use three-dimensional force sensors, joint torque sensors, or one-dimensional tension sensors simultaneously to meet different control requirements. For example, the Tesla Optimus may have added one-dimensional force sensors to the toes to obtain more point information in order to adapt to more complex ground scenarios.
Tactile sensors and electronic skin: In addition to force sensors, humanoid robots may also integrate tactile sensors and electronic skin to provide more comprehensive environmental perception capabilities.
06. Cost Analysis of Six-Dimensional Force Sensors
The cost of a six-dimensional force sensor includes material, manufacturing, and labor costs, with over 50% of the cost coming from strain gauges and manual processing.
As a core component of a six-dimensional force sensor, high-quality strain gauges are expensive. A single six-dimensional torque sensor requires at least 24 strain gauges. Considering requirements such as resistance to temperature drift and creep, the number of strain gauges in a single six-dimensional torque sensor is generally about 30-40, and the cost of a single strain gauge in a six-dimensional torque sensor is between 5,000 and 6,000 yuan.
The finished six-dimensional force sensor requires extremely high precision and accuracy, thus posing a significant barrier to manual processing, especially in the mounting and calibration stages, which require a large number of skilled workers for meticulous operation and cannot be automated in the short term. In the future, MEMS technology is likely to be the main direction for achieving efficient production of six-dimensional force sensors, and increased automation will help reduce costs.
Exploded view of a high-precision six-dimensional force sensor; strain gauges account for a high proportion of material costs. Sources: "A High-Precision Six-Dimensional Force Sensor_Wu Meizhen et al.", "A Miniature Strain Gauge Six-Dimensional Force Sensor and Its Calibration Device_Wang Peng et al."
In downstream applications of six-dimensional force sensors, industrial automation (including traditional robots) accounts for the largest share of application scale, but the humanoid robot industry has the fastest growth rate. According to MIR data, in 2023, six-dimensional force sensors were still mainly used in industrial automation, accounting for over 77% of the application scale, followed by the automotive industry; however, the application scale in the humanoid robot industry increased by 146.7% year-on-year, far exceeding the growth rate of other industries. With the expectation of accelerated mass production and urgent price reductions in humanoid robots, six-dimensional force sensors may usher in a price reduction trend.
The development of six-dimensional force sensors is difficult and has high barriers to entry, resulting in high prices, significantly higher than low-dimensional force sensors, especially given the current small market base. According to MIR research, the cost of a six-dimensional force sensor is several thousand yuan, while the unit price can reach 20,000-40,000 yuan, indicating a high profit margin and significant room for price reduction, which will benefit future cost reduction and enhanced market competitiveness. It is projected that the price of a single six-dimensional force sensor product may drop below 5,000 yuan within two years. This price reduction trend may attract more applications and market participants, driving rapid industry development and a new round of reshuffling.
07. Market Landscape and Typical Companies of Six-Dimensional Force Sensors
Currently, domestically produced six-dimensional force sensors have largely caught up with mainstream foreign products in terms of accuracy. However, gaps remain in sensitivity, crosstalk, overload resistance, and inter-dimensional coupling errors. Furthermore, the number of manufacturers truly capable of mass-producing products is still relatively small. According to MIR data, in recent years, domestic brands have leveraged their supply chain advantages to further reduce costs, offering lower-priced products and high-quality services while maintaining performance, gradually increasing the localization rate. However, the current market share of domestic manufacturers is only slightly above 30%, and the domestic six-dimensional force sensor market is still dominated by foreign brands, which hold a significant first-mover advantage in both products and applications.
In the domestic market, the six-dimensional force sensor market is highly concentrated, with the top five companies (CR5) accounting for over 50%. ATI (USA) holds an absolute leading position with a market share of 22.4%, followed by Yuli Instruments (China) with 12.2%, Epson (Japan) with 6.4%, Bluepoint Touch (China) with 4.8%, and Kunwei Technology (China) with 4.7%.
Source: Gongyan Industry Consulting (Gongyan.com)
In terms of representative companies, the global six-dimensional force sensor market is mainly divided into three camps: Japanese and Korean brands, European and American brands, and domestic brands. Each camp exhibits different characteristics in its product offerings.
In Japan and South Korea, six-dimensional force sensor manufacturers primarily supply local robot manufacturers. According to GGII, six-dimensional force/torque sensor manufacturers in Japan and South Korea mainly supply local robot manufacturers. For example, South Korean companies Robotous and Aidin Robotics' main partners include Doosan Robotics, Neuromeka, and Rainbow Robotics; Japanese companies Sintokogio and WACOH-TECH's main partners include FANUC, Denso, Mitsubishi, Nachi-Fujikoshi, and Yaskawa, while Epson's six-dimensional force sensors are mainly used in Epson's industrial robots.
In Europe and America, six-dimensional force sensor manufacturers can be divided into two categories: traditional sensor manufacturers and robot end-effector tool manufacturers.
Traditional sensor manufacturers include ATI, Bota Systems AG, ME-Meßsysteme GmbH, AMTl, Kistler, etc.
Globally renowned manufacturers of robot end effectors include SCHUNK, OnRobot, and Robotiq. In Europe and America, manufacturers primarily partner with collaborative robot manufacturers, including Universal Robots, Damin Robotics, and Omron.
In recent years, domestic manufacturers of six-dimensional force sensors have gradually grown, such as SRI, Kunwei Technology, Xinjingcheng, Haibosen, Bluepoint Touch, Shenyuansheng Intelligent, and Ruierte, all of which have launched related products and entered industrial applications. Other manufacturers, such as Chongqing Luban Robotics Technology Research Institute and Aili Intelligent, have developed their own force sensor technology and have the production capacity of six-dimensional force/torque sensors, with some product models beginning to enter the verification and testing stage by downstream users.
