# VFD proximity sensor stop.



## mksj (Feb 20, 2015)

I was interested in assessing how well a proximity sensor would work as an electronic stop on a  VFD controlled lathe instead of using a soft/hard stop or mechanically disengaging the feed. There are many different variants on proximity sensors, and I was looking for the following requirements:

1. PNP type to switch the power side of the VFD control inputs and normally closed (NC) type (i.e. conducts the VFD signals until it encounters and object).

2. Works off 12-24VDC (usual VFD signal or relay voltage) and capable of handling at least 200mA.

3. Quick response time of at least 200Hz and a sense distance of at least .3". Both are extremely important when you are looking at stop times of 1-2 seconds. Also high repeat accuracy, the default spec. is 10%.

4. Shielded sensor, so it is not affected by surrounding objects to the sides or behind the sensor.

There is a great review of these parameters here: http://www.automationdirect.com/static/specs/proxterms.pdf

The proximity sensor I ended up using is a PFK1-BP-3H from Automation Direct, which easily meets and exceeds most of these specifications. It is used with a M12 LED quick disconnect plug and cable  EVC178 which provides a LED status of the proximity sensor.  The sensor information is attached.

The application of this sensor is for VFD systems only, using either a 12VDC or 24VDC terminal input system using a latching power relay and a maximum current draw of 200mA. The VFD must use an external braking resistor and capable of stopping the lathe chuck in 1-2 seconds. The proximity sensor is being used like an E-Stop switch or brake switch, and is wired in series with these. It it's normal state it conducts the 12-24VDC signals/voltage, when triggered by the moving carriage into range (~0.4"), it goes to an open switch state. This breaks power to the power relay or forward/reverse relay, and the lathe quickly stops. It is imperative that there is some form of reset power button and/or requires the direction switch to be in the stop position when the proximity switch reverts to its NC state as the carriage is mechanically moved backwards. I recommend adding a momentary normally open bypass switch across the proximity sensor's switch wires, which allows one to reverse direction even when the proximity sensor is triggered in its open state. So for threading or machining to a stop, you feed the carriage until the proximity sensor engages, press the bypass switch momentarily and reverse the carriage. The proximity switch will reset to NC in about a second when feeding away from it.

I already had a hard stop, and wanted the proximity switch to work in conjunction with the hard stop micrometer (this is important for safety reasons). I machined a movable slider mechanism to hold the proximity sensor, which slides in a milled channel on round bushings, These set the distance for the outer brass plate. Brass slides a bit better against aluminum, but any kind of sliding mechanism will work. Since the mechanism slides forward as you turn the micrometer, you need a way to pull the mechanism back when you reverse the micrometer direction. I did not want to slot or drill a hole in the hard stop micrometer rod to secure it to the proximity holder, so I glued (epoxy) a small neodymium magnet into the cap. This secures the sliding mechanism to the end of the micrometer rod. The proximity sensor mechanism indexes with the hard stop with 0.001" resolution. The proximity sensor position is adjustable relative to the hard stop, the proximity sensor will activate ~0.1" before the hard stop and this is adjustable.

It should be noted that a hard stop is fixed, and is independent of speed, feed, system momentum and VFD braking capabilities. But it only stops the carriage when feeding. A proximity sensor will be affected by all these variables, so one needs to adjust the engagement point for a specific speed, feed and the momentum of the system.

So the first question is did it work? Yes. If you just look at the proximity sensor's ability to reproduce a stop at the same speed and feed, the reproducibility on multiple runs with this sensor at different RPMs and feeds had a range of 0.0006" using a WJ200 VFD with an external braking resistor (1 second stop time).  Small variations in speed and feed had a minimal impact on the stopping point. If one were to double the speed and increase the feed rate, I was able to see a ~0.050" increase in stooping distance, but the repeatability once zeroed for any specific speed and feed was better than 0.001" on repeated passes.  I have two short videos of the sensor mechanism and accuracy, but there seems to be no way to upload these videos. I will try to post elsewhere and provide links. I will post a schematic and suggested implementation to follow.


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## Ulma Doctor (Feb 20, 2015)

most of the processing equipment nowadays use sensors similar to what you have used.
they are very useful and surprisingly very rugged .
nice work!


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## Bamban (Feb 23, 2015)

Parts on the way from Automation Direct, will built one for my lathe.


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## UncleRuss (Mar 2, 2015)

Change your wiring to incorporate a momentary type of action.  E.g.: Push button on start up, relay pulls in and engages the sensor circuit.  When the sensor operates it, drops itself out of the circuit to be non functional.  Therefore illuminating the ills of the circuit you describe.  Sensor is non functional unless manually engaged.  You may get hold of me by message if you wish and I can help you draw and hook it up.  Piece of cake!


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## mksj (Mar 2, 2015)

Hi UncleRuss,

The implementation of the proximity sensor limit switch varies by VFD system and wiring. My PM1340GT lathe is relay controlled, the For/Rev direction relays are isolated/controlled by the proximity sensor. Power is always maintained to the sensor unless the E-Stop is engaged. It has no affect on the main power latch relay. Reset requires the direction switch is moved to the stop position for any system fault, there is no separate power engagement switch. This also holds true for another build system recently put together which we are also incorporating a proximity limit sensor. This is the build history for that system: http://www.hobby-machinist.com/threads/acer-trump-1236-vfd-conversion.32910/

There are various schematics, but in these schematics power is maintained to the sensor, only the load side is switched off which affects the direction controls. When the proximity sensor encounters and object it trips open stopping direction signals, the jumper bypass SW across the switch terminals allows one to momentarily bypass the sensor and power the machine to reverse direction. On a newer single relay schematic for the PM-1340GT lathe (which is different schematic than mine), the direction relay is also reset when the lathe direction lever is in the stop position. Sounds more complicated than it is, but implementation varies by machine and wiring.

This is the PM-1340GT lathe VFD single relay schematic with the proximity sensor limit switch:


This is the schematic we are using for the Acer Trump 1236 VFD conversion that incorporates momentary direction/stop buttons with Jog lockout and proximity sensor limit switch. He has been using this system, and it is working very well, but it was a bit of an unusual design. There is an extra set of relay contacts on each relay for possible future control of coolant system, DRO, etc. We use a separate main power relay with a power reset button, the proximity switch only affects the direction relays.


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