Machine Hardware and Electronics

Motors

Axis	Motor	Voltage	Specs
X1	Ametek Pittman M14437A170	24V
X2	Ametek Pittman M14437A170	24V
Y	Ametek Pittman M14437A170	24V
Z	maxon DC motor 148877 732925	48V	https://www.maxongroup.com/maxon/view/product/motor/dcmotor/re/re40/148877

The X1 axis

Encoders and axis movement

Axis	Encoder Counts per rev	Pinion teeth	Rack TPI	Counts per mm of movement (From original machine docs, after multiplying by four, so basically edges per mm)	Encoder counts per inch	Invert in HW?	Invert in SW?
X1	512	?	?	312.9	7947.66	Yes	No
X2	512	?	?	312.9	7947.66	No	No
Y	512	?	?	260.8	6624.32	Yes	No
Z	512	?	?	682.67	17339.818	No	No

All encoders are HEDL-5540 I11 1310A. To keep the software configuration simpler, we have "inverted" the X1 and Y axes encoders in the wiring, switching the A and B signals.

Homing switches

The homing switches are hall effect sensors, all Honeywell SS441-s04 (https://sensing.honeywell.com/honeywell-sensing-hall-effect-digital-position-ics-ss400-series-ss500-series-datasheet-32320997-b-en.pdf). There is no stock homing switch on the Z azis. We are using a Littlefuse 55140 03H02A 18320 (https://www.littelfuse.com/~/media/electronics/datasheets/hall_effect_sensors/littelfuse_hall_effect_sensors_55140_datasheet.pdf.pdf) to create one. These Hall effect sensors are similar.

Spindle brake

The spindle brake is pneumatic, controlled via a 24V solenoid. We've measured the solenoid to draw 25.1mA when activated.

The router head

Electronics

Homing switch circuit

The homing switches are 3-wire hall effect sensors as described above, operating voltage between 5VC and 24VDC and capable of sinking 10mA. A typical digital input circuit would have a 10k pull up resistor on the signal pin. In our case this would be difficult with 5V, since the Mesa 7i37TA has 4.7k of series resistance. We could use a 1k pull up resistor to get above the 4VDC threshold the Mesa card has to activate the input, but this leaves us with little margin for error or noise. Using a lower resistance starts to get close to the operating current of the sensor. Since we already have 24VDC on board to run the spindle Z brake solenoid, we can use 24VDC with a 10k pull up resistor, giving us a ~7.5VDC "off switch" voltage (24*4.7k/(10k+4.7k)) and 0VDC "on switch" voltage. Current through the sensor is about 2.4mA, well within the limit of both sensors.

This circuit could perhaps be simplified by inverting the logic. In general the behavior we want from limit+home switches is that a cut wire/loss of power should act like an "on switch" state. Since we are only using these switches for homing this is not a significant concern, but in the event we wanted to add limit functionality it would be nice to be able to do so without changing the circuit while still having the desired limit behavior. This leads us to the circuit diagram shown below:

State	Voltage
on switch	~0V
off switch	~7.5V
no power	~0V
all wires cut	~0V

Since the 7i37TA treats 0V as true, we invert the signals in the Hal configuration for the home switches.

We've created one of these circuits for each axis on a perf board.

Estop circuit

Our estop circuit is very simple, we are using a single-throw double-pole normally-closed estop button. We will use one of the poles for logic input and one for the motor power. Since the 7i37TA has series resistance appropriate to limit the current, we can simply put the estop button in series with the IN+/IN- wires. This provides the following states:

State	Voltage
switch deactivated	~24V
switch activated	~0V
no power	~0V
all wires cut	~0V

This is generally safe behavior for an estop button. Since the 7i37TA treats 0V as true, we invert the signal in the Hal configuration for the estop input.

Spindle Z brake circuit

The spindle Z brake is a pneumatic solenoid operated by a digital output. It pulls ~25mA and 24VDC which is well within the capabilities of the Mesa 7i37TA, so we can simply put the solenoid in series with the outputs. We have added a flyback diode as the solenoid is an inductor. This circuit is shown below:

Spindle on/off

The initial spindle will be a Hitachi router. The router pulls about 11 amps, and so we implemented a 30A contactor to turn the router on and off. The contactor is controlled by a smaller relay that is activated by a 24VDC signal controlled via a Mesa 7i37TA output signal.

A circuit diagram of for the spindle on/off signal