Servo drive

A servo drive is a drive with electronic position, speed or torque control (or a combination of these) with high to very high demands on the dynamics , the setting ranges and / or the accuracy of the movement. Servo drives are often used in machines in the manufacturing industry (e.g. in machine tools ) and in automation solutions ( packaging machines , industrial robots ).

commitment

Their use is characterized by the fact that they are often operated with strong speed and torque changes as well as high overload and holding torque at standstill. They are often used to move machine parts, such as grippers or robot arms. For applications with continuous rotation, the focus is on angle synchronization, e.g. B. in the various ink rollers of a printing machine.

Components

Servo axes generally consist of the main components

• Servo converter with
  • Power electronics
  • Control electronics: regulation, setpoint generation, monitoring
• Servo motor with
  • Active part with winding, iron and magnets for generating torque
  • Sensor / measuring devices for angle and speed feedback
  • Brake to hold the position at a standstill
• Gearbox for speed and torque conversion

The components form a coordinated mechatronic system that works together as a functional unit.

construction

Servo drives consist of a servo motor , the servo converter with power electronics and control and, if necessary, gears for speed adjustment or for converting the rotary movement into a linear movement. The servo converter supplies the servomotor with the current required for movement. In addition to the power electronics, the servo converter also contains a highly dynamic control for current, speed and position. The servo converter also includes evaluation electronics for the position encoder of the motor and an interface for data transmission / communication with the machine control.

Basically, monitoring devices against short circuit, overload or excess temperature are part of the equipment of a servo converter. The servo controller often also offers a certain range of control functions for motion control and for controlling part of the machine. Compared to standard solutions, the mechanical components of the gearbox for speed adjustment or for converting it into linear motion are usually designed with less play and higher load capacity to transmit the strong torque changes.

The servo motor for rotary servo drives is a synchronous or asynchronous motor with a usually slim design and high overload capacity. A high acceleration capacity is thus achieved. The servomotor has an integrated angle encoder to feed back the speed and position.

The control electronics of the servo device determine the angle of rotation and the speed of the motor from the signals from the angle encoder. An integrated brake in the motor is used to maintain the position of the motor even when the power is off, e.g. B. after switching off the machine, so that especially vertical axes remain in their position. Linear motors are also used for particularly precise linear movements. For servo applications, these are usually short stator motors with permanent magnet excitation.

Applications

Servo drives are used for highly dynamic and precise movements in modern production machines. Today they are used in a wide range of applications in handling and automation technology. They drive robots in the automotive industry, position portals in furniture production, wrap wires or cut and portion fish fillets . The servo drives ensure a high production speed through fast and precise movements in accordance with the control specifications.

Typical areas of application are positioning drives, coordinated drives, synchronous drives, cross cutters, flying saws and electronic cams.

Characteristic values ​​for the dynamic behavior

Summary

Servo drives are characterized by high dynamics and accuracy. The following table lists the minimum requirements for rotating servo drives in the torque range up to 50 Nm. The meaning of the characteristic values ​​is explained below. The values ​​in the table can be transferred to linear movements.

Parameter	Expression	Requirement for servo drives
Overload capacity	${\displaystyle c={\frac {M_{\text{max}}}{M_{\text{0}}}}}$	${\displaystyle c\geq 3}$
Cycle time setpoints and position control		${\displaystyle T_{{\text{C set}}\varphi }\leq 1{\text{ms}}}$
Closed loop bandwidth	${\displaystyle f_{\text{-3 dB I}}\approx {\frac {1}{2{,}5\cdot T_{\text{RI}}}}...{\frac {1}{2\cdot T_{\text{RI}}}}}$	${\displaystyle T_{\text{RI}}\leq 1{\text{ms}}}$, ${\displaystyle f_{\text{-3 dB I}}\geq 400{\text{Hz}}}$
Closed loop speed control bandwidth	${\displaystyle f_{\text{-3 dB n}}\approx {\frac {1}{2{,}5\cdot T_{\text{Rn}}}}...{\frac {1}{2\cdot T_{\text{Rn}}}}}$	${\displaystyle T_{\text{Rn}}\leq 3{\text{ms}}}$, ${\displaystyle f_{\text{-3 dB n}}\geq 150{\text{Hz}}}$
Bandwidth closed position control loop	${\displaystyle f_{{\text{-3 dB}}\varphi }\approx {\frac {1}{2{,}5\cdot T_{{\text{R}}\varphi }}}...{\frac {1}{2\cdot T_{{\text{R}}\varphi }}}}$	${\displaystyle T_{{\text{R}}\varphi }\leq 10{\text{ms}}}$, ${\displaystyle f_{{\text{-3 dB}}\varphi }\geq 50{\text{Hz}}}$
Speed ​​control loop performance	${\displaystyle T_{\text{min}}={\frac {2\pi \cdot n_{\text{N}}\cdot J_{\text{m}}}{M_{\text{max}}}}}$, ${\displaystyle f_{\text{P n}}={\frac {1}{2\pi \cdot T_{\text{min}}}}}$	${\displaystyle T_{\text{min}}\leq 15{\text{ms}}}$, ${\displaystyle f_{\text{P n}}\geq 10{\text{Hz}}}$

