Machine Design #04: Choosing a Motor for Machine Design – Part 2
This article is a set of engineering notes that continues the summary of motor-selection principles and characteristics in machine design, focusing on distinguishing and applying stepping motors and servo motors.
Contents
3. High-precision motor selection – distinguishing servo from stepper
- The stepping motor, driven by electrical pulses
- Torque characteristics of the stepping motor
- Environmental resistance of the stepping motor
- Representative makers and products
- The servo motor – standing out for high responsiveness
- Torque characteristics of the servo motor
- Environmental resistance of the servo motor
- Representative makers and products
- How to achieve high-precision positioning
- Open-loop and closed-loop control
- The role of the encoder in detecting position information
4. Learning the key points of motor selection and classification from application examples
- Motors in industrial robots – the core of FA
- Motors in machine tools for precision machining
- Motors in conveying equipment – handling logistics
- Motors in semiconductor production equipment
- Motors in next-generation vehicles: EVs and drones
- Motors in medical equipment – supporting life
5. Additional knowledge and auxiliary equipment needed for motor selection
- Using a gearhead to amplify torque
- The role of the "inverter" in converting between DC and AC
- Understanding torque and motor power (kW)
- Determining the direction of rotation
6. Summary of optimal motor selection and classification
3. High-precision motor selection – distinguishing servo from stepper
The stepping motor, driven by electrical pulses
A stepping motor is a motor that rotates through fixed angles (steps), one step for each input electrical signal (pulse). Because of this digital operating principle, it is also called a "pulse motor".
Its biggest advantage is that it can position accurately without a feedback sensor. As long as the controller counts the number of pulses it has sent, the system already knows how far the motor has turned, which makes the system configuration very simple and low in cost. In addition, even at a standstill, if current is still supplied, the motor produces a strong holding torque to maintain its position.
There are drawbacks, however. When the load exceeds the motor's capability, "step-out" (loss of synchronism) can occur: the motor fails to rotate the correct number of pulses, leading to a position error. For this reason, stepping motors are usually used in equipment such as 3D printers and analytical instruments — where the load is fairly stable and very high speed is not required.
Torque characteristics of the stepping motor
The most notable feature of the stepping motor is its ability to produce very high torque in the low-speed range. In particular, when stopped but still energised, it produces a "maximum holding torque" to lock its position. However, as the rotational speed rises, the torque falls off quickly, which makes it unsuitable for high-speed operation. Because of this characteristic, it suits intermittent positioning motions such as "move quickly over a short distance, then stop accurately".
Environmental resistance of the stepping motor
Thanks to its brushless construction, the stepping motor generates no dust from brush wear, so it suits clean environments. The motor body is fairly rugged, but to achieve high-precision positioning, temperature management is very important. If a large current is supplied continuously for a long time, the resulting temperature rise will affect performance.
Common countermeasures include: fitting a heat-dissipating plate, and using a current-down function when stopped. If the environment calls for dust and water resistance, choose a model with a high IP protection rating, or shield the motor with a protective housing.
Representative makers and products
Stepping motors are supplied by many makers, especially in the FA (factory automation) field.
Shinano Kenshi Co., Ltd. Develops the Plexmotion brand, focusing on convenience such as types with a built-in controller and driver. The CSA-UP series bundles the controller, driver, and motor together, making it easy to build a positioning system. URL: https://www.plexmotion.com/
Sanyo Denki Co., Ltd. The SANMOTION line is its flagship, standing out for high torque, low noise, and low vibration. In particular, the two-phase SANMOTION F2 system is widely used in fields that demand high reliability, such as semiconductor production equipment and medical devices. URL: https://www.sanyodenki.com/
The servo motor – standing out for high responsiveness
What is a servo motor? A servo motor is not merely a motor but a system made up of three parts:
- A main motor with high responsiveness.
- A feedback sensor (encoder) to monitor the motion.
- A driver (servo amplifier) — the controller that acts as the "brain", linking and controlling the other two.
The biggest feature of a servo motor lies in its closed-loop control based on feedback from the encoder. The driver continuously compares the "commanded position" with the "actual position reported by the encoder". If there is an error, it immediately adjusts the power supplied to the motor to eliminate that error.
Thanks to this mechanism, the servo motor achieves positioning at extremely high speed and extremely high accuracy. At the same time, even when an unexpected load appears, it automatically increases torque to hold the commanded position, so it is very tolerant of load fluctuation. It is a device indispensable in industrial robots and machine tools — where fast, powerful, and accurate motion is required.
Torque characteristics of the servo motor
The torque characteristics of a servo motor are the opposite of the stepping motor's. From low speed, through the rated speed, and into the higher-speed range, the servo motor can maintain large, stable torque across a wide band.
Thanks to this "flat" characteristic, it suits applications such as:
- Moving over long distances at high speed.
- Rotating fast while also needing large force, for example in cutting, milling, and grinding.
This makes it possible to perform dynamic motion very effectively.
Environmental resistance of the servo motor
The servo motor itself is mostly of brushless construction, and many models meet a high IP rating, so it is fair to say the motor's environmental resistance is quite high. However, when you look at the whole servo system, the most sensitive part is the encoder — the sensor that determines position.
