Servo Drives Explained
A Servo Drive is a special electronic amplifier used to power electric motors. The main task of a Servo Drive is to monitor feedback and continually adjust for deviation from expected behaviour. A Servo Drive is thus only one component forming part of a more comprehensive system referred to as a Servo System (servomechanism). Servo Systems includes four main components, a Servo Drive, a Servo Motor, a motion Controller, and a Feedback Device (usually referred to as an Encoder).
In a Servo System, a Servo Drive is responsible for powering the Servo Motor. The Servo Drive is an incredibly important component in determining the performance of the Servo System.
The motion Controller (sometimes referred to as the ‘brains’) determines what the Servo Motor must do and then triggers the Servo Drive (sometimes referred to as the ‘nervous system’) to send the necessary electrical energy to the Servo Motor to make the required movement/action happen (execute). In other words, the Controller calculates the path/trajectory/movement required and sends a low-voltage command signal to the Servo Drive, which in turn, sends the necessary voltage and current to the Servo Motor to achieve the required motion. Servo Drives can control torque, velocity, or position (torque control being the most common parameter).
Although Servo Drives can work on Analog or Digital signals, nearly all modern Servo Drives are Digital. Digital Servo Drives have internal processing capabilities that allow them to not only manage position, velocity, and torque loops, but also take over higher-level functions, such as trajectory generation. Digital Servo Drives also allow tuning to be done via software (versus manual tuning via potentiometers on Analog versions), and they can monitor internal functions and provide detailed fault diagnostics.
Analog Servo Drives might not have as many bells and whistles as the newer Digital Servo Drives, but do not require any processing time and are typically less expensive than the Digital alternatives. Analog Servo Drives are configured manually and rely on a centralised controller in multi-axis applications.
In other multi-axis applications, Controllers can communicate with Digital Servo Drives using network commands. Digital Servo Drives offer far greater configuration and performance capabilities over Pneumatic or Analog devices, and with the added intelligence of these devices, also offer far greater diagnostic benefits. Some digital Servo Drives are even advanced enough to store motion indexes and sequences in their memory and use input/output logic to operate independently without a dedicated Controller.
Today most industries rely on fieldbus networks that have become standardised for motion control. Industrial ethernet networks offers the ability to operate in time-sensitive applications. While there’s benefits to each network, certain products are designed specifically for certain fieldbus networks.
Servo Drives are also sometimes called amplifiers because they take the signal from the Controller and amplify it to deliver the required voltage and current to the Servo Motor. The term Servo Controller is also sometimes used as an umbrella term for the parts of the Servo System responsible for controlling the Servo Motor (consisting of the Servo Drive and the motion Controller).
Servo Drives were originally stand-alone components, separate from the Servo Motor and Controller, and although most references to the components of Servo Systems are as individual pieces of hardware, these are often integrated (to various levels). In fact, almost all Servo Motors include an integrated encoder for feedback (Feedback Device). Similarly, Servo Motors with integrated Servo Drives are extremely common, with some designs integrating traditional motion Controller functions as well. These integrated designs are sometimes called Integrated Motors or simply called Drives for their axis-driving function. These designs tend to reduce wiring, make sizing and selection easier, and save considerable space and setup time.
There are also several types of Servo Drives that all serve slightly different purposes. The most common type of Servo Drive is a torque-mode amplifier which converts the command signal from the controller into a specified current and sends it to the Servo Motor. The Servo Drive is controlling the amount of torque that the Servo Motor can produce. A linear Servo Drive works slightly differently because the current is proportional to the force rather than the torque. This directly drives the Servo Motor’s force output.
There are also many types of Servo Drives, classified based on their application such as current type (AC or DC), motor’s rotating field (synchronous or asynchronous), type of communication being used, whether the motor use brushes or not, etc.
For more information regarding Servo Systems, please refer to the following related blog posts: