My Robotic Arm

I designed and built a robotic arm for a friend who had built a clever telepresence robot based on a Roomba vacuum.  His software development was brilliant, but he lacked the facilities to construct the mechanical parts, like the arm.  A primary requirement for the arm is for it to be lightweight, for the Roomba lacked the weight and power to maneuver with a heavy arm.  It was also critical that as much of the mass as possible was located close to the center of gravity so that the lightweight robot did not lose its balance.  Therefore, I located all of the heavy servos in the shoulder, and made the wrist and claw as light as possible.  Strong Kevlar strings transmit the motion from the servos to the joints.

The shoulder:  5 servos and several pulleys packed tightly together.
Servo 1: (large one center left)
Arm shoulder joint up/down
Servo 2: (large one top left)
Elbow joint up/down.
Servo 3: (small one bottom right)
Claw open/close
Servo 4: (small one above Servo 3)
Wrist clockwise/counterclockwise.

The wrist and claw.  Everything possible was done to reduce its weight. The aluminum parts were lightened by by milling large holes in them, and by keeping the parts count low.  The hinge halves and the wrist housing are one piece.  The wrist pulley and lever are also one piece.
 This was made on my Harbor Freight mill using a rotary table, since turning this piece on a lathe would not be possible.

This pulley/lever combination opens and closes the claw using a stiff wire which passes through the hollow wrist axle.

 The wrist is rotated by a pulley driven by one of the Kevlar strings. This string pair, and the claw string pair run through the lower PVC pipe.  There are pulleys in the hinged pipe joints to ensure that the string tension and position remains constant throughout the range of elbow movement.

The wrist pulley is held to the wrist axle by a wire pin to make removal for servicing easy.




The new robotic hand was far smaller and lighter than the original hand, which had its servo coupled directly to the claw. The original hand did not have an articulated wrist.   Since the original arm was being left on the robot, I copied the style by also using a pair of 1/2 inch PVC pipes for the "bones"

The lower elbow joint has 3 pulleys inside a very small space.  One bronze pulley guides the wrist strings, and the other bronze pulley guides the claw strings.  The center pulley is locked to the outer half of the joint, and carries the string which is attached to the elbow assist spring.   Top left in picture: The center pulley being turned from 1/2 inch diameter steel stock.  Top center: the steel pulley and one bronze pulley.  There was not enough space for a V groove in the bronze pulley, but since the bronze pulley extends into the relief cut in the sides of the steel pulley, the string stays on the pulley, and does not get caught in the space between them.   Note that the center of the steel pulley is threaded.  This pulley does not turn relative to the outer half of the elbow joint.  

The upper elbow joint is a completely different design, for its sole purpose is to move the elbow.  The pulley in this one was machined as one piece with the hinge half, and designed to take the large amount of torque required to actuate the elbow while having a lever arm length of only 3/8 inch.  The joint has a huge mechanical disadvantage of about 20:1.  As a result, about 20 foot pounds of torque is required for the arm to lift a 1 pound weight.  This was a consequence of choosing styling over  engineering.  I wanted to create an arm which moved without any external mechanisms visible.  However, it also met my requirement that the weight be kept very low.  The arm worked well, and met its design goal of being able to lift a cup of water.  Designing an arm which could lift more weight would have been pointless, because the robot would have lost its balance if it tried to lift more weight.

 The string was tensioned by turning this nut.  The string had to be kept very tight for proper operation. That was easy to do with the Kevlar string. I used multiple strings to achieve a tensile strength of about 200 pounds.  Manually moving the arm would not break the string.

The underside, or "armpit" or the arm.    There is very little wasted space.  Note how closely packed the  shoulder rack and pinion is.  The end of the rack is contoured to clear the curved end of the shoulder servo. It appears that there is no room for it to move, but it can move  to its full travel limits.  This gives the arm about 10 degrees of left/right motion. I chose this range of motion to keep the design simple. 

This photo shows how the 10 degrees of movement was easily done.  The arm can move that much while retaining a simple belt drive for the up/down movement, and all the servos can be mounted in the same frame.  This is only slightly more complex than not having any left/right movement at all.  A lightweight, compact design was more important than having a larger range of movement. 

Bottom view of the belt drive.  The servo pulley is also custom made. The splines in the servo pulley were filed by hand. 

The robot I bult the arm for is called MAYA, and it is the brainchild of Ben Hylak, who came up with the idea of hacking a Roomba robot and using it as the motive power for a low cost telepresence robot. It is Ben's telepresence robot concept and software development that is the reason for the national recognition he has received and his subsequent invitation to the White House.  My arm merely went along for the ride.  While I have never received, nor expect to receive an invitation to the White House, at least I can say that something I had a part in making was a guest of the President!

Stancor Ultralinear Amplifier

The Stancor original Williamson and Williamson Ultralinear amplifiers were kits marketed by Stancor to showcase the performance of their transformers.  They used a separate power supply chassis connected by a 4 conductor cable. A pair of these makes a great sounding stereo power amplifier.  The Ultralinear is much preferred because the power output is much greater. 25 watts vs. 8 watts. 

