The International Financial Review (IFR) has predicted that the service robot industry will continue to grow as it penetrates the market, with sales expected to exceed 500,000 units and revenue reaching $27.7 billion by 2023. BuyShares' forecast is even more optimistic, predicting that the market will grow by 30% within two years, with revenue exceeding $30 billion.
Compared to industrial robots whose applications are limited to specific locations such as factories, service robots, which focus on high-tech perception, decision-making, and execution to perform service work that benefits humans, will cover a wide range of scenarios and have more room for imagination.
As the power source of robots, motors directly affect key parameters such as effective load and working radius. The technical choices and performance trade-offs in their design will directly affect the performance and application scope of robots. The potential applications in the service robot market will also place more demands on motor design.
Based on years of experience in motor research and product design, we have found that brushless motors are widely used in the robotics field due to their advantages such as small size, light weight, high output, fast response, high speed, low inertia, smooth rotation, stable torque, intelligent operation, flexible commutation, maintenance-free operation, high efficiency, low operating temperature, low electromagnetic radiation, long lifespan, and applicability to various environments. They also have promising prospects in the application of service robots.
Generally speaking, when designing motors for service robots, the following three points require special attention.
Image | Jicui Intelligent Manufacturing's self-developed permanent magnet brushless torque motor
01 Both good looks and performance are essential – hollow design for maximum output
Power density is the amount of power generated per unit volume of a motor (energy transfer time rate). The greater the power and/or the smaller the size of the motor, the higher the power density.
Currently, due to requirements for product miniaturization and lightweighting, the space for robot motors is generally quite limited. Therefore, when designing robot motors, our goal is to provide the highest possible power output within the smallest possible space; thus, power density is one of the important factors we consider in motor design.
Image | Motor drive system
In general, when designing service robots, frameless motors are the mainstream choice for motor design in order to reduce size and weight and save layout space.
According to the calculation formula, the air gap radius is positively correlated with torque; the larger the air gap radius, the greater the torque value. The square of the torque is also positively correlated with the cube of the air gap radius. Therefore, the selection of the air gap radius is crucial for improving power-torque density.
Figure | Method for calculating air gap radius
02 Two key advantages – stable low speed and strong explosive power
Considering the application scenarios of service robots, the low-speed stability and short-term overload capacity of the motor generally need to be given more consideration.
Regarding the important performance indicator of low-speed smoothness, the main factor affecting it is the pulsating torque of the motor during low-speed operation, including ripple torque and cogging torque caused by non-sinusoidal electromotive force or current. Among them, the ripple torque caused by non-sinusoidal current is mainly caused by current commutation, electromagnetic factors, armature reaction, etc.
The influence of cogging torque in permanent magnet motors can be mitigated through motor design. Currently, the main methods for improvement in motor design include: (1) optimizing the combination of pole and slot numbers; (2) using skewed poles and slots; (3) optimizing the pole arc coefficient; (4) using unequal-thickness magnetic poles; and (5) segmenting the magnetic poles. Other methods are less commonly used due to cost and other factors.
In the current service robot field, especially in applications such as legged robots, there are higher requirements for the short-term overload capacity and peak torque density of motors. In other words, when selecting such motors, a key consideration is how the motor designer handles the variable of temperature.
When choosing a cooling solution, the motors of service robots prioritize torque output density, which is the output power per unit volume or mass. This means that cooling methods that increase the overall mass, such as oil cooling or water cooling, are unlikely to be preferred.
In legged robot scenarios, compared to hydraulic and pneumatic transmission methods, electric drive still has an inherent disadvantage in terms of density output.
Currently, the market generally improves output density by increasing magnetic flux density, reducing current load, using lightweight design, and improving material properties. Many compromise solutions have emerged in the market for improving the torque output density of electric drives, and have achieved good results. However, how to improve the torque density of motors will remain a hot research topic in the industry.
03. Joints must be flexible – Measuring servo bandwidth
In servo systems, servo bandwidth is a measure of the system's ability to respond quickly and suppress disturbances, directly determining the acceleration performance of the joints. The servo bandwidth parameter measures its response speed to changing input commands, and also determines how quickly it can respond to changes in position, speed, or torque parameters such as feedback and errors.
For motor design, the mechanical time constant is an important reference quantity when calculating servo bandwidth parameters. It refers to the time delay caused by the mechanical inertia of the mechanical system when it accelerates from zero to its rated speed.
at last
Currently, the motors meticulously crafted by the Jicui Intelligent Manufacturing team for collaborative robots, legged robots, medical robots, and flying robots possess certain advantages in terms of reliability, control sensitivity, smooth operation, and low-speed start-up performance, comparable to similar products from abroad.
Our lightweight collaborative robots utilize self-developed motors and are widely used in service scenarios such as new retail. Jicui Intelligent Manufacturing has long focused on robot R&D, possessing extensive experience in everything from integrated drive and control joints and high-precision magnetic encoders to lightweight collaborative robots, unmanned disinfection and sterilization vehicles, and the creation of automated production lines. We welcome your inquiries and discussions.
