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A servo drive is an electronic device that is an integral part of a closed-loop servo system, in which it receives command signals and compares them with the feedback signal provided by the servomechanism to produce the voltage and current required to rotate a servo motor and correct any deviations from the set status. Thus, for a better understanding of what a servo drive is, let’s discuss its role in a servo system.
A servo system is an automated closed-loop motion control system, which instead of controlling devices by applying variable input signals, controls them using a feedback signal generated by comparing the reference input signals and the output signals from the device being controlled. A typical servo system includes four main components, namely:
Servo Motor: This is the muscle of the servo system, as it produces the torque and velocity required to accelerate and move the connected load based on the supplied voltage and current or as commanded by the servo controller. Most servo motors normally rotate only 90° in either direction for a full 180° range of movement.
Servo motors are available in three basic types, namely Linear, Continuous Rotation, and Positional Rotation. But they come in a variety of technologies including AC brushless designs, brushed AC or DC permanent magnet motors, etc.
Servo Drive: This is part of the brain of the servo system. It acquires a command signal for current, velocity, or position from the servo controller and amplifies or adjusts it to deliver a definite amount of current and voltage to the servo motor in order to achieve the required motion. A servo drive also compares the servo system’s closed-loop feedback data and command reference signal to verify that the servo motor is rotating as commanded. The command reference signal can be supplied from a variety of sources, but it’s mainly supplied from a Computer Numerical Control (CNC), Programmable Logic Controller (PLC), or motion controller.
Simple servo drives mainly control current to produce the necessary torque for operating the servo motor as required, but higher-level servo drives provide additional features for control of torque and/or speed, and they can be configured for position control with additional programming capabilities.
Servo Controller: This is the brain of the servo system. It makes use of a programming environment to enable a variety of control options for machine control, operation of outputs and inputs and is connected to a Human Machine Interface (HMI) to allow users to interact with the servo system. Servo controllers in a servo system can be integrated with the servo drives or used as stand-alone units.
Essentially, a servo controller functions as the link between the system’s servo motor and a high-level control such as Computer Numerical Control (CNC), Programmable Logic Controller (PLC), or motion controller. It adapts the original command signal from the high-level control and forwards it to the connected servo drive. This way, it enables extremely precise control of the servo motor’s torque, speed, and position.
It also regulates the current draw used to drive the servo motor, as power is constantly applied to servo motors in a servo system. More specifically, the servo controller is responsible for calculating the motion trajectory required of the servo motor and sending low-voltage command signals to the servo drive for controlling the connected servo motor.
Feedback Device: The feedback element in a servo system can be a linear transducer, potentiometer, encoder, resolver, tachometer, Hall Effect sensor, or even a vision system (a more advanced form of feedback). It’s normally integrated into the servo motor, or used as an independent unit that’s installed in a remote location away from the servo motor.
The role of the feedback element is to evaluate the relation between the control input and the actual position of the servo motor. It then sends this relation as a feedback signal to the servo drive via its own control loop and/or to the servo controller. Thus, feedback elements in servo systems generate feedback signals for the speed, torque, and position of the servo motor.
Cabling and Communication: The servo system elements are interconnected so the communication, feedback, and power cabling become the nervous system that connects together the servo controller, servo drive, servo motor, and the feedback device. Communication to and from the servo controller and the other elements of the servo system can be via a simple analog and digital I/O or through digital Fieldbus communications such as CANopen, EtherCAT, and many other industry standard communication protocols.
To better understand how a servo drive works, it’s important to first understand the working principle of the complete servo system.
Any automatic motion control system built with servo motors, servo drives, and servo controllers regulates motor operation in a closed loop. It receives feedback on the actual torque output, speed, or position of the servo motor; next, it compares the received value to the command value and computes the subsequent errors between the two values. The servo drive then uses this error information to correct the operating parameters of the servo motor in real-time, to make sure that the servo system can attain the required performance. This cycle of feedback detection, error computation, and error correction is known as closed-loop control, which is an operating characteristic of servo systems.
In closed-loop servo systems, either the servo controller or servo drive processes the control loop or both of them based on the required control. The control loops for torque, speed/velocity, and position are used independently to attain the required servo motor operation. However, not all servo applications require the three control loops altogether. For example, some servo applications will only require the torque control loop. While other applications will require a current and velocity control loop for motor speed control, but still in other applications, all three control loops are required to control the position of a servo motor.
A servo drive starts operating on receiving command signals from the host controller to control the rotation speed, output torque, or position of a servo motor. It controls the motor torque, speed, or position according to the input signals from the servo controller, feedback device (i.e. encoder, resolver, tachometer, etc.), and from the servo motor itself. Using this input information, the servo drive supplies the appropriate amounts of electrical energy to the servo motor at the appropriate times to obtain the motion profile required at a given time.
The basic operating principle of a servo drive is similar to that of an inverter drive or a Variable Frequency Drive (VFD), in which an electric motor is operated by converting fixed frequency AC supply voltage to DC voltage and then to an adjustable AC output voltage with the required frequency.
