Technical Explanation for Servomotors and Servo Drives CSM_Servo_TG_E_1_1 Introduction Sensors What Is a Servomotor and What Is a Servo Drive? A servomotor 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. The servo system vary the controlled amount, such as position, speed, or torque, according to the set target value (command value) to precisely control the machine operation. Switches Safety Components Relays Control Components Automation Systems Motion / Drives Servo System Configuration Example (1) Command section (2) Control section (3) Drive and detection section Outputs command Controls the motor Drives the controlled object and signals for operation. according to commands. detects that object. Servomotor Power transmission Encoder mechanism Target values Motor power signals Ball screw Table Feedback signals Feedback signals Controller Servo drive Servomotor Energy Conservation Support / Environment Measure Equipment Power Supplies / In Addition Others Common 1 Technical Explanation for Servomotors and Servo Drives Features Sensors Precise, High-speed Control • Servomotors excel at position and speed control. • Precise and flexible positioning is possible. • Servomotors do not stall even at high speeds. Deviations due to large external forces are corrected because encoders are used to monitor movement. Switches Safety Components Relays Control Components Automation Systems Motion / Drives Fully-closed Loop Open Loop The most reliable form of closed loop. A fully-closed loop is A stepper motor is used instead of a servomotor. There is no used when high precision is required. feedback loop. The motor is controlled while directly reading the position of The overall configuration is simple. Positioning can be the machine (workpiece or table) using a linear encoder and performed at low cost, but gear and ball screw backlash and comparing the read position with the command value (target pitch errors cannot be compensated. When a stepper motor value). Therefore, there is no need to compensate for gear stalls, an error will occur between the command value and the backlash between the motor and mechanical system, feed actual movement. This error cannot be compensated. screw pitch error, or error due to feed screw torsion or Open loop control is suitable for low-precision, low-cost, low- expansion. speed, and low-load-change applications. Fully-closed Loop System Configuration Example Open Loop System Configuration Example Servomotor Stepper motor Power transmissi Ball screw mechanism Table Ball screw Table Controller Stepper motor drive Controller Servo drive Linear encoder Semi-closed Loop This method is commonly used in servo systems. It is faster and has better positioning precision than an open loop. Typically an encoder or other detector is attached behind the motor. The encoder detects the rotation angle of a feed screw (ball screw) and provides it as feedback of the machine (workpiece or table) travel position. This means that the position of the machine is not detected directly. Energy Conservation Support / The characteristics depend on where the detector is installed. Environment Measure Equipment Opposite of Installation location of Motor side of Behind motor motor side of detector feed screw feed screw Compensation Compensation Gear backlash required not required Ball screw or nut torsion Affected Hardly affected Ball screw expansion or Power Supplies / Affected In Addition contraction Compensation Ball screw pitch error required Semi-closed Loop System Configuration Example Servomotor Others Ball screw Table Controller Servo drive Common 2 Technical Explanation for Servomotors and Servo Drives Principles Sensors Servo Operation and Configuration Encoder A system built with servo drives and servomotors controls Servomotors are different from typical motors in that they motor operation in closed loop. The actual position, speed, or have encoders. This allows high-speed and high-precision torque of the servomotor is fed back to compare to the control according to the given position and speed commands. command value and calculate the following errors between Encoders are one of the hardware elements that form the core them. Then the servo drive corrects the operation of the of a servo system, and they generate speed and position Switches Safety Components Relays Control Components Automation Systems Motion / Drives servomotor in realtime using this error information to ensure feedback. In many cases, the encoder is built into the that the system can achieve the required performance. This servomotor or attached to the servomotor. In certain cycle of feedback, error detection, and correction is called applications, the encoder is an independent unit that is closed-loop control. installed away from the servomotor. When the encoder is The control loop is processed by either of servo drive or installed in a remote location, it is used for related parameters motion controller, or both depending on the required control. in addition to for control of servomotor operation. The control loops for position, speed, and torque are Encoders are divided into two kinds. independently used to achieve the required operation. • Incremental encoders Applications will not always require all three control loops. In • Absolute encoders some applications, only the control loop for torque control will Multi-turn absolute encoders are typically used for be required. In other applications, current and speed for servomotors. speed control are required, and in still other applications, Refer to the Technical Explanation for Rotary Encoders for three control loops for position control are required. more information on encoders. Torque control Speed loop gain Servo Drive Speed loop integral time constant Speed control Servo drives follow commands from the host controller and Control Error/ Error speed Drive control the output torque, rotation speed, or position of section Oscillator counter conversion Motor motors. The position, speed, or torque are controlled according to inputs from a motion controller, feedback encoder, and the Frequency/ servomotor itself, and the servo drive supplies the appropriate Position Current control speed amounts of power to the servomotor at the appropriate times. Position conversion feedback Speed Position loop gain command feedback Speed The basic operating principle is the same as for an inverter, in command Multiplication which the motor is operated by converting AC power to DC Position feedback Encoder power to be a certain frequency. Servomotor Fixed frequency Required frequency The most common types of industrial servomotors are those (50/60 Hz) (0 to 400 Hz) based on brushless motors. The rotor has a powerful Converter Smoothing Inverter permanent magnet. The stator is composed of multiple section circuit section section conductor coils, and the rotor spins when the coils are powered in the specified order. The movement of the rotor is Energy Conservation Support / determined by the stator’s frequency, phase, polarity, and Motor Environment Measure Equipment current when the correct current is supplied to the stator coils at the appropriate time. PWM Speed, Power Supplies / position, Control circuits In Addition or torque Encoder Encoder Motor shaft Servo drive Case A servo drive also has the following functions. • Communications with the motion controller • Encoder feedback reading and realtime closed-loop control Others adjustment • I/O processing for safety components, mode inputs, and Rotor Permanent Stator coils operating status output signals magnet Common 3 Technical Explanation for Servomotors and Servo Drives Explanation of Terms Performance Sensors Effective Torque Regeneration Resistance A value of the average torque (RMS) that is produced during A resistor that absorbs regenerative energy. Regenerative operation of a motor. energy is the energy generated by a motor when the motor A motor with a larger value than the effective torque must be operates. chosen. A servo drive uses internal regenerative processing circuits to The unit is N·m. absorb the regenerative energy generated by a motor when Switches Safety Components Relays Control Components Automation Systems Motion / Drives the motor decelerates to prevent the DC voltage from Torque Constant increasing. When a current flows to a motor, the current and the flux If the regenerative energy from the motor is too large, an produce a torque. overvoltage can occur. The torque constant is the relationship between this current To prevent overvoltages, the operation pattern must be and the produced torque. The higher the torque, the smaller changed to reduce the regenerative energy or an external the controlling current. regenerative resistor must be connected to increase the The unit is N·m/A. capacity to process regenerative energy. Power Rate Vibration Class The power rate is given by this formula: A class based on the value of the vibration measured at the 2 -3 Power rate = (Rated torque) /Rotor inertia x 10 . shaft of a motor rotating at the rated speed without a load. The higher the value is, the better the response is. There are five vibration classes into which the measured total The unit is kW/s. amplitudes are divided. Rotor Inertia Position Control Mode The moment of inertia of the rotor, expressed in Jm. A control mode in which positioning commands are input from The smaller the value is, the quicker the