Notes:  : Maximum torque  : holding torque  : Engine mass moment of inertia  : Rated speed ${\displaystyle M_{\text{max}}}$ ${\displaystyle M_{\text{0}}}$ ${\displaystyle J_{\text{m}}}$ ${\displaystyle n_{\text{N}}}$

Explanation

Servo drives are made for movements with high precision and dynamics. The quality of a servo drive is therefore described by characteristic values ​​for the dynamic behavior and the precision for the mechanical variables of torque, speed and position. The following illustration focuses on rotating drives. Linear actuators are treated in the same way. Applications for servo drives are characterized by the cycle time of the machining process and the required accuracy of the movement.

This leads to the following questions, which should be answered by suitable characteristic values:

• Is the servo drive able to follow the setpoints of the motion control with a defined accuracy?
• Can the servo drive provide the power to accelerate and position the machine with the required cycle time?

With a view to dynamic movements, the behavior is described by the small-signal behavior. In order to achieve fast dynamic response times of the drive, short cycle times are required for the setpoint specification and the control loops.

Ultimately, a setpoint cycle time is:

${\displaystyle T_{{\text{C set}}\varphi }\leq 1{\text{ms}}}$ required.

At the same time, the position control loop must:

${\displaystyle T_{{\text{C}}\varphi }\leq 1{\text{ms}}}$ work.

The following applies to the maximum cycle time of the speed controller:

${\displaystyle T_{\text{C n}}\leq 0{,}25{\text{ms}}}$ .

According to IEC 61800-4, the dynamic behavior is determined by the characteristic values

Response time, rise time and settling time are described.

The response time

${\displaystyle T_{\text{R}}}$

is a particularly good size to use.

For specific drives, the time for the current control loop is less than 0.7 ms.

The minimum requirement is:

${\displaystyle T_{\text{RI}}\leq 1{\text{ms}}}$ .

The response time is closely linked to the control range. The bandwidth is the frequency range in which the gain and the phase response are within the limits of ± 3dB and ± 90 °.

The bandwidth can roughly be derived from the response time:

${\displaystyle T_{\text{R}}}$ according to the equation

${\displaystyle f_{\text{-3 dB}}\approx {\frac {1}{2{,}5\cdot T_{\text{R}}}}...{\frac {1}{2\cdot T_{\text{R}}}}}$

can be determined if the response time is for an increase to 90% of the final value. The equation is based on the PT2 behavior of the closed control loop with a large and a small time constant. The bandwidth can also be roughly determined from the parameters of the closed control loop.

Ultimately, only the total inertia of the motor and machine are:

${\displaystyle J_{\text{g}}}$ as well as the proportional gain ${\displaystyle K_{\text{Pn}}}$

necessary.

If the integral component of the controller is neglected, the following equations roughly describe the relationships in the control loop:

• Torque: ${\displaystyle M_{\text{m}}=K_{\text{Pn}}\cdot (n_{\text{set}}-n_{\text{i}})}$

• Mechanics: ${\displaystyle 2\pi \cdot n_{\text{m}}={\frac {1}{s}}{\frac {M_{\text{m}}}{J_{\text{g}}}}}$

• Closed loop: ${\displaystyle G={\frac {n_{\text{i}}}{n_{\text{set}}}}={\frac {1}{1+{\frac {J_{\text{g}}}{K_{\text{Pn}}}}s}}}$

• Bandwidth for the closed loop: ; ${\displaystyle f_{\text{-3dB}}={\frac {1}{2\pi }}{\frac {K_{\text{Pn}}}{J_{\text{g}}}}}$ ${\displaystyle \omega _{\text{-3dB}}={\frac {K_{\text{Pn}}}{J_{\text{g}}}}}$

A bandwidth of at least 150 Hz is required for the speed controller for a dynamic drive. The position controller requires a bandwidth of at least 50 Hz. These values ​​apply to a drive that works without manipulated variable restrictions. This means that neither maximum current nor maximum torque, maximum speed or maximum voltage are reached during the movement.

