Rotary table home

Hello - I am building an indexing machine with a stepper driven table. The machine will be loaded and unloaded from one position and then index. I am planning on 6 stations so as mentioned 1 will be load / unload. Station 4 will be the machine action and all other stations will be idle or no action to the product.

When the machine is started, I need to find a home position since a stepper looses this at shutdown or worse power failure. I am planning on a mechanical limit switch that will be triggered by a peg as the table spins.

Is there a better way of doing this? Do I need to worry about wear on the switch? If I go with non contact type probes, how repeatable will they be?

Thank you,

Anthony

On my polar machine I use a QRE1113 reflective opto sensor to sense a black line to home the machine. Very repeatable and nothing to wear.

Nice and reasonably priced

Thank you,

Anthony

Ground fungus - did you buy a bread out board or just the chip. If just the chip, what other components did you wire with it.

Anthony

Just to add information, I saw a video testing many different switches. They all broke well into the 1,000s to 100,000s. I bet your machine won’t home that much. Although you should pull the switch away when not homing to increase lifespan.

You are right. It will home only once or twice a day but I was planning on leaving it hit the pin on every rotation. I guess I could put an actuator to remove it but this will probably add play to the system.

To use the QRE1113 you need one resistor (I used 470 Ohms) to limit current through the LED. The pull up on the collector of the phototransistor can be the internal pullup on the digital pin used to read the phototransistor.

aiannar974:
You are right. It will home only once or twice a day but I was planning on leaving it hit the pin on every rotation. I guess I could put an actuator to remove it but this will probably add play to the system.

I doubt a solenoid will add enough play to the system that it will effect the ability to press the 0.1 psi or less of the button/Micro switch.

Ground fungus - I would have thought that would have given you and analong output, not digital.

aiannar974:
Ground fungus - I would have thought that would have given you and analog output, not digital.

With a breakout board that this appears to come on, there is circuitry that converts the sensor into a digital circuit.

I have seen there are two versions of the breakout board but I think ground fungus is using the chip only with a resistor

oh, well there must be enough difference to allow the digital pin to round the output up or down.

there must be enough difference to allow the digital pin to round the output up or down.

That is the case. The path that the sesor is over is reflective, to IR, except for a narrow black bit. The input is LOW (phototransistor conducting) except when it is over the black line where the input is pulled HIGH (phototransistor not conducting).

aiannar974:
Hello - I am building an indexing machine with a stepper driven table. The machine will be loaded and unloaded from one position and then index. I am planning on 6 stations so as mentioned 1 will be load / unload. Station 4 will be the machine action and all other stations will be idle or no action to the product.

When the machine is started, I need to find a home position since a stepper looses this at shutdown or worse power failure. I am planning on a mechanical limit switch that will be triggered by a peg as the table spins.

Is there a better way of doing this? Do I need to worry about wear on the switch? If I go with non contact type probes, how repeatable will they be?

Thank you,

Anthony

If you decide on a microswitch, use one with a 'leaf' actuator. The correct way to use the switch is NOT to detect the closing of the switch as the home position, but to detect the opening of the switch AFTER it has closed. You do that by checking the switch and then stepping ONE position and repeat until switch opens.

Paul

Will the machine produce any shavings or crumbs or suchlike? Any big current spikes? If so, you should take care to mount the sensor in a manner where these things won't interfere with actuation or cause false actuation (muy bad!).

The machine will be filling fluid in bottles. There should be no shavings but the lens will still need to be clean or have the ability to be cleaned.

Paul - I thought the back off method you mentions was for linear slides where there is a physical barrier that you don’t want to jam up against. In this case, the machine will not have that issue and will only rotate one direction. Am I missing something? Are the switches more repeatable when the switch is releasing?

aiannar974:
Paul - I thought the back off method you mentions was for linear slides where there is a physical barrier that you don’t want to jam up against. In this case, the machine will not have that issue and will only rotate one direction. Am I missing something? Are the switches more repeatable when the switch is releasing?

When starting a machine, the moving part can be anywhere along it's path. If the program moved one step and tested for the switch , then did it again, and again, the reset time could take many seconds, minutes.

So, a rapid movement until the switch is detected, then step by step the other direction until the switch is opened will take just a very few iterations.

My CNC soldering machine is that way. My electronic driveway gate is that way.

Paul

Paul - that is very creative. Makes a lot of sense. Thank you, anthony

Why do you need a limit switch?

It does no harm to the stepper motor itself to be driven (repeatedly) against a mechanical stop. You must of course, make the mechanical assembly sufficiently robust - but that would always be a basic requirement.

Paul_KD7HB:
If you decide on a microswitch, use one with a 'leaf' actuator. The correct way to use the switch is NOT to detect the closing of the switch as the home position, but to detect the opening of the switch AFTER it has closed. You do that by checking the switch and then stepping ONE position and repeat until switch opens.

Paul

Most microswitches have both options. There's a bonus.