Mardus-Kreutz Unipolar Micro-stepper. 3rd Part.

Translator's Block Diagram

In this part of the series we will open the translator black box and explain what is inside each of the smaller boxes.

Opto-Isolator Stage.

The opto-isolator stage's function is isolate the signals coming into the translator, specially, isolate the PC Ground from the motor power supply ground.

Isolation is used to protect the Lpt port of the PC from dangerous voltages and current leakage from the motor power supply, but its main function is really protect the Pc user from nasty/dangerous electrical shocks should the protective ground connection, on the motor power supply circuit, fail by any reason (the protective ground is the green wire connected to the building's ground).

Micro Controller Circuit.

The micro-controller used in our design is the ATtiny2313-20 from Atmel. This circuit includes, the micro-controller chip, Reset circuit, 20 Mhz oscillator's Xtal, Some LEDs for status display and the programming Jumpers connected to some of the chip's I/O pins.

Reference signal Processing.

The output of the micro-controller to the reference processing circuit are two 39.2 Khz 16 bits hardware generated PWM signals. Both signals have to be processed in order to obtain two analog waveforms which will respectively become the Channel A and Channel B current reference waveforms.

The processing circuit is a Second order low pass filter (two cascaded first order filters).

Another logic level signal coming from the micro-controller (STANDBY) will reduce the reference voltage to 50% after 1 second inactivity. This is done by means of a switched resistive voltage divider, the switching is done by means of two low power Mosfets.

Half of the Reference signal processing circuit is located on the Translator board, the other half is located on the Power Stage board. The reason of the splitting is simple, the reference signals are sent to the Power stage by means of a ribbon cable, noise captured on the cable is attenuated by the second low pass filter, located at the power board, before the analog signal amplitude is divided at the respective current setting potentiometer (more detailed explanation on this subject will be offered later).

SYNC signal.

This is one of the most important signals, its function is to serve as reference frequency for the choppers at the Power Stage, but it serves a second and very important function: It defines the "blanking time" on the chopper comparator circuits. We will be talking about the "blanking time" when we get to the chopper's circuit description, and a little more when we discuss the blanking time relationship with the minimum step current regulation.

The Sync signal is generated on a hardware 8 bits PWM generator inside the Micro-controller Chip, the "blanking time" is adjustable in the firmware. More information about this subject is going to be posted in the next parts of this blog.

Micro-controller's Watch-Dog.

The internal watchdog time base generator of the micro-controller is used as reference for a software controlled speed measurement circuit, it is also used as reference signal for the 1 Second inactivity Standby signal control.

The Speed measurement circuit is the responsible of controlling the variable morphing reference waveform versus motor speed, details about both features will be discussed on the firmware description part of this series.

FIRST PROTOTYPE PHOTOS

Translator and Power stage (click to enlarge)

Translator's bottom side (click to enlarge)