The encoder contains precise optical components and electronic circuits, which are easily affected by:
- Strong vibration
- Mechanical shock
- Condensation
- Adhering oil and grease
These factors can lead to failure. Therefore, in a harsh environment, choose a model that uses a resolver (a magnetic sensor) with higher environmental resistance, or protect the motor with a suitable shielding solution.
Representative makers and products
The high-efficiency AC servo motor is a field where the leading FA companies compete to develop products.
Mitsubishi Electric Corporation Its general-purpose servo line MELSERVO is widely recognised as an industry standard. The new-generation MELSERVO-J5 achieves industry-leading response speed and integrates a high-resolution encoder, contributing to higher equipment productivity. URL: https://www.mitsubishielectric.co.jp/fa/products/drv/servo/
Yaskawa Electric Corporation The world's first company to commercialise a fully digital servo. Its Σ (Sigma) line is famous for high reliability. The new-generation Σ-X not only improves motion performance but also integrates a predictive-maintenance function based on sensor data. URL: https://www.yaskawa-global.com/product/servomotor
How to achieve high-precision positioning
When you want to achieve accurate positioning, the two main options usually used are the stepping motor and the servo motor. Which one to choose depends on the technical requirements of the equipment.
First, consider the cost and complexity of the system. The stepping motor works on the open-loop control principle, needing no feedback mechanism, so the whole system is simpler and lower in cost than a servo. If the load can be predicted in advance and there is little risk of step-out, the stepping motor is the cost-effective choice.
Conversely, a servo motor should be chosen when higher-level performance is required. For example:
- Continuous, high-frequency acceleration and deceleration.
- Applications where the load may fluctuate greatly during operation.
- Or situations where step-out is absolutely not allowed.
In those cases, a servo motor with closed-loop control delivers the reliability and response speed required.
Stepper vs servo – a quick comparison
- Control method: stepper — open-loop (no feedback); servo — closed-loop (encoder feedback).
- Positioning: stepper — accurate at low speed via pulse counting, but can step out under overload; servo — high accuracy maintained even under load fluctuation.
- Torque vs speed: stepper — high torque at low speed, falls off quickly at speed; servo — large, flat torque across a wide speed band.
- System cost and complexity: stepper — simple and low cost; servo — more complex and higher cost.
- Best fit: stepper — predictable loads, short accurate moves; servo — dynamic, high-frequency motion where step-out is unacceptable.
Open-loop and closed-loop control
Motor control methods split into two broad types:
Open-loop control The controller only sends commands one way to the motor and does not confirm them with feedback. The system is cheap and simple. The stepper is the representative example: send 100 pulses and it is assumed the motor has turned exactly 100 steps. But if an unexpected load makes the motor slip (step out), the controller cannot detect it.
Closed-loop control Uses a feedback signal from an encoder or sensor. The controller continuously compares the commanded value with the actual value and adjusts to eliminate the error. The servo motor is the classic example: it constantly monitors the "target" and the "actual position" to achieve high accuracy and reliability.
The role of the encoder in a servo motor
The encoder acts as the "eyes" of the servo system. Mounted on the motor shaft, it detects the rotation angle and speed and feeds them back to the driver as electrical signals. It is the encoder that makes closed-loop control possible.
There are two main types:
Incremental encoder
- Outputs pulse signals in proportion to the amount of rotation.
- Tells you the relative position from an origin point.
- Simple construction, low cost.
- Drawback: when the power is turned off, all current position information is lost, so a homing operation must be carried out at start-up.
Absolute encoder
- Outputs a signal unique to each rotation angle.
- Always knows the absolute position, even after a power loss.
- No homing operation needed at start-up.
- However, the construction is more complex and the cost is higher.
4. Motor selection and classification through real-world applications
Motors – the heart of the industrial robot in FA
In the field of factory automation (FA), the industrial robot is seen as the crystallisation of motor technology. A robot has many joints, like a human arm, and when each joint moves in coordination with high precision and high speed, it can carry out complex tasks such as welding or assembly.
To drive a robot's joints, high-efficiency AC servo motors are used almost without exception. The reason is that the load acting on each joint constantly changes with the posture of the robot arm, yet the robot must still follow the specified trajectory with absolute accuracy and at high speed. With its closed-loop control, the servo motor is highly tolerant of load fluctuation and responds quickly to control signals. It is fair to say that a robot's performance is almost entirely determined by the performance of the servo motors at its joints.
Motors in machine tools – the foundation of precision machining
CNC machine tools such as the machining center are the "mother machines" — they create precise parts out of blocks of metal. Here, two types of motor with different roles work together:
Axis drive motor: Used to move the table or the tool precisely along the X, Y, and Z axes. Because sub-micron positioning accuracy is required, AC servo motors are used as standard. The ability to hold an accurate position even under the large load fluctuation caused by cutting forces is essential.
Spindle motor: Used to rotate the cutting tool quickly. The dedicated type is the built-in spindle motor (a form of AC motor), designed to produce high torque and power stably across a wide range of rotational speeds.