I rebuilt a set of these, and can attest to their fine performance. 

The number one problem with any old electronic equipment is the electrolytic capacitors. I removed the metal can capacitors and replaced them with modern ones.  The modern ones are much smaller than the originals, and are designed for mounting on a printed circuit board.

I cut pieces of circuit board material into the shape of the original capacitor bases, and drilled holes for the new capacitors.

At Right:  The new next to the old.

Below: An amplifier chassis with the new capacitors.

One of the originals was a dual capacitor, so my replacement has 2 capacitors, too. 

 The power supply chassis has 3.   The 3 capacitors combined with one of Stancor's chokes do a great job of filtering hum from the 440 volt supply.

 I made new cables to connect the power supplies to the amplifiers.  The plugs and sockets are the same as early 4 pin tubes, like the type 80 rectifier.  I needed a replacement plug, so I took the base off an old Philco 80 tube, and made an aluminum cap for it.

The chassis had no bottoms, but they have threaded holes to attach bottom covers.
I made covers from sheet steel and attached rubber feet to them

I modernized the chassis by adding power sockets and replaced the original 2 wire lamp cord with 3 wire grounded cords.
 The modern cords and the steel bottom covers make these amplifiers much safer.

The amplifier schematic.
  Ultra-linear circuits are easy to identify by the number of transformer leads going to the output tubes.  Ultra-linear circuits have 2 wires to each tube, while other circuits have just one.  If you find a Stancor chassis with the labels missing, this is how you can tell which version you have.

The amplifier has sockets to measure and balance the plate currents on the 807 output tubes.  This is important for two reasons. One, balanced current is important to achieve the lowest distortion. Another good reason is to verify that the tubes are not using too much current.  This happened to me. C4 was bad.  If C4 or C5 are leaky, the grid bias voltage will go positive, and cause the tubes to draw excess current.  Drawing only a little too much will cause the plates to glow red to an excessive degree..

In this picture, the tube in the foreground is drawing too much plate current.  The one in the background is ok. Its plate is slightly red, that is acceptable.  The blue glow on the glass is acceptable, too.  A gassy tube has a glow inside the tube.  These are factory new Raytheon tubes.

The power supply is simple.  The supplies are only large enough for a single amplifier, and will overheat if two amps are connected to one power supply. 

1942 Philips GM 3155B Kathograph II Oscilloscope

     About 35 years ago, an older gentleman I worked with gave me this Philips oscilloscope.  It was given to him many years before, and that man told him that it came off a German U-Boat.  It is possible, but I can't prove it.  Apparently a GI brought it home as a war souvenir.  This sat among my collection of radios until today, when I decided to see if it worked.  Why did I wait 35 years?  Because I was happy to have it on my shelf, and did not want to risk blowing the rare Valvo tubes.  But  now I need to sell it and other things in my collection, and I felt that if it were operable, I could get a higher price.
     I first had to tackle a problem that I have known about for 35 years, and was one of the reasons I simply put it on the shelf. Someone in the past tried to repair it and made a mess of things.

The Y gain control potentiometer failed, and someone decided to "fix" it with a volume control from an old radio. Trouble is, the volume control had a SPST switch at the full CCW position, while the original had a SPDT switch to change the range of the gain control.  To solve that problem, the repairman put a toggle switch inside the cabinet.  I knew it couldn't be wired right because he had the the wire from the braided shield connected to the switch, when it must go to ground.
 

At that point, I did not have schematics, so I took my best guess. I also rooted through my collection of potentiometers and found one with a SPDT switch. It was a 2 gang pot, but I will just use one.  I later found out it is the exact value I need, 500k ohms.

I connected the wires, and was ready to try it out.  All I needed was a power cord.  I did not want to solder wires to the mains socket as was done before, so I went back to my supply of electrical hardware. It turns out that an American appliance connector is a close fit for this vintage European connector.

This oscilloscope is very versatile, and can be used nearly anywhere in the world, thanks to it's multi-tap transformer and voltage selector switch.Both sides of the mains are fused. 

I verified I had the right voltage, and plugged it in.
Nothing. No lights, no hum, not even any smoke.
I discovered that the power switch is bad.  Both sides of the mains are switched, and both switches were stuck open.  I soldered jumpers across the switch, for I want to leave the original switch intact.  This way everything is original except for the Y gain control.

I tried it again.  I should mention that I don't just plug it in, rather I raise the voltage slowly using a variable transformer.  This way I can catch a little problem before it becomes a big one.