The limitation of the current, the torque or the speed leads to the performance bandwidth. The power bandwidth describes the drive's ability to transmit power to the machine for a sinusoidal movement. In order to transmit power at a sinusoidal speed, both real and reactive power, the drive must work below the manipulated variable limits.

The manipulated variable limits are the maximum torque:

${\displaystyle M_{\text{max}}}$,

the maximum speed:

${\displaystyle n_{\text{max}}}$

and the maximum voltage that affects the maximum torque slope:

${\displaystyle w=\max({\dot {M}})=\max \left({\frac {\mathrm {d} M}{\mathrm {d} t}}\right)=\left({\frac {M_{\text{max}}}{T_{\text{max}}}}\right)}$

with the rise time: ${\displaystyle T_{\text{max}}}$

for the torque increase from 0 to maximum torque: leads. ${\displaystyle M_{\text{max}}}$

The maximum power that can be exchanged between the motor and the machine is calculated as follows . ${\displaystyle {\hat {S}}_{\text{W}}={\hat {\omega }}_{\text{W}}\cdot {\hat {M}}_{\text{W}}=2\pi \cdot {\hat {n}}_{\text{W}}\cdot {\hat {M}}_{\text{W}}}$

The torque must be sinusoidal and is limited by the maximum torque and the maximum torque slope:

${\displaystyle M_{\text{m}}={\hat {M}}_{\text{m}}\cdot \sin(\omega \cdot t)}$

With:

${\displaystyle {\hat {M}}_{\text{m}}=\min \left({\frac {w}{\omega }};M_{\text{max}}\right)=\min \left({\frac {M_{\text{max}}}{\omega \cdot T_{\text{max}}}};M_{\text{max}}\right)}$

Together with the inertia of the machine and the motor, the maximum acceleration and the maximum speed are defined. The greatest power can be exchanged between the machine and the motor if the inertia of the machine is the same as the inertia of the motor.

This results in the maximum acceleration:

${\displaystyle {\hat {\alpha }}={\frac {{\hat {M}}_{\text{m}}}{2\cdot J}}}$ and the maximum speed . ${\displaystyle {\hat {\omega }}_{\text{W}}=\min \left(\omega _{\text{max}};{\frac {\hat {\alpha }}{\omega }}\right)}$

The torque for the machine results from the speed and the torque calculated in this way . ${\displaystyle {\hat {\omega }}_{\text{W}}}$ ${\displaystyle {\hat {M}}_{\text{m}}}$ ${\displaystyle {\hat {M}}_{\text{w}}={\hat {M}}_{\text{m}}-{\hat {\alpha }}_{\text{w}}\cdot J_{\text{m}}={\hat {M}}_{\text{m}}-\omega \cdot {\hat {\omega }}_{\text{w}}\cdot J_{\text{m}}}$

The maximum power for low frequencies is . ${\displaystyle \omega \approx 0}$ ${\displaystyle S_{\text{w max}}={\hat {\omega }}_{\text{w}}\cdot M_{\text{max}}=2\pi \cdot n_{\text{max}}\cdot M_{\text{max}}}$

The power decreases with increasing frequency . The power bandwidth is given by the frequency at which the power is half the maximum power (−3 dB). ${\displaystyle {\hat {S}}_{\text{W}}={\hat {\omega }}_{\text{W}}\cdot {\hat {M}}_{\text{W}}}$ ${\displaystyle \omega _{\text{Pn}}=2\pi \cdot f_{\text{Pn}}}$ ${\displaystyle {\hat {S}}_{\text{W}}}$ ${\displaystyle S_{\text{w max}}}$

The performance range is calculated from the above expressions for and to ${\displaystyle {\hat {\omega }}_{\text{W}}}$ ${\displaystyle {\hat {M}}_{\text{W}}}$

${\displaystyle \omega _{\text{Pn}}=2\pi \cdot f_{\text{Pn}}={\frac {1}{T_{\text{min}}}}}$, with . ${\displaystyle f_{\text{Pn}}={\frac {1}{2\pi \cdot T_{\text{min}}}}}$ ${\displaystyle T_{\text{min}}={\frac {2\pi \cdot n_{\text{max}}\cdot J}{M_{\text{max}}}}}$

For servo drives applies or . ${\displaystyle T_{\text{min}}\leq 15{\text{ms}}}$ ${\displaystyle f_{\text{Pn}}\geq 10{\text{Hz}}}$

See also

• Servo
• Servo motor
• Position control

literature

• Manfred Schulze: Electric servo drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich 2008, ISBN 978-3-446-41459-4 .

Individual evidence

1. ↑ Servo drives  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
2. ↑ IEC 61800-4 chapter 7.2 dynamic performance