Block diagram of the Mardus-Kreutz drive (Click to enlarge). COMPONENTS OF THE M-K Drives.
What is a Translator?
A "Translator" is the part of the Micro-Stepper Drive circuit whose function is to "translate" Step and Direction input signals into the proper sequence of digital and analog signals required to drive the power output stage.
Basically, the translator contains the brain and soul of the stepper drive. The brain is a cheap but powerful micro-controller, the ATTiny2313 from Atmel. The soul is a simple but effective firmware (micro-controller software) able to handle a maximum step frequency of 100 Khz and providing all the micro-step functionality, as well as current waveform morphing with speed, standby motor current reduction, and automatic torque compensation. The translator includes also the opto-isolation circuit for the Step and Direction signals coming from the PC Lpt port or controlling device.
The Firmware.
The firmware is written in BASCOM AVR, a simple to understand flavor of the Basic language for the AVR micro-controllers, which is also free (the free version is limited to 2K bytes of code, just the capacity of our controller chip). Why Basic and not C or assembly?. We used Basic, as programming language, because the code was intended to be used as a learning tool, and well commented Basic code is simpler to digest.
Power Stage.
The output signals of the "translator" are logic level signals which can't provide the voltage and current levels necessary to drive most stepper motors. The power stage is the part of the circuit that provides the the power handling on the drive. It includes the current control as well as the coil switching, motor power supply filtering, and safety circuits.
The current control circuit is the heart of the Power stage. In our case we selected a fixed frequency chopper circuit. A detailed explanation of the discrete chopper circuit will be addressed in the respective circuit description. On the power stage there are also two voltage regulators: +5 Vdc regulator for the logic circuits, and +9 Vdc for the gate driver chips. The former is also used by the translator board.
Stepper Motor and Micro-stepper Theory
Stepper Motor theory is beyond the scope of this blog, there is a fine tutorial on the subject I would like the reader to familiarize with before continuing reading this blog.
We will be referring to some concepts explained on that tutorial on the description of the circuits on the following posts. My intention is to go deeper into each block (of our Block diagram), opening its contents into increasingly finer details, finalizing at the component level.
I will also talk about design decisions and compromises, as well as the solutions we implemented, in order to overcome some of the fixed frequency chopper inherent dis-advantages.
What I want to make clear is that there are many ways to get any stepper motor running, our design objectives were always focused into micro-stepping driving unipolar motors, but also to get the most of them, in terms of power, accuracy, efficiency and performance.
Our design is neither a mere copy of a text book circuit nor just a discretization of an integrated controller, it was intended as a design exercise with the objective of using it as a learning tool, nevertheless, the final product has an exceptional quality, as you can witness on the many videos taken during the early tests.
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