Flexible Digital Servo Drives Speed Machine Control Design
- Wednesday, 01 October 2008
Advances in digital hardware technology and software innovations now allow a single digital servo drive to be configured to work in a variety of machine control architectures.
For the machine builder, that means only having to work with one drive from a single vendor to fulfill various applications. Not only that, but setting up the drive for a particular machine control type requires simple configuration in the drive’s setup software. There is no traditional programming language to learn, write or debug.
In the machine control structure, the servo drive provides the link from the motors and I/O to the machine’s central controller, typically a PLC (Programmable Logic Controller) or IPC (Industrial PC). Traditionally, the servo drive provided the power conversion and contained the servo current and velocity loops.
With the incorporation of digital technology, servo drives can now control the servo position loop, have more digital and analog I/O, can communicate on a bus network, and can accept multiple feedback types. That said, in any given application, the servo drive capabilities that are utilized depend primarily on the machine’s control architecture and the other components that are specified prior to the drive.
Because it serves as the “heart of the control,” a machine designer will often select the PLC or IPC control platform and software prior to drive selection. Key factors for selecting the machine controller are:
- Ability to integrate an HMI
- Ability to integrate I/O
- Programming language/capability
- Execution capability
- Connectivity to higher-level controllers
- Ability to close servo loops
- Application need for centralized or decentralized control
- End customer preference
The motor type may also be selected prior to the drive, as the motor must be able to meet the mechanical and dynamic motion capabilities of the application. For example, a linear motor would be used if the application requires high dynamic indexing greater than can be achieved with a rotary motor with a ball screw or bell and pulley, in order to convert the rotary to linear motion. Alternatively, if a motor is needed that will mate well with a gearbox in order to obtain good mechanical advantages, a traditional rotary-style servomotor would be selected.
Other key motor selection factors include:
- Accuracy, repeatability, torque density, torque ripple
- Mounting configurations and physical constraints of the application
- Feedback types: dig enc, sine encoder, resolver, encoder with halls
The servo drive must be compatible with the motor and/or controller, which have often already been selected. Based on the capabilities of the PC or PLC controller, the servo drive will provide many of the following functions, in addition to basic power conversion and current loop control:
- Compatibility to the feedback device
- Velocity loop servo control
- Position loop servo control
- Machine I/O control (motion related) Travel Limit switches home switch, etc.
- Controller interface part (digital, analog, Bus) with commands and status information flowing
- Motor brake control
- Profile generation
Today’s high performance servo drives are capable of far more than simply being configured to fit into the control scheme of the machine and performing basic functions. They can actually increase machine performance, shorten the time it takes to get the machine up and running, and lower total machine cost.