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Servomotors / Servo Drivers

Introduction Features
Principles Classifications
Engineering Data Further Information
Explanation of Terms Troubleshooting

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Primary Contents


Effective Torque

A value of the average torque (RMS) that is produced during operation of a motor.
A motor with a larger value than the effective torque must be chosen.
The unit is N・m.

Torque Constant

When a current flows to a motor, the current and the flux produce a torque.
The torque constant is the relationship between this current and the produced torque. The higher the torque, the smaller the controlling current.
The unit is N・m/A.

Power Rate

The power rate is given by this formula:
power rate = (Rated torque) 2/Rotor inertia x 10-3.
The higher the value is, the better the response is.
The unit is kW/s.

Rotor Inertia

The moment of inertia of the rotor, expressed in Jm.
The smaller the value is, the quicker the response is.
The unit is kg・m2.

Applicable Load Inertia

The range in which a drive can control the load inertia.
The range is limited by the gain adjustment range and the energy absorption capacity. The unit is kg・m2.

Rated Output

The rated output (P) is the mechanical power that a motor can output.
The rated torque (T) and the rated speed (N) are related to the rated power as follows:

Electrical Time Constant

The transient response time to the current that flows to the armature of a motor to which a power supply voltage is applied.
It is expressed by this formula: Electrical time constant = Armature inductance/Armature resistance.
Because a smaller value enables the current wave to rise more quickly, the transient response time to the current is faster.


The mechanical system has a dead zone between forward and reverse.
A gear that changes from forward to reverse must turn by the amount of the dead zone before turning the specified amount.
This movement is called the backlash.
backlash is given in minutes. One turn is 360 degrees. One minute is 1/60 of 1 degree.
The smaller the backlash is, the less the dead zone is.

Regeneration Resistance

A resistor that absorbs regenerative energy. regenerative energy is the energy generated by a motor when the motor operates.
A servo drive uses internal regenerative processing circuits to absorb the regenerative energy generated by a motor when the motor decelerates to prevent the DC voltage from increasing.
If the regenerative energy from the motor is too large, an overvoltage can occur.
To prevent overvoltages, the operation pattern must be changed to reduce the regenerative energy or an external regenerative resistor must be connected to increase the capacity to process regenerative energy.

Vibration Class

A class based on the value of the vibration measured at the shaft of a motor rotating at the rated speed without a load. There are five vibration classes into which the measured total amplitudes are divided.

Position Control Mode

A control mode in which positioning commands are input from a controller and positioning is controlled using the target values in the commands.

Closed Loop

A control method that compares the position commanded by the controller and the actual motor position.
An error signal is returned to the controller and used to give the system the correct position.
Closed-loop control can be performed based on the speed, acceleration, or torque in addition to the position.
The motion control method without using feedback is called open loop.

Open Loop

A control method in which the results of movement are not compared with the actuator reference.
When the controller commands the motor to move, it is assumed that the requested movement will be completed.

Control Loop

In process control, a control loop adjusts a target variable by adjusting other variables using feedback and error correction.
In motion control, control loops are set for speed, acceleration, position, or torque.


Realtime Autotuning

Realtime autotuning estimates the load inertia of the machine in realtime, and operates the machine by automatically setting the gain according to the estimated load inertia.
At the same time, it can lower the resonance and vibration if the adaptive filter is enabled.

Manual Tuning

A gain adjustment method used when autotuning cannot be performed due to the restrictions of the operating pattern or load conditions or when maximum responsiveness needs to be obtained for individual loads.

Notch Filter

A notch filter is used to eliminate a specified frequency component.
The notch filter can restrict a resonance peak, and it allows a high gain setting and vibration reduction.

Disturbance Observer Function

The effect of disturbance torque can be lowered, and vibration can be reduced by using the disturbance torque value.

Friction Torque Compensation Function

A function that reduces the influence of mechanical friction.

Hybrid Vibration Suppression Function

A function that suppresses the vibration that is caused by the amount of the torsion between the motor and the load.

Feed-forward Function

A function that increases the responsiveness of the control system by adding the feed-forward value to the command value.

Instantaneous Speed Observer Function

This function uses a load model to estimate the motor speed.
It improves the speed detection accuracy and can provide both high responsiveness and minimum vibration when stopping.

Safe Torque OFF Function

The safe torque OFF function (STO) is used to cut off the motor current and stop the motor through the input signals from a safety device, such as a safety controller or safety sensor.

Regenerative Energy Absorption

A servo drive absorbs regenerative energy internally with the built-in capacitor.
If the regenerative energy cannot be completely absorbed with the built-in capacitor, it is absorbed with the internal regeneration resistor.

Regenerative Energy

Power produced by a motor for a generator.
The regenerative energy is produced by the external forces or gravity during Servomotor deceleration.
In this case, measures for design must be taken to keep the energy within the energy absorption capacity.

Forward and Reverse Drive Prohibit

A function that prevents the servomotor from rotating outside of the operating range of the device by connecting limit inputs.
When the Forward Drive Prohibit Input or Reverse Drive Prohibit Input turns OFF, the Servomotor will stop rotating.

Damping Control

A function used to reduce vibration when using a low-rigidity mechanism or equipment whose ends tend to vibrate.

Internally Set Speed Control

A function that controls the speed of the servomotor using speeds set in the internal speed setting parameters.

Electronic Gear

A function that rotates the servomotor for the number of pulses obtained by multiplying the command pulses by the electronic gear ratio.
The electronic gear is used to synchronize the position and speed of two lines, to enable using a position controller with a low command pulse frequency or to set the machine travel distance per pulse, to 0.01 mm for example.

Torque Limit

A function that limits the output torque of a motor.
The torque limit is used for pressing a moving part of a machine (such as a bending machine) against a workpiece with a constant force, or for protecting the servomotor and mechanical system from excessive force or torque.

Position Command Filter

A function that performs soft start processing for the command pulses using the selected filter to gently accelerate and decelerate.
The filter characteristics for the position command filter are selected using the position command filter Time Constant Setting.
This function is effective when there is no acceleration or deceleration function in the command pulse (controller), when the command pulse frequency changes abruptly, causing the achinery to vibrate during acceleration and deceleration, or when the electronic gear setting is high.

Position Loop Gain

Servo systems with a low loop gain have a low response and can increase the positioning time.
The higher the position loop gain, the shorter the positioning time. If the setting is too high, however, overshooting or hunting may occur in the system.

Incremental Command

An incremental command determines the travel amount between the present position and the target position.

Absolute Command

An absolute command determines the travel amount from a command value that is based upon the origin.
Thus the command value is different from the travel amount unless the motor is at the origin.

Error Counter

An up/down binary counter that counts the difference between the position command pulses and the position feedback pulses. is converted by an D/A (digital/analog) converter and becomes the speed command voltage.
The accumulated pulses is converted to an analog voltage by an D/A (digital/analog) converter and becomes the speed command voltage.

Absolute Position

Position information that fully describes a position within a space without referencing a previous position.

Absolute Positioning

Directly moving devices or materials to a specific position in a space without referencing the previous position.

Positioning Completion Signal

A signal that occurs when positioning is completed.
This signal turns ON when the following error is within the inposition range set in the parameter.
This signal is primarily used to start any of the following operations after positioning.
This signal is also called the in-position signal (INP).

Motor with Brake

A motor with an electromagnetic brake.

Brake Interlock

A function that sets the output timing for the brake interlock output (BKIR) signal that activates the holding brake when the servo is turned ON, when an alarm occurs, or when the servo is turned OFF.
The output timing is set in the parameter when a motor with a brake is used.
A holding brake is used in applications, such as for a vertical axis, to prevent the workpiece from falling.

Dynamic Brake (DB)

A brake that converts the rotational energy into heat by shortcircuiting the terminals of the servomotor through a resistor to quickly stop the motor when a power is interrupted or a servo amplifier failure occurs.
Larger brake torque can be obtained than with an electromagnetic brake.
However, there is no holding torque when the motor is stopped, so a mechanical brake must be applied to hold the motor.
Dynamic brake is used for mechanical protection.

Free Run

A status in which a motor continues to rotate due to its inertia when servo is turned OFF.

Immediate Stop Torque

When an error is detected, the motor is stopped with the torque set in the parameter.


Servomotor with Absolute Encoder

A servomotor with an absolute encoder has an encoder in which a disk rotates to tell the servomotor the position when the power is turned ON.
A servomotor with absolute encoder that is used in an industrial robot or multi-axis transfer system needs to know the position when the power is turned ON to continue operation quickly after a power interruption or to prevent mistakes in operation.
A servomotor with an absolute encoder needs a backup battery for operation.

Servomotor with Incremental Encoder

A servomotor with an incremental encoder does not know the position when the power is turned ON.
Instead, it needs to perform an origin search to enable positioning.

Encoder Dividing

A function that sets the number of pulses for the encoder signals output from the servo drive.
Encoder dividing is used for a controller with a low response frequency or for setting a pulse rate that is easily divisible.


A device that is a structural unit of a servo system and is used with a servo drive.
The servomotor includes the motor that drives the load and a position detection component, such as an encoder.

Servo Drive

A device that is a structural unit of a servo system and is used with a servomotor.
The servo drive controls the servomotor according to instructions from a PLC or other controller and performs feedback control with signals from an encoder or other component.


A power transmission mechanism that decreases motor speed and increases torque.
If the reduction ratio is 1/R and the decelerator efficiency is η, the speed will be 1/R, the torque R×η, and the load inertia 1/R2.

Winding Resistance

The line resistance of a coil.


A device that generates mechanical motion using air pressure, water pressure, or electricity.
Industrial actuators are commonly driven by electric motors.

Ball Screw

One of the lead screws.
The threads of the screw are pulled with ball bearings in a carriage.
Its high mechanical efficiency and low energy consumption result in high rigidity and high reliability.
Ball screws are mainly used in high-speed and high-precision machines.

Rack and Pinion

A device that converts rotary motion into linear motion.
Normally a rack and pinion is composed of a gearwheel (pinion) and a flat toothed bar (rack).

Shaft Bearing

A part that supports a shaft that rotates or performs reciprocating operation.


A part that is used to connect shafts together.

Timing Belt

A power transmission mechanism that converts rotary motion into linear motion in conjunction with pulleys.
If the pulley diameter is D, the travel distance per rotation is πD.
Timing belts are usually toothed belts that mesh with pulleys to prevent slipping.


A rotary part that transmits rotary motion to a belt.


A machine part that fits between stationary parts and rotating parts to support the rotating parts

Synchronous Motor and Induction Motor

Synchronous Motor:
A motor that has magnetic poles in the motor rotor and moves synchronously with the behavior of the magnetic field.
Induction Motor:
A motor whose movement is delayed in respect to the behavior of the magnetic field.
The rotor is constructed of a non-magnetic material, such as aluminum or copper. A magnetic field created in the stator induces a current in the rotor. Rotation of the rotor results from the interaction of the magnetic field created by the rotor
current with the magnetic field of the stator.


The property of an object to retain its original shape when an external force is applied.
The higher the stiffness, the higher the ability of an object to retain its original shape.
The lower the stiffness, the more easily an object is stretched or compressed by an external force.


The property of an object to maintain its current state of motion.
Inertia is dependent on an object’s mass, shape, and axis of movement.