Motors in conveying equipment – the pillar of logistics
In logistics centers and factory production lines, conveying equipment uses many different types of motor depending on the role:
For a belt conveyor, the requirement is continuous operation at a stable speed, prioritising durability and low cost. A simple, rugged AC induction motor is therefore the optimal choice.
By contrast, in equipment such as packaging machines and labelling machines, accurate positioning and intermittent operation are needed (for example, pulling film to the right length, or applying a label at the right position). In this case, a stepping motor is often used, because it can easily achieve high accuracy with open-loop control.
Motors in semiconductor production equipment – special requirements
Semiconductor production takes place in a clean room, where even the tiniest dust or vibration is not allowed. Therefore, in the robots that transport silicon wafers and in the exposure machines, special types of motor are used.
Instead of a motor with a gearhead — which can wear its gears (creating dust) and introduce mechanical backlash — the following are used:
Direct-drive (DD) motor
Linear motor
A linear motor produces linear motion directly by electromagnetic force, without going through a mechanism that converts rotation into translation. This removes mechanical error and achieves accuracy down to the nanometre scale, while still ensuring high speed.
Motors in next-generation vehicles: EVs and drones
The performance of an electric vehicle (EV) or a drone is directly affected by the motor it is fitted with:
EV: The drive motor must convert the energy from the battery into motion as efficiently as possible. For this reason, many models use a permanent-magnet synchronous motor (PMSM), which is compact and light while also achieving high efficiency and high power.
Drone: It needs an extremely light motor with instant responsiveness to control its flight attitude accurately. Therefore, almost all drones use a brushless DC motor (BLDC) — a compact, light, high-efficiency motor that helps extend flight time.
Motors in medical equipment – where lives are at stake
In medical equipment, especially machines that relate directly to a patient's life, the motor must have absolute reliability and safety.
For example, in a surgery-assist robot, where the doctor operates remotely, the arm joints use a DC servo motor (mainly the brushless type) capable of extremely precise control. They run smoothly even at low speed, without unwanted vibration, faithfully reproducing the delicate movements of the doctor and making possible operations that demand very high precision.
In addition, in a heart-lung machine or a blood pump, a motor that can run continuously for a long time with high durability is needed. A brushless DC motor is therefore used, thanks to its low risk of wear-related failure and its ability to control rotational speed stably, helping to sustain the patient's life.
5. Auxiliary equipment and additional knowledge needed when selecting a motor
Using a gearhead to amplify torque
When selecting a motor, there are cases where the motor alone cannot meet all the requirements. In particular, when you need large torque but not high rotational speed, a gearhead (reducer) is the solution.
A gearhead reduces the motor's rotational speed through gear pairs while amplifying the torque. For example, with a 10:1 reduction ratio, the rotational speed drops to one tenth but the torque increases roughly tenfold. This lets a compact, high-speed motor meet applications that call for low speed and large torque.
Another important role of the gearhead is to reduce the moment of inertia seen from the motor shaft down to the load, by the square of the reduction ratio. This is especially useful in servo-motor design, greatly improving the inertia ratio and raising the stability of the control system.
Converting DC and AC – the role of the inverter
An inverter can be understood simply as a device that converts and controls electric current. Its main function is to convert direct current (DC) into alternating current (AC). Thanks to the inverter, the range for selecting and controlling motors becomes wider and more flexible.
Understanding torque and power (kW)
During motor selection, engineers often have to convert and calculate between torque and motor power (kW). This is basic knowledge, but very useful when you need to compare and cross-check many motors from different makers to choose the best option.
Being aware of the motor's direction of rotation
Motors have symbols for the direction of rotation, such as CW (clockwise) and CCW (counter-clockwise). These are important terms at the design stage. Understanding them and using them consistently makes communication and coordination between engineers smoother and more accurate.
6. Summary of optimal motor selection and classification
This article has explained comprehensively how to select and classify motors in machine design. To choose the optimal motor, you first need to grasp the overall picture.
Motors are classified by energy source (AC/DC) and by control method (servo/step).
- AC motor: runs on the commercial mains, rugged; the representative example is the induction motor.
- DC motor: easy speed control; the brushless DC motor is the mainstream today.
The basic principle in selection is to work from three figures: torque, rotational speed, and inertia. A selection procedure based on accurate, systematic load calculation helps avoid mistakes.
For accurate positioning, the choice is usually a stepping motor or a servo motor.
- Stepping motor: open-loop control, simple and low cost.
- Servo motor: closed-loop control, achieving high accuracy and high responsiveness.
Which one to use depends on the balance between performance requirements and cost.
A motor's performance depends heavily on the driver (the motor controller). The encoder acts as the "eyes" of the servo system, ensuring accuracy. The gearhead is an indispensable part for amplifying torque and improving the inertia ratio.
Each field — FA, semiconductors, home appliances, automotive, medical, and so on — has its own suitable type of motor. Through the real-world examples of each industry, you can understand why that type of motor is chosen.
Motor selection is truly a central concern in design, deciding both the performance and the cost of the whole machine.
The end.
A note from MINATA
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