06-24-2005, 12:59 AM
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| Gold Member | | Join Date: Mar 2003 Location: United States
Posts: 2,717
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1) The 'best' frequency is a balance between two opposing goals; motor ripple current heating decreases with increasing frequency while the drive switching losses decrease with decreasing switching frequency. Throw in the acoustic element (you don't want to listen to the drive squeal) and the number that satisfies the three goals best is around 20kHz.
2) The duty cycle adjusts itself (servos) to what is required to keep the desired current flowing in the motor at any given moment. In a good drive it will range between 0% and 100% over the normal operating range of the motor.
3) 'Chopper' drives are technically asynchronous circuits. They have two free-running oscillators (one for each winding) and usually are of the 'constant off-time' switching topology. The L297 / L298 and the Allegro offerings are the most common examples.
The circuits are very simple and easy to build but have irritating drawbacks, the most obvious is the squealing, hissing and grunting that everyone loves. Other less obvious drawbacks are poor reference tracking at all but the lowest of speeds which keeps the motor from being a smooth as it could be. These are unavoidable because of a phase-locking phenomena inheirent with this switching method.
Synchronous switching drives avoid these drawbacks by using a common fixed-frequency oscillator for both windings but are much more difficult to design. They are more properly called pulse-width modulated (PWM) drives. No single-chip solutions for this type are marketed to my knowlege.
Mariss  |
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