Our hackerspace is adding some smarts to the laser cutters to prevent stupid and expensive mistakes.
Another guy is doing the logic (in hardware -- for resilience).
My job is the airflow sensor.
After discussing a few options (including hot wire sensors and the like, I have sorta settled on a pair of thermistors in thermally connected enclosures, one of which will be in the air path.
The circuit is as follows. (It's very conceptual at present -- I'm not really sure what voltages I have available).
Does anyone want to cast their beady eye over it and make comments?
The delay between activation/loss of air and reporting can easily be a couple of seconds, but I'm aiming on making it sub-second, but slow enough that mains interference and other noise will be filtered out as "high frequency".
I'll also let you read the summary I sent to our members:
And here are the conceptual schematics:
Any other thoughts? Perhaps power supply out of tolerance (e.g. power disconnected, low input voltage, etc?)
Another guy is doing the logic (in hardware -- for resilience).
My job is the airflow sensor.
After discussing a few options (including hot wire sensors and the like, I have sorta settled on a pair of thermistors in thermally connected enclosures, one of which will be in the air path.
The circuit is as follows. (It's very conceptual at present -- I'm not really sure what voltages I have available).
Does anyone want to cast their beady eye over it and make comments?
The delay between activation/loss of air and reporting can easily be a couple of seconds, but I'm aiming on making it sub-second, but slow enough that mains interference and other noise will be filtered out as "high frequency".
I'll also let you read the summary I sent to our members:
As promised, while at the buck's night last night I put more thought into the airflow sensor. But not much...
This morning I discovered I have only 2 platinum resistance thermistors, so the thought of burning on out (to determine the max current carrying capacity) and then using two of them was not going to be an option.
Instead I pulled out some 470 ohm NTC thermistors and determined that at 50mA (where the resistance falls to around 140 ohms) they are very sensitive to any air movement (and surprisingly fast to respond). The power dissipated is around 350mW per thermistor at equilibrium (falling from 1W at cold startup).
One possible problem is that the air coming from the compressor will be at a temperature different from ambient. In some circumstances this might be a problem. One option is to mount each thermistor in the airflow, but having one given a highly restricted airflow. Another option is to mount both inside metal enclosures that are thermally connected, one open to the airflow, the other closed.
My original idea was to set these up as a voltage divider driven by an AC source to give a variable amplitude AC output. I'm now considering each thermistor having its own constant current source and comparing the two outputs to generate a signal.
In the proposed arrangement any common mode noise will be pretty well cancelled out, so shielded twisted pair cable (similar to audio cable) should be ample to reduce noise. Our interest in low frequency signals (determined principally by our requirement for a particular reaction time) means we can further attenuate signals above (say) 1Hz without any real degradation.
What are the voltages easily available inside the laser cutter? Is there a 12V rail?
Attached are three images.
LaserAir-1 is the circuit to generate a pair of filtered and buffered voltages, one from the reference thermistor, the other from the thermistor in the airflow.
LaserAir-2 is the error outputs. I can detect either input pulled too close to the upper or lower rail (failure of current source or thermistor) and the reference thermistor returning a voltage lower than the airflow sensor (probably misalignment or thermistor failure). These can be combined into a single active low output.
LaserAir-3 contains the circuit for reporting airflow.
It is likely that any or all of the conditions (error and/or air) could be reported on startup. If the controller has suitable capability, it should wait until a few seconds after the power is applied, then, if there is no error, turn the air on and off to validate the operation of the solenoids, availability of air, and the ability of the circuit to detect airflow.
I'm going to pass this conceptual circuit pass a few people for comments before I start thinking about component values. Knowing the power supply rails available and the required specs for the outputs (these can't drive more than a couple of mA) will be of great assistance.
This morning I discovered I have only 2 platinum resistance thermistors, so the thought of burning on out (to determine the max current carrying capacity) and then using two of them was not going to be an option.
Instead I pulled out some 470 ohm NTC thermistors and determined that at 50mA (where the resistance falls to around 140 ohms) they are very sensitive to any air movement (and surprisingly fast to respond). The power dissipated is around 350mW per thermistor at equilibrium (falling from 1W at cold startup).
One possible problem is that the air coming from the compressor will be at a temperature different from ambient. In some circumstances this might be a problem. One option is to mount each thermistor in the airflow, but having one given a highly restricted airflow. Another option is to mount both inside metal enclosures that are thermally connected, one open to the airflow, the other closed.
My original idea was to set these up as a voltage divider driven by an AC source to give a variable amplitude AC output. I'm now considering each thermistor having its own constant current source and comparing the two outputs to generate a signal.
In the proposed arrangement any common mode noise will be pretty well cancelled out, so shielded twisted pair cable (similar to audio cable) should be ample to reduce noise. Our interest in low frequency signals (determined principally by our requirement for a particular reaction time) means we can further attenuate signals above (say) 1Hz without any real degradation.
What are the voltages easily available inside the laser cutter? Is there a 12V rail?
Attached are three images.
LaserAir-1 is the circuit to generate a pair of filtered and buffered voltages, one from the reference thermistor, the other from the thermistor in the airflow.
LaserAir-2 is the error outputs. I can detect either input pulled too close to the upper or lower rail (failure of current source or thermistor) and the reference thermistor returning a voltage lower than the airflow sensor (probably misalignment or thermistor failure). These can be combined into a single active low output.
LaserAir-3 contains the circuit for reporting airflow.
It is likely that any or all of the conditions (error and/or air) could be reported on startup. If the controller has suitable capability, it should wait until a few seconds after the power is applied, then, if there is no error, turn the air on and off to validate the operation of the solenoids, availability of air, and the ability of the circuit to detect airflow.
I'm going to pass this conceptual circuit pass a few people for comments before I start thinking about component values. Knowing the power supply rails available and the required specs for the outputs (these can't drive more than a couple of mA) will be of great assistance.
And here are the conceptual schematics:
Any other thoughts? Perhaps power supply out of tolerance (e.g. power disconnected, low input voltage, etc?)