Electronics
OpenAFM Electronics
Voice Coils
Each coil is controlled by a pair of push-pull amplifiers. One of each pair has a constant output voltage of 2.5v
The other member of the pair is controlled by the output of the voltage adder, which takes inputs from the potentiometers. The 2.5v source from the voltage divider is also used to provide the virtual ground of the adder The voice coils are low impedance (4 Ohm?) so a small voltage creates a large current. Transistors in the current prototype are rated for ~1A which is more than enough to cook the coil. Large displacements will heat the coil and introduce drift
Op-amp Bits
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Voice coil voltage adder
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Piezo stage amplifier
U12.1 and U12.2 sum the coarse and fine inputs for focus and tracking. Input of 2.5v to + means that the difference of each input from 2.5v is summed and inverted. e.g. 2.7v (+0.2) and 2.4v (-0.1) input would produce 2.4v (-1*(0.2 - 0.1)) output. Coarse/fine sensitivity is determined by ‘squashing’ the range of the potentiometer by using resistors either side (make it a proportionally less important in the voltage divider)
Bypassing capacitors should be added to the output of these potentiometers in future, creating and RC low pass filter to remove some mechanical noise from the potentiometers
Opposing outputs (e.g. +x and -x) are created by inverting and non-inverting amplifiers from the same DAC output. These have a gain of ~2, giving a range of 0 - 10v and 0 - -10v respectively
Laser Power
LM317 is an adjustable voltage regulator which can be used as a constant current source. The regulator will adjust itself such that the voltage drop across R7 and J1 (normally shorted) is 1.25v. If we want 60mA current we need 1.25 = 60/1000 * R so R ~ 20 Ohms J1 allows for insertion of a variable resistor to increase resistance and lower current Reverse breakdown voltage of the laser diode is ~2.2v, and the current necessary to cause permanent damage is tiny. A 3.5v TVS diode is in parallel with the diode to suppress any spikes. It is also important to realise that if the laser is not plugged in, U33 faces an open circuit and will raise the voltage to ~8V in an attempt to pass current. This will charge the output capacitors, which would destroy the laser when it was plugged in if they were not discharged through R8.
General This is a sensitive circuit with both analog and high speed digital components (I2C, arduino + clock) The current paths of these components must be kept separate, else voltage drop from one may induce noise in the other. Return current paths must also be considered. Ground/power planes mitigate voltage drop by providing low impedance paths. An attempt was made in the current prototype to decouple analog from digital and PSU noise using ferrite beads (low impedance to DC but high impedance to high frequency) and multiple regulators. Effectiveness of this is probably limited whilst the arduino is involved, since we have no direct control over the power supplied to the arduino ADC Best solution in the future: optocouplers to isolate USB, onboard microprocessor with DAC and ADC controlled via I2C. Grounded metal case. However it is important to note that electrical noise has not been a problem so far
PCB Design • • • • •
Use mounting holes Planes can aid with routing as well as noise Make the board future-proof and testable Consider the effort involved in assembly Each unique component and inch of board area carries a cost
Things to Change
Voice Coils
Each coil is controlled by a pair of push-pull amplifiers. One of each pair has a constant output voltage of 2.5v
The other member of the pair is controlled by the output of the voltage adder, which takes inputs from the potentiometers. The 2.5v source from the voltage divider is also used to provide the virtual ground of the adder The voice coils are low impedance (4 Ohm?) so a small voltage creates a large current. Transistors in the current prototype are rated for ~1A which is more than enough to cook the coil. Large displacements will heat the coil and introduce drift
Op-amp Bits
•
Voice coil voltage adder
•
Piezo stage amplifier
U12.1 and U12.2 sum the coarse and fine inputs for focus and tracking. Input of 2.5v to + means that the difference of each input from 2.5v is summed and inverted. e.g. 2.7v (+0.2) and 2.4v (-0.1) input would produce 2.4v (-1*(0.2 - 0.1)) output. Coarse/fine sensitivity is determined by ‘squashing’ the range of the potentiometer by using resistors either side (make it a proportionally less important in the voltage divider)
Bypassing capacitors should be added to the output of these potentiometers in future, creating and RC low pass filter to remove some mechanical noise from the potentiometers
Opposing outputs (e.g. +x and -x) are created by inverting and non-inverting amplifiers from the same DAC output. These have a gain of ~2, giving a range of 0 - 10v and 0 - -10v respectively
Laser Power
LM317 is an adjustable voltage regulator which can be used as a constant current source. The regulator will adjust itself such that the voltage drop across R7 and J1 (normally shorted) is 1.25v. If we want 60mA current we need 1.25 = 60/1000 * R so R ~ 20 Ohms J1 allows for insertion of a variable resistor to increase resistance and lower current Reverse breakdown voltage of the laser diode is ~2.2v, and the current necessary to cause permanent damage is tiny. A 3.5v TVS diode is in parallel with the diode to suppress any spikes. It is also important to realise that if the laser is not plugged in, U33 faces an open circuit and will raise the voltage to ~8V in an attempt to pass current. This will charge the output capacitors, which would destroy the laser when it was plugged in if they were not discharged through R8.
General This is a sensitive circuit with both analog and high speed digital components (I2C, arduino + clock) The current paths of these components must be kept separate, else voltage drop from one may induce noise in the other. Return current paths must also be considered. Ground/power planes mitigate voltage drop by providing low impedance paths. An attempt was made in the current prototype to decouple analog from digital and PSU noise using ferrite beads (low impedance to DC but high impedance to high frequency) and multiple regulators. Effectiveness of this is probably limited whilst the arduino is involved, since we have no direct control over the power supplied to the arduino ADC Best solution in the future: optocouplers to isolate USB, onboard microprocessor with DAC and ADC controlled via I2C. Grounded metal case. However it is important to note that electrical noise has not been a problem so far
PCB Design • • • • •
Use mounting holes Planes can aid with routing as well as noise Make the board future-proof and testable Consider the effort involved in assembly Each unique component and inch of board area carries a cost
Things to Change
