Electricity + Control July 2016

DRIVES, MOTORS + SWITCHGEAR

Figure1: The mechanical part of the test rig.

Matching load and servo motor sizes Glyn Craig, Techlyn

This article, the fourth in the series, deals with stability in practical systems.

T he effect of different load inertias is illustrated with experiments using real life components. In addition, tuning the controller is covered in detail. Previous articles described systemtesting, servo operating principles and a practical mechatronics case study. This article describes the effect on the system when the load inertia is changed. Equipment used For the test, a brushless motor was mounted on a chassis and coupled to a Techlyn brushless drive. Test response was measured using a Personal Computer (PC) coupled via a serial port to the drive. Step re- sponse was displayed while the tuning parameters were interactively changed. Tests were carried out on the motor with no load, as well as with flywheels dimensioned to provide load inertia of 3 X and 10 X motor rotor inertia. Conventional wisdom states that inertia matches of up to 10:1 are acceptable, with the optimum point being at 3:1. Our experiments sought to see if this was indeed true. Our drive is controlled by a Microchip PIC18F2331 microcontroller. This device is highly optimised for motion control with all time intensive functions being performed by hardware registers. Moves are commanded via a serial port or step and direction digital inputs. The step and direction inputs make the system resemble a step motor system (with encoder position maintenance). The PC test program sends move commands and graphs the response with respect to time.

2,3 kg.cm² (10:1 inertia match). In total, six tests were performed and graphed.

What do the parameters on the graph mean? Ideally, the July 2015 Electricity+Control article (Brushless servo operating principles) should be to hand [2].

Figure 2 • On the lower left is the Servo Parameters Pane

• Kp is the Position Gain • Kd is the Derivative Gain • Ki is the Integral Gain • Il is the Integration limit

• The top half shows the rotor position with respect to time • At the lower right is the Configuration Pane which is self explana- tory. The horizontal dotted lines are the position thresholds which are used to measure times. The rise time for instance is the time measured fromzero time to the first crossing of the lower threshold PID Servo Control These are the control filter parameters. The output to the motor amplifier is the sum of three components: • One, proportional to the position error providingmost of the error correction (Kp) • One, proportional to the rate of change in the position error which provides a stabilising damping effect

Tests The test motor had a rated rotor inertia of 0,23 kg.cm². Two flywheels were machined with inertias of 0,69 kg.cm² (3:1 inertia match) and

Electricity+Control July ‘16

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