How to implement the CNC machineâš“
A microcontroller is obviously necessary. For it, 8-bit types with built-in USB interface, like AT90USB162, are sufficient. For more performance, necessary for parallel kinematics like this one, a 32-bit controller like AT91SAM7S should be used. Also, when encoders and therefore feedback loops (with PI controller) are used, a 32-bit controller should be used.
Motor controlâš“
All what a CNC constructor has to do is to create a microcontroller firmware that exposes a suitable USB sound card to the PC, and interpret the incoming data accordingly. Despite the relaxed sample rate (incoming data rata is only 3 kByte/s), the firmware can easily create high-speed, high-resolution step pulses for the stepper motors, because:- Mach3 (i.e. the built-in Trajectory Planner) will ensure that speed ramps (i.e. accelerations) are limited.
- Mach3 will sum up all the pulses for the correct position, therefore, there is no need for the microcontroller to deal with (wide-bitness) positions.
- Incoming speed data can be easily converted to equally-distanced step pulses and microsteps, it's not a great task. Three hardware CTC will ease this, but one single timer is still sufficient.
For built-in safety, a disrupting waveform data stream will lead to an emergency stop, which is mostly intended.
An emergency stop removes power from all motors and activates all available brakes. The exception from this rule are motors or solenoids holding a load that will otherwise fall down and make additional damage. References are lost, and a synchronization movement is necessary in front of next positioning.Incoming invalid data, such as 80h-centered sound data, must be ignored by that firmware, to prevent inadvertent movements by a mis-configured media player.
A normal stop speeds down the motors as fast as possible, then holding them for some microseconds to ensure braking, and later reduce or remove power. References are kept.
See also: USB2LPT and CNC
Switches and feedbackâš“
All switches are relatively low-speed and reported by an HID interface. This will include all end switches and some human-interface switches, possibly an extra Emergency Stop contact. (Of course an Emergency Stop should interrupt the power supply for all drives – in wired hardware!) This is to be specified!
Construction of controller firmware
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