Watson Electronics

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This page is intended as an initial review of the current state of the art in reprap electronics. The ultimate goal is to look for areas where the quality of the electronics can be improved, the features expanded and the cost maintained or reduced.

Stepper Drivers

X axis, Y axis, Z axis, E1 axis, E2 axis will cover most situations. An upgrade of the stepper drivers to 1/16 stepping might be implemented. Also, it would be nice to programatically control max stepper current rather than messing with single turn potentiometers. It seems that a lot of folks don't have the tools or knowledge to set the stepper current using a DVM and a potentiometer which can lead to frustration with under/over driving the steppers.

Temperature Measurement

Thermistor

Sticking with thermistors is probably the lowest cost solution. Though the range of accurate measurement is small thermistors are still pretty effective. There will be need for improved A/D and signal conditioning. Reading accuracy and repeatability should be close to 0.5 degrees C or better in the operating temperature range. Gen6 electronics seem to vary more than this.

Heater Control

Bang Bang

Bang Bang control is a method of on / off power control. The on and off time are controlled proportionally to maintain the average power delivered and needed to maintain a certain temperature. The reality is that this doesn't differ from PWM control except for the fact that bang bang control is typically implemented with on off cycles with a period of hundreds of milliseconds, or many seconds.

PWM

PWM control is an efficient method of proportional control which is much more power effecient than linear control. The disadvantages are that it can be audible at low frequencies (which is dwarfed by the audibility of the PWM stepper motors in most reprap designs) and at high frequencies switching losses can become significant. That said PWM control can easily be 90% efficient. One of the problems seen in several of the heater controller designs has been use of arbitrarily high frequencies. Frequencies above a few hundred Hz or a few KHz, although potentially audible, can not be exceeded without proper MOSFET drivers to pump the MOSFET gate capacitance. The higher the frequency, the more time the MOSFET spends in it's switching transition state which causes excess power loss. This is especially problematic when using microprocessor I/O pins to drive a MOSFET gate as they can not deliver enough current quickly enough to charge the gate and switch the MOSFET rapidly.

Future Direction

It would seem that the continued use of PWM control, with sufficiently sized MOSFET along with a low frequency (100Hz with capacity to utilize up to 1KHZ is a good target for heater circuits.

Heaters for 2 extrudes and 1 heated bed would seem to cover the most ambitious of reprap users.

Hot end targets will be for 5A delivery for extrusion heads allowing for as little as 2.4 ohms and a maximum power of 60W. This is likely overkill and will be rolled back as competing interests (such as cost, board real estate etc. come into play)

Heated bed targets will be for 15A at 12V delivery for a maximum power of 180W. Separate power input for the heated bed may be considered.

Computer Interface

It may be possible to create a simple modular communication interface that would allow for a plugin "option" for users to select which computer interface they preferred whether it be USB, RS232, or Bluetooth. This would likely increase overall system cost slightly (an additional daughter board would be required along with connectors to implement each of the 3 options.

External Storage

SD

Adding SD is the lowest cost and easiest approach, the increased board cost is minimal and the cost of a connector is only a few $.

MicroSD

microSD can be handled with use of an adapter. There might not be any reason to consider a dedicated microSD slot unless board space were an issue.

User Interface

Character LCD

A character display would likely be included as an addon. It would be valuable to have some feedback during a print, especially if external storage is used and standalone operation is desired. This may be implemented as an addon module to the base electronics as a plugin type character display is pretty easy to obtain and implement.