A linear stepper motor, also called a linear stepping motor, generates rotation through interaction between the magnetic rotor core and the pulsed electromagnetic field produced by the stator. It converts rotary motion into linear motion inside the motor. Linear stepper motors can directly produce linear or reciprocating linear motion. If a rotary motor is used as the power source, conversion to linear motion requires mechanisms such as gears, cams, belts, or steel wire. Linear stepper motors were first introduced in 1968. The figure below shows several typical linear stepper motors.

The result is a greatly simplified design, enabling the direct use of linear stepper motors for precision linear motion in many applications without the installation of external mechanical linkages.

Structure of external drive linear motor

The following figure shows the basic structure of an externally driven linear motor:

Compared with rotary motors, linear stepper motors can move directly in a straight line, which can make the machine smaller and make the design simpler.

Basic principles of externally driven linear motors

The rotor of an externally driven linear stepper motor is a permanent magnet. When current flows through the stator windings, they generate a vector magnetic field. This field rotates the rotor through a certain angle so that a pair of its magnetic poles aligns with the stator field. When the stator's vector magnetic field rotates through an angle, the rotor follows it through an angle. Each input electrical pulse rotates the rotor by a certain angle and advances it one step. The output angular displacement is proportional to the number of input pulses, and rotational speed is proportional to pulse frequency.

Change the order in which the windings are energized and the motor will reverse direction. Therefore, the rotation of the stepper motor can be controlled by controlling the number of pulses, frequency and the energization sequence of each phase winding of the motor.

The motor uses the screw as the output shaft, and an external drive nut is engaged with the screw outside the motor to prevent the screw and nut from rotating relative to each other, thereby achieving linear motion. The result is a greatly simplified design, enabling the direct use of linear stepper motors for precision linear motion in many applications without the installation of external mechanical linkages.

Advantages of externally driven linear motors

Precision linear screw stepper motors can replace cylinders in certain situations to achieve the advantages of precise positioning, controllable speed, and high precision. Linear screw stepper motors are widely used in many high-precision fields including manufacturing, precision calibration, precision fluid measurement, precise position movement, etc.

▲High precision, repeatable positioning accuracy can reach ±0.01mm

The linear screw stepper motor reduces the problem of interpolation lag due to its simple transmission mechanism. The positioning accuracy, repeatability, and absolute accuracy are generally higher than that of "rotary motor + screw" and are easier to implement. The repeatable positioning accuracy of the ordinary screw of the linear screw stepper motor can reach ±0.05mm, and the repeated positioning accuracy of the ball screw can reach ±0.01mm.

▲ High speed, up to 300m/min

From the comparison of speed and acceleration, the linear screw stepper motor has considerable advantages. The linear screw stepper motor speed can reach 300m/min, and the acceleration is 10g; the ball screw speed is 120m/min, and the acceleration is 1.5g. Moreover, the speed of the linear screw stepper motor will be further improved after the heating problem is successfully solved, and the "rotary servo motor & "Ball screw" is limited in speed and it is difficult to increase it much.

▲ Long service life and simple maintenance

Because there is an installation gap between the moving parts and the fixed parts, the linear screw stepper motor has no contact and will not be worn due to the high-speed reciprocating motion of the mover. There is no change in the motion positioning accuracy after long-term use, so it is suitable for high-precision occasions. Ball screws cannot ensure accuracy in high-speed reciprocating motion. Due to high-speed friction, the screw nut will be worn, affecting the accuracy requirements of the motion, and cannot meet high-precision requirements.

Selection of externally driven linear motors

When developing products or solutions related to linear motion, we recommend that engineers focus on the following points:

1.What is the load on the system?

The load of the system includes static load and dynamic load. The size of the load often determines the basic size of the motor.

Static load: the maximum thrust that the screw can withstand in a static state.

Dynamic load: the maximum thrust that the screw can withstand when it is in motion.

2.What is the linear running speed of the motor?

The running speed of the linear motor is closely related to the lead of the screw. When the screw rotates once, the nut advances one lead. For low speeds, a screw with a smaller lead should be selected, and for high speeds, a larger screw should be selected.

3.What are the accuracy requirements of the system?

Screw accuracy: Screw accuracy is generally measured by linear accuracy, that is, the error between the actual stroke and the theoretical stroke after the screw rotates hard for a circle.

Repeated positioning accuracy: Repeated positioning accuracy defines the accuracy of the system's ability to repeatedly reach a specified position. It is an important indicator for the system.

Backlash: The relative axial movement amount of the backlash screw and nut when they are stationary. As the working time increases, the backlash will also increase due to wear. Backlash compensation or correction can be achieved by anti-backlash nuts. When bidirectional positioning is required, backlash needs to be concerned.

4.Other options

You also need to consider the following issues when selecting: Does the installation of the linear stepper motor comply with the mechanical structure design? How would you connect the moving object to the nut? What is the effective stroke of the screw? What kind of driver does it match?

Article source: Shanghai MOONS

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