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Results for tag "coil"

13 Articles

Test Equipment Upcycling – Variable Attenuator Module

A while back I found myself in the need of an adjustable RF attenuator capable of high-GHz operation. As luck would have it I had an old Spectrum analyser on the shelf at work, which we had retired quite some time ago.

Spectrum analysers being quite capable test instruments, I knew that the input attenuation would be done with a standalone module that we could recover for reuse without too much trouble.

The attenuator module

Here’s the module itself, with the factory drive PCB removed from the bottom, showing the solenoids that operate the RF switches. There are test wires attached to them here to work out which solenoid switches which attenuation stage. In the case of this module, there are switches for the following:

  • Input select switch
  • AC/DC coupling
  • -5dB
  • -10dB
  • -20dB
  • -40dB

For me this means I have up to -75dB attenuation in 5dB steps, with optional switchable A-B input & either AC or DC coupling.

Drive is easy, requiring a pulse on the solenoid coil to switch over, the polarity depending on which way the switch is going.

Building a Control Board

Now I’ve identified that the module was reusable, it was time to spin up a board to integrate all the features we needed:

  • Onboard battery power
  • Pushbutton operation
  • Indication of current attenuation level

The partially populated board is shown at right, with an Arduino microcontroller for main control, 18650 battery socket on the right, and control buttons in the centre. The OLED display module for showing the current attenuation level & battery voltage level is missing at the moment, but it’s clear where this goes.

As there weren’t enough GPIO pins for everything on the Arduino, a Microchip MC23017 16-Bit I/O expander, which is controlled via an I²C bus. This is convenient since I’m already using I²C for the onboard display.

Driving the Solenoids

A closer view of the board shows the trip of dual H-Bridge drivers on the board, which will soon be hidden underneath the attenuator block. These are LB1836M parts from ON Semiconductor. Each chip drives a pair of solenoids.

Power Supplies

The bottom of the board has all the power control circuitry, which are modularised for ease of production. There’s a Lithium charge & protection module for the 18650 onboard cell, along with a boost converter to give the ~9v rail required to operate the attenuator solenoids. While they would switch at 5v, the results were not reliable.

Finishing off

A bit more time later, some suitable firmware has been written for the Arduino, and the attenuator block is fitted onto the PCB. The onboard OLED nicely shows the current attenuation level, battery level & which input is selected.

Wheelchair Motor Service Part 1: Teardown & Inspection

Trolley Propulsion System: Wheelchair Motor Units

So it’s time to get the propulsion system underway for the trolley, a pair of wheelchair motors were sourced for this, from HacMan. Since I don’t know how many hours are on these units, or how they’ve been treated in the past, I’m going to do a full service on them to ensure reliability. I decided on wheelchair motors due to their extreme ruggedness & heavily built components – this project when complete is going to weigh in at about 150kg!
I suspected something was amiss with one of the motors from running them under no load: the left hand wheelchair motor was heating up to the point of being too hot to touch, so this one at the very least needed some investigation.

Motor Disassembly & Assessment

Rear Cover Removed

Rear Cover Removed

With the back cover removed from the motor the electromagnetic brake is revealed. This engages when power is removed to stop the motor freewheeling, which even though it’s a wormdrive box, it will do readily if backdriven.

Electromagnetic Brake Assembly

Electromagnetic Brake Assembly

The brake is rated 6.7W at 24v DC.

Brake Disc

Brake Disc

The brake disc is just visible between the plates of the brake here, with some green dust worn off the disc. When power is applied, the top disc, just under the magnet on top, is pulled upward against spring pressure away from the brake disc, which is attached to the motor armature.

Brake Disc

Brake Disc

Here’s the brake disc, removed from the motor. There’s only a little wear here, as I’d expect – these brakes don’t engage until the motors have come to a complete stop.

Brake Actuator

Brake Actuator

The steel disc above the magnet acts as one of the friction surfaces of the brake.

Brake Solenoid

Brake Solenoid

Finally, the solenoid is at the back, partially potted in resin. The strong coil spring in the centre applies the brakes when power is disconnected.

Gearbox Grok

Gearbox Grok

Removing the top of the gearbox reveals the state of the internals – There’s no wear at all on the gearset, but the lubricant is totally manky. The external oil seals have been leaking for some time, letting water in and grease out. The emulsified result is revolting! These gearboxes have a wormdrive first stage, the worm gear is underneath the left hand gearset. Steel spur gears then do the final gearing to the output shaft. The output gear is splined onto the output, and can slide along the shaft out of mesh – this is the freewheel clutch mechanism. At the moment it’s all obscured by the disgusting lubricant.