The working principle of a servo drive in a servo system can be summarized as follows:
Therefore, the role of the servo drive in a servo system is to regulate the difference between the actual status of the servo motor with the required motor status by making the necessary voltage or current adjustments. This form of servo system operation differs from that of an open loop motor control system where the motor can rotate at the wrong RPM regardless of the feedback. But servo drives enable servo motors to respond with feedback gain, stiffness, and damping depending on the operation requirements of the servo system. For this reason, a servo system is referred to as a closed-loop system because its elements can read and respond to feedback regarding the motor’s operating status.
All servo drives have two major components in common: (i) The Power Stage; (ii) Control Loops (Servo Loops). The power stage receives input power from a DC or AC power supply, it then makes use of transistors in an H-bridge topology to transmit that power to the servo motor. The transistors function as power switches that facilitate current and voltage flow through the servo motor in either clockwise or counter-clockwise direction, providing reverse or forward motion.
The servo loops or control loops provide proportional control of the servo motor as per the reference input signal/command signal. A simple servo drive normally consists of one servo loop for controlling torque. While more advanced servo drives have an additional velocity/speed loop and may also incorporate a position loop. In a fully integrated servo drive system, a low-power digital command signal from a motion controller can command the desired motion profile by applying all three servo loops if tuned for optimum servo performance. In such a case, each control loop signals the subsequent servo loop and continuously monitors the appropriate feedback devices to make real-time adjustments to match the commanded operating parameters.
To get more technical, once a command signal or reference input signal is applied to the servo drive system, it’s compared with an output reference signal produced by the output sensor of the servo system, while the feedback element of the servo motor produces a third signal known as the feedback signal. The feedback signal acts as an input signal to the device being controlled by the servo system. This input signal to the controlled device is present as long there exists a logical difference between the command signal and the output signal of the servo system.
But after the controlled device achieves its desired motion profile, there is no longer any logical difference between the reference input signal/command signal and the output reference signal of the servo system. So, the feedback signal resulting from the comparison of the aforementioned two signals will not remain enough to continue operating the controlled device and produce an additional output signal until the next command signal or reference input signal from a high-level control is applied to the servo drive system via the servo controller.
From the discussion in the previous sections, the main functions of a servo drive in a closed-loop servo system can be outlined as follows:
Note: A servo drive is sometimes referred to as a servo amplifier, because it receives a control signal from the servo controller, amplifies it, and then transmits a specific amount of electric current and voltage to a servo motor so as to produce a motion that’s proportional to the received control signal. But when compared to standard power amplifiers, servo amplifiers offer a wide range of benefits for automated machining systems, including superior speed and motion control as well as precise positioning.
There is a wide variety of different types of servo drives that differ in regard to electrical ratings, features, operating parameters, and configurations. The electrical ratings include AC or DC supply voltage, rated power consumption, peak output current, maximum output voltage, and continuous output current. Thus, several servo drives are available in different voltage, power, and current ratings. Also, some servo drives use either three-phase or single-phase AC inputs at a frequency of 400 Hz, 50 Hz, or 60 Hz.
The most common features that differentiate the various types of servo drives include soft-starting; auto-tuning, status monitoring, and self-diagnostics; auxiliary I/O or brake outputs; alarms for fault conditions such as overvoltage; and injection, dynamic or regenerative braking. Operating parameters are the setup and control specifications. Some servo drives are set up and controlled using manual controls like jumpers, DIP (Dual In-line Package) switches, potentiometers, or knobs. While others have a digital control panel, slots for Personal Computer Memory Card International Association (PCMCIA) cards, a joystick, or a computer interface. Web-enabled and wireless controls are also available in some servo drives.
Besides, most servo drives allow storage of control programs on removable, non-volatile storage media like USB flash drives and SD cards. Moreover, various servo drives support the use of compatible hand-held devices for remote programming. Servo drive configurations include the different mounting options for installing a given servo drive. Most servo drives mount on a DIN rail, Printed Circuit Board (PCB), rack, chassis, panel, or wall. Thus, you can come across PCB-mounted servo drives, Panel-mounted servo drives, etc. Stand-alone servo drives and Integrated Circuit(IC) servo drive chips mounting on PCBs are also available.
Note: Servo drives control a wide range of servo motors that require position control, velocity control, and/or torque control. This brings about another classification of servo drives in terms of supported control loops, with the common variation being the torque-mode servo drive. This drive type converts the reference input signal from the servo controller into a given amount of current that’s applied to the connected servo motor. And since the input current is directly proportional to output torque, the torque-mode servo drive ends up controlling the amount of output torque the servo motor produces.
In addition, servo drives are also categorized based on the type of commutation they prompt, and in this category, we have trapezoidal and sinusoidal servo drives. The commutation sequence prompted by the servo drive determines the type of servo motor the drive can operate. For example, Brushless DC servo motors can be driven by either sinusoidal or trapezoidal servo drives.
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