TrolCat

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‘TrolCat is an open project by Michael Dennis.[edit | edit source]

In Norwegian folklore, a troll cat can transform itself to do its master’s bidding. My “Cat” is designed to deliver a solid base for custom tooling, so that its master can concentrate on design of the tool without reinventing motion. It is a robust XYZ gantry controlled by open hardware/software that can understand standard CNC commands, and a standard interface for tooling control.

A pair of Origa belt drives form the Y axis, and are synchronized by a drive shaft. The X axis is a Pacific Bearing screw drive unit. Z is a generic linear drive. It’s a cross-section of commercially available linear actuators from the late twentieth century.

All axes are driven by Teknic M2330 motors. These are brushless servos cabled to Sst1500 amplifiers. The amplifiers take step/direction inputs and control the motors. They are “stepper killers” in that they offer more power, speed and accuracy than a stepper of the same size. And they can be digitally tuned to the application.

An Arduino supplies the step/direction signals. The Arduino runs GRBL, a G-Code interpreter that forms the core of most 3D printers. GRBL needs a host; a PC or Mac to run the front end.

There is a box full of power supplies - 85V and 12V. There is a box full of controls - GRBL and some switches.

Because this is a “public” tool, there are limit switches to protect users and hardware. The amplifiers shut down motion when a limit is detected. They will pass the signal to GRBL, which will suspend move signals.

Status[edit | edit source]

10 August 16[edit | edit source]

Everything moves, but...

GRBL handles limit switches in a most peculiar way. Usually, the limit switches on a commercial machine are Normally Closed (NC) - that way if they fail they will most likely fail in a safe fashion. And since there is a signal on the line during normal usage, NC switches are more noise resistant. GRBL 8.x wants Normally Open (NO) switches. GRBL 9.x allows NC switches, but they have to be wired in series. This creates a ground loop in the Teknic amplifiers, such that if one limit is triggered, the Teknic sees both limits as tripped. Of course, the Teknics want an NC switch, so I can either rewire the limits to the Arduino and depend on it to shut down an errant axis, or I can eliminate the limit pass-through and let the Arduino continue to process GCode even though the Teknic has detected a limit.

Or adopt my current strategy, which is to pass the "-" limit to the Arduino so it can do a homing routine, and then set soft limits. The "+" limit switch is still connected to the Teknic, so that if soft limits fail for any reason, the Teknic will still stop on the limit trigger.

Oh, and GRBL doesn't know if a switch is NO or NC; it only sees a change of state. So if you do run into a limit, it locks itself. Then after a reset, when the axis moves off the limit, it locks again. Irritating.

16 August 16[edit | edit source]

The frame has been modified by moving the legs. This allows for more vertical travel in the adjustable bed. Work has started on the bed, which will be a box structure skinned in plywood or MDF. Since the bed is a box, one of you bright makers could convert it into a vacuum hold-down.

Flags need to be installed for the Y and Z limits.

30 August 16[edit | edit source]

The bed is framed, and ready for a top panel. The top panel will have T-slots. So it will be plywood to stand the stress. Since LaserCube is stalled until after Labor Day, this weekend shall be dedicated to TrolCat, finishing up the little things that need to be done:

  • Limit flags all axes.
  • Limit switches on Z.
  • Final tuning.
  • Design of the tool plate.

24 Oct 16[edit | edit source]

Schematic

An interlude. Designed a circuit to operate a pen holder salvaged from an HP Plotter. Pretty simple; a pot is wired as a voltage divider to supply a voice coil that holds the pen. More volts, more pressure on the pen. The coil loop is interruptible using a solid state relay (SSR), so 0V raises the pen, and pot voltage lowers it. The SSR is wired to the Spindle pin of GRBL. Work-ish, but at 24V tends to draw enough current to smoke the pot, so a 12V supply is a better bet. A snubber capacitor is probably in order, too. All this mounts to a DIN rail, so the wiring is tidy and easy to mount to the 'trolCat.