Tag Archives: position detector

The position regulator

The position regulator takes the output from the position sensor, and converts it into a voltage drive signal. Nothing fance-it is just an opamp implementation of a PID regulator.

levi_position_reg

Brief description:
The position signal is connected to JP1-8, and converted to a 0-referenced voltage, relative to the desired position setpoint (R13, 10-turn pot), using IC1A, 1B and 1C.
JP7-1 provides this shifted signal for monitoring (position error signal). Opamps IC2A, 2B and 2D for the proportional, differential, and integral parts of the PID regulator. The P-component is made user-settable via R14 – a 10-turn pot. The other components only need some initial adjustment, depending on the coil you are using (C5 and C8 might also need adjustment; use high-quality capacitors, Mylar or similar). IC1D sums all the regulator components, and has R12, another 10-turn used-adjustable pot for total gain adjustment. Total gain is the most critical adjustment, if you change the type/weight of the suspended part.
IC2C adds a constant offset-just to prepare the signal (available at JP1-3) for the current regulator. JP1-5 has the intermediate current regulator setpoint, for monitoring purposes.

The position detector

The position detector module senses the exact position of the object suspended below the magnet, using a modulated infrared (IR) light beam, and a frequency-selective phototransitor amplifier, and converts it into a voltage signal. The range is a few mm, so there will be a need to adjust the position of the detector, if you want to change the distance from the magnet.
The circuit is pretty fast, and can detect position changes in the range of a few micrometers easily, and at kHz bandwidth. Also, the output (voltage vs. gap) is very linear, this has been checked with the help of a micrometer stage, used to block part of the beam.

levi_position_sensor

Brief description – lower part, the IR emitter driver:
This part of the circuit uses one half of a 324 quad opamp – IC1D is running as an oscillator, and IC1C, as a current regulator for the IR emitter, a Fairchild QED233 (940 nm, 40 degrees emission angle, 1.5 A peak forward current). R11 is the current sensing resistor – a BUZ11 is used as a driver. The QED233 is attached to JP1-1 (cathode) and JP1-2 (anode). R12 and C7 de-couple the IR emitter supply, to avoid interference with the receiver circuit.

The upper part, the IR receiver: active element is a Fairchild QSD124 phototransitor (narrow angle, 24 degrees, IR diode with daylight filter), attached to JP1-5 (collector) and JP1-6 (emitter).
The phototransitor output first passes through a high-pass filter, R6/C3, and then through a rectifier, D1. The resulting signal is then low-pass filtered to remove the ripple (R8 added to reduce noise/oscillations), and amplified (IC1A). IC1B does some further clean-up, and provide a nice drive signal for the other circuits. R1 really serves no function other than being a handy test point for troubleshooting.

The coil, the circuits

Others have explained at length the theorical background of magnetic leviation, and the associated regulator maths. I don’t want to get into this here – feel free to ask, I will explain it to you.

The goal: an apparatus that can hold a sphere of at least 1 kg mass (about 60 mm diameter, 2 1/2″) suspended from the pole of the magnet, in a distance of at last 25 mm/1″. It should be adjustable to hold also other parts, and not restricted to magnets (many of the available hobbyist leviation circuits can only hold magnets).

With the goal set, and after some calculations, related to electric magnet, heat dissipation, allowable maximum temperature for free air operation, time constants – the coil: about 10 kg of copper, of roughly 3 mm diameter wire. I selected a high-temperature type W210 isolated copper wire, this will withstand long term use even if the coil is operated permanently at full current. Don’t worry about the space the 10 kg will take, copper is very dense… Winding it can be a bit tough, as always, ask a friend for help, and best wind it with the help of a lathe (by manually turning it), or some other makeshift fixture. Winding it free-hand is not a good choice, because you won’t get a nice and tight coil.

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For the circuits, these were build in modular style:

(1) The position detector – based on an infrared LED, and infrared photodiode, and fully independent of the lighting conditions. Is uses a modulated emitter, and a frequency-selective detector. It needs to have a reasonably fast response, and low noise/drift, otherwise, the regulation loop will be unstable – the sphere will start oscillating, and drop off.

(2) The position regulator – this is a classical PID regulator, build using a few TL074 opams. It compares the position setpoint and acutal position (from position detector, item 1), and determines the current needed to hold the sphere stable (current setpoint signal).

(3) The current regulator – this circuit converts the current setpoint signal into a drive current for the coil. To avoid undue powder dissipation in the regulator, this is achieved use a classic TL494 PWM regulation scheme. There is also a current limiting circuit, build-in, with a 0.050 Ohm 4-lead sense resistor in the coil current loop, and an AD620 instrumentation amplifier.

(4) Some auxilliary circuits – to switch off the magnets by disabling the current regulator, if there is no object attached. Otherwise, the magnet would run at full current, if the sphere drops off, and dissipate a low of heat for no good reason. Some power supply circuits – to provide 12 V power for the regulators, and unregulated 35 V to drive the magnet.