TechArchive

Original article date: October 1998

The SMT-BD1 digital servo amplifier is the latest development from Infranor. As its name suggests it uses a digital signal processor (DSP) to control all the functions of the amplifier and this results in a drive that is extremely easy to set up. New control functions have been introduced within the software of the control architecture.

A Windows-based graphical program for automatic setting of the amplifier parameters is supplied with the SMT-BD1 meaning that specialist servo-knowledge is not required. The auto-tuning functions enables the amplifier to be commissioned quickly and easily. There is an auto-phasing function which enables the drive to be used with other motors including frameless types and can also compensate for wiring and motor set-up problems such as a mis-adjusted resolver. Beign digital all parameter settings are accurate and repeatable: the parameters are saved to the EEPROM on the board and they can also be saved to file for quick commissioning on identical machines.

The embedded control system has within it a number of application derived programs – these are functions that would normally require the use of dedicated controllers. A software switch is requird to access some of these functions and in some cases a daughter board does need to be fitted. Options available thus far are:

• Networking – communjcate to a number of drives via RS232 or RS485 serial link
• CAN bus networking – communicate to the drives as part of a CAN system
• Software oscilloscope – on-screen digital storage oscilloscope (DSO)
• Simulation – change system parameters and check response
• Cogging torque compensation – maps out and significantly reduces torque ripple on brushless motors
• Stepper motor emulation – uses pulse and direction signals to control the motor
• Software gearbox – to drive the motor at a ratio relative to another axis
• Tension control – uses an input from a load cell to control the tension on a fibre or a web
• Spindle motor option – uses the drive to stop the motor in set positions
• Registration – uses an optical input to correct the pitch of products on a conveyor belt
• Unwinding/rewinding – similar to tension control but with a variable diameter spool
• Positioner – a full single axis positioner integrated into the drive.

Each amplifier is supplier with the basic BPCW setup setup and tuning software. The standard communications to the drive is via an RS232 serial link which is normally removed once the drive is set up. For communications over longer distances an RS485 option is available. In some cases it is possible to keep the drive in communication with a host computer and perform the setup and running of a number of drives via a simple network arrangement. Each card has an address and the host computer sends the instructions for the maximum speed and currents as well as the command to auto-tune the drive. For simple applications running in speed control the speed and direction signal could be sent down the serial link. If there is an error on the drive a signal will be sent to the host and a software reset can be attempted.

For systems where the drive is to be more fully integrated into the control system bus networks are becoming more common. Due to its low cost and high level of data security Infranor has chosen to implement CANbus on its drives (see page 000). Two protocols are available one aimed at OEMs which wish to design their own control system. Alternatively a protocol is offered which was derived by the Can in Automation (CIA) motion control forum. CanOpen is the standard protocol that should be implemented on host controllers such as PLCs and motion controllers. Cooperation between Infranor and some motion controller manufacturers means that a CAN output can be specified to talk directly with Infranor SMTBD drives on their products.

With CANbus the system can send commands and data over the network to each individual drive and also interrogate them for any errors that may have occurred. System wiring is also greatly reduced.

The SDMT-BD1 drive is available in rack-mounted or monoblock formats with power outputs from 2 to 35A continuous and 4 to 100A peak.

• Infranor
• Brian Webb
• 01403 223500

October 1998