First and foremost is safety. We are dealing with a high powered and invisible laser. It cuts, it burns, it slices and dices skin, eyes, and anything else in it’s path. Should I have the machine open to replace materials or debug and accidentally place my hand on the control panel, I could accidentally turn on the unit and fire the laser. A software glitch or a pet might also accidentally start the laser unexpectedly. We need to fix this.
To solve the issue of safety, a few additions/replacements need to be made:
System ESTOP Button
An ESTOP button is a big red mushroom shaped button that latches when you push it in. They are used in industrial machines as a way to stop and de-energize a system. For my purpose, I will use it in one of two ways.
1) If the mains power consumption is low enough and I can find a contact that can support the current needed, I will wire the mains power through the button. That is, if I press the button, the 120VAC being fed to the power supply will be cut off. I will also wire the laser ON/OFF circuit through a second contact so that the laser cuts out before the power supply has a chance to drain any stored power.
2) If the mains power consumption is significantly high, I will wire just the laser ON/OFF circuit through the button so that it acts as a safer laser ON/OFF switch. A circuit through the second contact would also be wired to the new control board to stop motor movement.
If I can, I will shoot for option 1. Given the tube fires at a max of 40W, that will give me around 0.5A at 120VAC with the power losses in the power supply (I’ll measure and report this later). I don’t trust the engineers who designed the laser power supply to have a fail safe system that won’t continue firing the laser after the signal was removed or that won’t fire on its own. There is too much at stake if they screwed up either the engineering or the wiring. It also allows me to stop the machine quickly and easily should something go massively wrong like a fire.
Laser Enable Key Switch
In addition to the ESTOP button, I will be replacing the laser ON/OFF toggle switch with a key switch. In a nutshell, you need a key to turn the laser on. Once you turn it off and remove the key, the laser
will not should not fire. This is great for a couple reasons. 1) Since there isn’t a button you can accidentally push, you can’t accidentally push it. 2) If I have company over that gets a little drunk and curious, I can remove the key and hide it so they can’t burn a hole in their fingernails or set fire to my apartment.
Covered Test Fire Button
Alright, alright, part of this is me wanting a goofy movie-style “FIRE!!!” button, but there is a practical use. I don’t want to accidentally push it. If the button has a cover that keeps you from pushing it, again, you can’t accidentally push it. So, double whammy – I get to live my action movie dream every time I use it and it increases the safety of the machine.
Current Settings and Measurement
Looking through schematics others have posted online, the current control knob on the machine is just a basic voltage divider circuit. Turning it one way or the other will sweep the voltage read by the power supply between 0V and 5V. Potentiometers are great for inexact things like volume control, but not for laser power control. They have two issues for this application. 1) They can wear out over time or oxidize. This means that there is no guarantee of a smooth transition of power as you rotate it. A bare spot internally might cause an intermittent short and therefore a massive jump in the laser power or else a short that takes out the power supply. I don’t like either. 2) They are not precise. When it comes to engraving, I want to have the ability to dial in the power for any given material. Once I find a setting that works, I want to be able to return exactly back to that setting.
To fix the potentiometer issue, I will be adding a small Arduino board to control a DAC (digital to analog converter) alongside either an LCD or 7 segment display. The DAC will allow me to send a precise and repeatable value from 0-5V and therefore a precise and repeatable power level. The power level would be displayed on the display. The actual input device will be a rotary encoder read by the Arduino.
Current Sensor Replacement
Analog meters are cool, but they kinda suck. They aren’t usually calibrated well (if at all), and can fail quickly due to their mechanical nature. I’ve used them in novelty clock projects and they just don’t last that long. To replace it, I have two main options.
The first is to place a 1Ohm resistor where the analog ammeter is and measure the voltage across it (it would have to be amplified to read well). I’m not a huge fan of this option. Should the laser tube fail or short for some reason, I will be left with around 40,000V being sent to a control board which will result in a fun explosion and an unsafe machine.
The second option is to utilize a hall-effect current measurement board. These come in two flavors. The first flavor is a non-contact sensor that wraps around the current-carrying wire and sense the magnetic fields there. The second flavor is placed in series with the circuit. It has a thick, half-circle shaped loop with the hall effect sensor underneath. As current passes through the half circle loop, it creates a magnetic field that is measured. Both are safer options, but they are susceptible to electromagnetic noise which the laser and its power supply will make a lot of. They can also lack in accuracy for lower currents which we will be using for the laser.
At the moment, I am leaning towards the second option – a hall effect sensor based unit. Should I have a failure somewhere, I’m much less likely to fry everything. I need to do more research to figure out what sensor units are available. Regardless of which I pick, they will be read by the same Arduino that is running the DAC and the value displayed on an LCD or 7-segment display.
Water Flow, Control, and Temperature
I’ve seen a couple examples of people adding temperature and water flow sensors to their machines. I like this quite a bit. It is extremely important to keep the laser tube cool. Should it overheat, it will go down the tubes. Knowing the temperature of the water reservoir and knowing that the water is in fact flowing is very helpful. I will be adding temperature and flow indication to the unit. The temp will be found with a thermocouple and the flow with a simple flow switch. Both can be found online for cheap.
I will be adding to these as well. A liquid level sensor in the reservoir will tell me if the water is getting low. A DPDT (double pole, double throw) relay will be placed in the laser enable circuit as well as the control board ESTOP circuit to stop the laser and relay if the water isn’t flowing, the temp gets too high, or the water level too low. A button added to the panel will be added to the panel to turn the water on/off via a dedicated relay.
I want to use a relay instead of just wiring a bunch of sensor switches in series is for a couple reasons. First, wiring a bunch of sensor switches is begging for some horrible debug issues. Should any one go down, you have do dismantle the entire system to find the problem. Having everything go back to a master node that monitors things and either lights up indication LEDs or outputs on a screen makes things a lot easier. Second, it allows me to use analog sensors and tweak them as time goes by. If I don’t like an enable setpoint, I simply change the code. It also allows me to add warnings and alarms in the future should I want them.
I may also add a pressure sensor that would be used shut off the flow in the event a hose gets disconnected.
Fire Detection and Ventilation Control
Should the laser cutter catch fire, I would like the system to stop running and cut off the air ventilation system so it doesn’t make the fire burn hotter. Normal methods of fire detection become difficult in laser cutting applications. Smoke detectors will not work as the unit creates smoke during normal operation. IR (infrared) detectors don’t work because we are cutting with IR and burning the material we are lazing. UV (ultraviolet) detectors don’t work for reasons similar to IR. What is left is internal ambient temperature. A thermocouple placed in the machine will watch for temperatures to get too high which would indicate a fire.
If a fire does occur and the temp increases, a relay would remove power from the blower and a second relay would put the system into a soft ESTOP just like with the water monitoring system.