SEED Guides Sub-System Specification - Planar Mechanisms <

DIRECT ELECTRICAL DRIVES

Electrical drives using servo motors generate controlled output movements with greater flexibility of timing than is possible with a conventional mechanism since they operate on-demand and are readily re-programmed. Coarse control, i.e. repeatability to within 1 mm, is obtainable by direct switching: accurate control of position, velocity and acceleration requires microprocessor control. In 1988 the cost of the electrical system is several times that of the equivalent mechanism which also has greater load/speed capacity.

These generate accurate increments of rotation (typically 0.9š; 1.8š; 2.5š) in either direction. The number of input pulses determines the angle of rotation whilst controlled variation of pulse frequency produces an output acceleration analogous to a cam. Some types generate a positive holding torque. Positional control is lost if the load exceeds the motor rating: the nominal 1000Hz limit of pulse rate can be raised by an order of magnitude at the expense of greatly increased current consumption.

'Software cams' are 'intelligent', pre-programmed microprocessor controls driving a 4-quadrant motor having a linear torque/speed characteristic and braking capacity. Input torque control facilitates jerk-free acceleration. These drives have the merit of initiating a controlled sequence of events upon demand without mechanical engagement or liability to cumulative error. A typical operating speed is 200 cycles/min, compared with 2000-5000 cycles/min possible with a mechanical device.

The eccentric disc, centre E, rotates about 0 to drive the rocker PR. The broken lines OE and part of ER indicate the equivalent crank-rocker linkage.

This mechanism is advantageous when the eccentricity is small, then the eccentric can be machined integral with the shaft and the imbalance is relatively small. (NB, the opportunity for partial balance by removing material). Unless it can be assembled from one end the disc and sheave must be split; the sliding velocity may be significant. In compensation the mechanism has inherent force closure, can be stiff and strong and is suitable for inaccessible locations.

Figure 15 Typical Acceleration Curves Slider Crank Mechanism & Scotch Yoke RISE DWELL RETURN DWELL +1 0 360 CAM ROTATION (degrees) -1

NOTE: Whilst every effort has been made to ensure that the information given is correct and up-to-date, SEED the publishers and authors cannot beheld responsible for any errors or omissions that might occur in this Guide. Use of the methods or data on projects for application outside the educational environment should be justified and validated during the course of the designer's normal professional duties. Copyright © G. Druce