Input Shaft Seal

Input Shaft Seal

Here’s the failed seal on the left hand gearbox, the face damage was done by petrol immersion to clean everything up. (The seal is already compromised, so I’m not fussed about solvents eating the remaining rubber). The motor shaft is joined to the gearbox input by a rubber coupling.

Output Shaft Seal

Output Shaft Seal

The output shaft seals seem to be still OK, there has been some seepage past the collar that the shaft rides in, but nothing more. This can be resealed with some Loctite bearing sealant. The sleeve is held into the gearbox by the wheel hub when in operation, but this doesn’t seal the gap unfortunately. I don’t know why the manufacturer didn’t just machine the shaft to that larger diameter, instead of using an extra sleeve to accommodate the seal.

Bore Seals

Bore Seals

The bore seals covering the ends of the shafts are also fine, which is a good thing, since I can’t seem to find replacements for these anywhere. The input shaft seals will be replaced on both gearboxes though.

Motor Contamination

Motor Contamination

The oil seal must have been leaking for a long while! This is the gearbox end of the wheelchair motor frame, completely clogged with grease. Luckily only a small amount has made it down past the armature to the brushgear.

Damaged Commutator

Damaged Commutator

The commutator of this motor is badly damaged, and the brushes are very worn. This has been caused by the gearbox oil seal failing, and contaminating the motor internals with lubricant. The undercut between the segments is all but gone – filled with an abrasive mixture of brush dust, copper dust & old lubricant. Some repair work will be required here.

Second Motor

Second Motor

Here’s the brushgear removed from the second wheelchair motor, this one looks much more normal, and there’s not as much wear on the brushes or the commutator. Just the usual coating of brush dust.

Armatures

Armatures

Here’s both armatures together, with the contaminated one on the right, after some cleaning to remove most of the greasy old grok & brush dust from everything. The windings on the damaged left hand wheelchair motor haven’t darkened, which I would expect from severe overheating damage, so I’m hoping this armature is OK, and won’t require a rewind. Using an ohmmeter on these windings doesn’t tell me much – there’s only 7 turns of 0.86mm (20AWG) magnet wire in each coil, so they read as a dead short anyway. There was some leakage between the windings and the core before I cleaned things up – this was in the high (28+) megohms range, but this seems to have cleared now I’ve given things a real good cleaning.

More to come when new bearings & seals arrive!

Aspen Universal Condensate Pump

Universal Peristaltic Condensate Pump

Universal Peristaltic Condensate Pump

Here’s another piece of commercial gear, from an industrial air conditioning unit. These pumps are used to drain the condensate from the evaporator unit, so water doesn’t end up raining down from the ceiling.

Pump Head

Pump Head

This is a peristaltic pump, with a silicone hose forming the pumping element.

Rear Panel

Rear Panel

The test switch & electrical connections are on the back, along with the data label.

Power & Sensor Socket

Power & Sensor Socket

The electrical connections are all on a single 5-pin socket. Along with 240v AC mains, there are a pair of thermistors connected to the unit, which switch the pump on when a 5°C temperature difference across the evaporator coil is detected. When air is cooled, it’s capacity for moisture drops, so the water condenses out on the coil.

Roller Wheel

Roller Wheel

Here the front cover has been removed from the pump, showing the silicone tube & roller wheel. The wheel was originally Cadmium-plated, but exposure to the elements has oxidized this into highly toxic Cadmium Oxide.

Pump Rollers

Pump Rollers

Here you can see the rollers. These pinch the tube at the inlet, and the rotation carries a slug of liquid through the tube to the outlet side.

Pump Tube

Pump Tube

Here’s the tube itself, the main wearing part of the pump. This is replaceable as a spare part.

Motor & Gearbox

Motor & Gearbox

Inside the casing is a shaded-pole motor, connected to a large gearbox, to give the slow rotation for the pump head. The rated speed is 51RPM.

Control PCB

Control PCB

There’s not much to the control PCB. The large resistor forms a voltage dropper, to reduce the mains 240v to a more suitable level for the logic. There’s a TL062C Low-Power JFET Op-Amp & a CD4060BCM 14-stage binary ripple counter forming the logic. The set point is adjustable via the potentiometer.

Pump Triac

Pump Triac

The pump motor is switched via this Z7M SMD triac, not much switching power is needed here as the motor is only a very small shaded-pole type.