Posted to News on 3rd Sep 2007, 19:42
Low-cost stepper controller delivers servo performance
Festo is launching an intelligent stepper motor controller that employs encoder-based feedback to deliver true servo functionality using low-cost stepper motor technology. The new CMMS-ST motor controller, in conjunction with the company's new EMMS-ST series stepper motors, enables machine designers and builders to implement high-accuracy positional control systems extremely cost-effectively.
Designed primarily for cost-sensitive applications that demand servo functionality but without needing high peak speeds – such as actuators driven via toothed belts – the CMMS-ST controller and EMMS-ST stepper motor combination delivers up to 6.5Nm of torque and speeds up to 2000rpm. It is suitable for single- and multi-axis applications with loads up to 20kg.
A key advantage of using closed-loop feedback is that the stepper motor is always operating at maximum efficiency and cannot suffer step loss; if the motor is overloaded, the position controller automatically compensates for following error and maintains position tracking. This means that, unlike an open-loop stepper system, it is not necessary to oversize the motor to handle an application's peak torque demands, thereby enabling the use of smaller, less expensive units. Other operational benefits include better smoothness, lower acoustic noise levels, and an absence of mechanical resonance. Although many stepper drives offer the options of closed-loop facilities, these generally only offer alarm functionality in case of step error. However, the CMMS-ST is claimed to offer true servo functionality with full control of current, speed and position.
All-in-one
The CMMS-ST is a complete ready-to-run unit, comprising a positioning controller, power output stage and all necessary control interfaces. It features a built-in CANopen fieldbus interface as standard, with all bus settings controlled via front-panel DIP switches. DeviceNet and Profibus interfaces are also available, and are supplied as field-replaceable plug-in modules. The controller is very easy to commission: all set-up parameters – including complete move profiles defining velocity, acceleration and deceleration – are developed off-line on a PC using Festo's intuitive Windows-based FCT software, and then downloaded to the controller. As many as 64 move profiles can be stored on the controller, and linked together, so that entire motion control sequences can be executed entirely independently of the host PLC. Additional move profiles can, of course, also be specified via a fieldbus whenever required.
A novel feature of the CMMS-ST controller is that all parameters and controller firmware can be stored on a plug-in SD card. This option provides a particularly effective service aid (a replacement drive can be installed and the SD card swapped into the new drive, thereby obviating any need for re-commissioning).
The CMMS-ST controller accepts industry-standard step-and-direction or +/-10V analogue speed-direction inputs, and features particularly extensive I/O facilities, including dedicated inputs for jog and high-speed position capture signals. All the I/O facilities are designed for robustness: they are fully protected against short-circuits, over-voltage and reverse voltage, and use 24V levels throughout to ensure a high degree of noise immunity and compatibility with virtually all PLCs on the market. RS232 and RS485 serial communications are provided as standard.
Position, velocity and torque control
Capable of operating in position, velocity and torque control modes, the controller also supports master/slave synchronisation and interpolated positioning for multi-axis applications. It is designed for use with standard two-phase 1.8degree (200 step/rev) hybrid stepper motors, and offers full-step, half-step, micro-step and smoothing modes of operation. The controller's power output stage accommodates motor supply voltages between 24 and 75V DC and can generate up to 12A peak current per phase. An integral braking resistor can handle up to 500W peak regenerated power.
Festo's new EMMS-ST series stepper motors are available in three frame sizes and two different stack lengths. Depending on the model, the motors can provide a holding torque of 0.5-8.6Nm and accommodate speeds up to 3000rpm. Each model is available with or without the integral encoder option and with or without a holding brake.
The launch of the CMM-ST stepper motor controller marks the latest move by Festo to expand its presence in the electrical drives and controls market. Well known as a leading manufacturer of pneumatic automation components and systems, the company now also produces a growing range of compatible electrical automation products, thereby providing customers with a highly convenient, single source of supply for hybrid systems.
