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Why use a stepper gearbox motor​?

Have already visited: 146405/25/2020  

Why use a stepper gearbox motor?

Stepping motors are known for their precise positioning capabilities and high torque output at low speeds, but they need to be carefully dimensioned to ensure that the motor matches the load and application parameters to minimize the possibility of out of step or motor stall. Adding gearbox in a stepping motor system can improve motor performance by reducing the inertia ratio of load and motor, increasing load torque and reducing motor oscillation.

Reduce the motor load inertia ratio

One reason for the lack of step distance in the application of the stepping motor is inertia. The ratio of load inertia to motor inertia determines how the motor can drive or control the load, especially in the acceleration and deceleration part of the motion curve. If the load inertia is significantly higher than the motor inertia, it will be difficult for the motor to control the load, and overshoot (Overspeed steps out of the command range) or undershoot (undershoot steps) may occur. A high load inertia ratio can also cause the motor to draw too much current and stall.

J L = load inertia

J M = motor inertia

One way to reduce the inertia ratio is to use a motor with larger inertia. But this means higher costs, more weight, and trickle action on other parts of the system, such as couplings, cables, and drive assemblies. Instead, a gearbox is added to the system, which reduces the square of the gear ratio by the inertia ratio of the load to the motor.

I = gear reduction

Increase load torque

stepper gearbox motor

Another reason to use the stepper gearbox motor is to increase the torque that can be used to drive the load. When the load is driven by a motor gearbox combination, the gearbox multiplies the torque of the motor by an amount proportional to the gear ratio and gearbox efficiency.

T o = torque output on the transmission shaft

TM = torque output on motor shaft

η = transmission efficiency

However, despite the increased torque in the transmission, they reduce speed. (that's why they are sometimes called "gear reducers" or "reducers.". )In other words, when the gearbox is connected to the motor, the motor must rotate faster - equal to the gear ratio - to transmit the target speed to the load.

N o = speed output on the transmission shaft

N m = speed output on motor shaft

Moreover, due to braking torque and other losses, the torque of the stepping motor usually decreases rapidly with the increase of speed. This inverse relationship between speed and torque means that it is only feasible to increase the speed by a certain amount before the motor can not transmit the required torque (even if multiplied by the gear ratio).

Note that the continuous torque (green) of the stepper motor decreases rapidly as the speed increases, unlike the continuous torque (blue) of the servo motor, which is relatively flat even at high motor speeds.

Reduce resonance and vibration

But accelerating the motor does help. When the gearbox is installed, the additional speed required by the motor means that the motor operates outside its resonance frequency range, in which oscillation and vibration will cause the motor to lose a step or even stall.

A stepper gearbox motor (top) shows significantly less vibration than a gearless stepper motor (bottom), especially at low speeds in the resonant frequency range. In addition to ensuring that the gearbox has the correct torque, speed and inertia values, it is also important to choose a high accuracy, low backlash gearbox - especially when connecting the gearbox to the stepper motor. Step motor operates in the open-loop system, and the clearance in gearbox reduces the positioning accuracy of the system, without feedback to monitor or correct the positioning error. This is why stepping applications often use high-precision planetary gearboxes with clearances as low as 2 to 3 minutes of arc. Some manufacturers offer stepper motors with harmonic gears that can achieve zero backlashes in most applications.

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