 Amazingly, all the tubes lighted up, and after perhaps 50 years, the oscilloscope came to life again! Soon I had a trace on the CRT.
I did not know it, but I was soon in for a shock!  A 610 volt shock, to be exact.  See the 4 filter capacitors in a row? Unlike what is commonly found, where the metal cans are grounded to the chassis, these are on insulated bases, and the can voltage is very high!
From left to right, the first one is at chassis ground, no surprise there. However, the second one is at +205 volts. The third one is at -405 volts!  Between capacitors 2 and 3 is a potential of 610 volts!  Cap #4 is "only" -262 volts.  So, if you ever encounter one of these oscilloscopes, do not touch the capacitors!

The two clear glass tubes are 1876 rectifiers, and the gold tube is a 4673 pentode for the Y amplifier.

 Impressively, all of these high voltage capacitors appear to be good.

A view inside the cabinet.
That little rectangle of paper is very important!
That is all that is preventing the capacitors,
and their high voltages, from touching the metal cabinet.

Here is a view of the other side:  The clear tube is a 4890 triode oscillator, and the shielded tube is a 4673 pentode amplifier for the X axis.

Underneath the chassis, it looks like the day it was manufactured, 72 years ago.

 A side view of the cabinet.  The 72 year old decals are still in fair condition:

Finally, here is it in operation for the first time in 50 years.  Most of the controls work well, and the trigger is surprisingly stable.

The trigger is disabled by pulling this jumper out:

 This simple scope does not have centering controls, and I did not see any trimmers inside.

 I have to translate the German language instruction manual to see if there is a way to center the image.  Unfortunately, two of the knobs were missing, and I had to substitute the ones seen in the first picture.

The Kathograph has found a new home at Volker K's Oscilloscope Museum. http://www.oscilloscopemuseum.com/
Should you have any questions about an old oscilloscope, I highly recommend his website.

Lorex Camera Repair

I have a Lorex security camera system that works very well.  However, after a year in operation, two cameras developed blurry areas in the bottom 1/3 of the image, and the colors were not as good as they used to be.  At first I suspected moisture, since these are outside cameras.  But when I disassembled them, I found the day/night filter had come loose and fell out of place. This caused the blur at the bottom.

Here is how to disassemble and repair the camera. First, pry out the three white plugs in the lens, and then remove the screws.  One of the white plugs can be seen under the screw on the left. Then gently pry out the lens. I put a screwdriver in the holes and rock the lens loose, a little at a time. I change holes and rock some more. Do not exert too much pressure or you will crack the lens.

     The lens and electronics come out as a single unit.  There is an O ring around the lens and each screw.  These are critical, even in indoor use, to prevent moisture from fogging the camera.

     Remove the two screws in the black plastic plate, and the lens and infrared LEDs will come loose.  Unplug the LEDs and set them aside.
     Remove the center screw in the white plastic, and the two black screws close to the motor, which is centered above the white plastic. 

     Turn the camera over and lift off the motor and filter assembly. Be careful not to lose the filter!  it may fall out as soon as the screws are loosened.  Notice how the filter has one half that is red when viewed under certain lighting angles.

On my camera, the red half is on the right in the disassembled picture. It is important that it is oriented properly. The filter has a front and back. The small side fits into the plastic frame.  It appears that it was intended to be a snap fit, since I do not see evidence of glue. However I have to now glue mine to secure them in place.  Use very small amounts of glue and do not get any on the lens or the filter.  If you do, the camera will be ruined.

Reassembly.
Stretch the O ring over the lens. Try to get it on without it twisting.  It will seal better if it is back on the way it was originally.
Press the lens back into the body, and see that it is uniformly bottomed out. Do not use the screws to pull it in place, or the lens may crack. Tighten each screw a little at a time.  Your camera should now be as good as new.
Here are some tips on identifying the camera models. There are 50 viewing angle and 90 degree viewing angle cameras that look nearly the same.  You can tell them apart by looking at the lens.
      The 50 degree camera is on the left, and has a small lens in the rear, behind the day/night filter. In these cameras, if the filter is loose, you can see it.

  The 90 degree camera is on the right.  It's wide angle lens is close to the front, and the day/night filter is behind it.  You cannot see the day/night filter without disassembling the camera.
The electronics are different, too.

Here is the back of the 50 degree camera.  Prominently missing is the white plastic piece, which is a heat sink.  That heat sink on the 90 degree cameras makes a huge difference in night vision performance.








 Here are pictures taken at night from each camera. They are both looking at my driveway from the same distance. The driveway is barely visible in the picture taken with the 50 degree camera, while it is easily visible in the picture from the 90 degree camera.

Daytime performance of the two cameras is similar, but I prefer the wider viewing angle of the 90 degree camera.

The 50 degree cameras had IR LED boards that could hold 30 LED's, but only had 18.  The 90 degree boards have 18 LED's and no extra holes. I decided to experiment, and I took one of the 50 degree boards and added 12 more LED's. These LED's I mounted at an angle to reduce the bright spot in the center of the night view.  Notice in the picture below how the LED's are angled out.


I installed this board in one of the 90 degree cameras, and it made a huge difference.







Compare this picture to the other night pictures. It is unfortunate that Lorex did not use 30 LED's for they would have a much better product.