andypugh wrote:
AThayer wrote: So what controls on long each phase is on or off?
How fast you are moving.
The pattern is also related to how the phases of the stepper motor are energised.
OK, so the drive transistors are to stay on in their respective state until the next step is made. But, there's more. The Bridgeport drives have a magnetic amplifier (saturable reactor) that drops the voltage to the motor to a low value when moving at very low speeds or standing still. These drives have no resistance other than the motor winding, and apply a pretty high voltage from the transformer. There is a transistor (one per axis) that sends current through a small winding on the saturable reactor to saturate the core with a magnetic field. When this transistor is on and the core saturated, the full supply voltage is delivered to the drive. So, you need some circuit with a delay to shut the transistor off when standing still for a moment, but to turn it on immediately when a step is requested.
As for the 2-wires or 4-wires, the computer could drive them with just two signals. You would then need external circutry to convert each phase signal into two complementary signals for the two transistors. You should also add some RC-diode delays so that the transistor that is to be turned off shuts off immediately, but the transistor that is to be turned on has a delay of somewhere between 10 us to 100 us. This is to prevent the transistor that is going off to try to short the transformer action through the windings from the transistor turning on.
Also, know that the Bridgeport drives are absolutely HORRIBLE, very poor electrical and thermal design from the early 1960's. Every possible "disease" of early stepper drives is manifested in them. Almost any modern stepper drive will greatly outperform them. Microstepping was the greatest advance, as a good microstepping drive gets rid of the resonance issues that plague full- and half-step drives.
Jon
