WinchBot

The video about WinchBot

Parts list

My WinchBot originates from a cooperation with RS-Components which is why links in the table point to their online shop.

Component	Number	RS Components	Remark
Arduino Uno	1	715-4081
Standard servos	5	781-3058
Geard DC motors	5	321-3158	Drives for winches
H bridges	3	636-384	I purchased the bare ICs and soldered the H bridges. If you are not skilled in soldering electronic circuits, buy assembled boards with a L298N chipset instead.
Diodes	12	781-5616	Only needed if you create the H bridges from scratch.
Transmissive optical sensors	6	919-0165
Raspberry Pi	1	832-6274	Optional, the WinchBot can be commanded from any Linux PC.
Touchscreen for Raspberry Pi	1	899-7466	Optional, the WinchBot can be commanded from any Linux PC.
Polycarbonate plates 625x305x5mm	1	769-8749	Use 5mm plywood instead if you don't have access to a CNC router.
Ball bearings 10x9x30mm	3	618-9985
Gear wheel 80 teeth, module 1	3	521-7691	Needed if no access to a CNC router given.
Gear wheel 12 teeth, module 1	3	521-6193	Needed if no access to a CNC router given.
Threaded rod M6	1	530-321
Nuts M6	1 Package	525-919
Screws + Nuts M3	multiple
Aluminum square tube Dimensions: 15x15x1000mm	1		Central mount of servo arm
Aluminum or steel square tube Dimensions: 20x20x1000mm	5		Triangular frame
Cord or steel cable	10m		I am using thin steel wire usually for jewelery.
Small parts like cables, washers and so on.

Mechanics

Figure 1:
I carved all parts with my CNC v3.2 from 5mm polycarbonate plates. Use 5mm plywood if you don't have access to a CNC router, because polycarbonate is tricky to process manually.

Figure 2:
The drums have ball bearings of the dimensions 10x9x30mm
Don't forget to drill a 1.5mm hole on the bottom disc (bottom left of the picture) that is used to tighten the cable.

Figure 3:
Use a tapper to cut 6mm threads in a couple of drill holes. The alignment of the taps becomes simple when using a manually operated (unplug the machine) drill press.

Figure 4:

Figure 5:

Figure 6:

Figure 7:

Figure 8:

Figure 9:
After the first test runs I have replaced the 30 teeth sensor disc by a 12 teeth version with 7mm tooth width. The distance from the center lines of the optical sensors is 10 to 11mm.
A 8 teeth gear is glued to the output shaft of the motors.
Instead of the gears carved with my CNC machine, you can buy module 1 gears (see parts list).

Figure 10:
The frame is made of 20x20x1000mm steel square tubes, because I had them in stock. Use aluminum square tubes to get a lightweight frame.

Figure 11:
The WinchBot is hanging on three cords on the ceiling of my photo studio.

Maths

Figure 12:
To operate the WinchBot, the X/Y/Z coordinates have to be transformed into cable lengths. Furthermore, the angle of the central square tube in the X/Z and Y/Z plain are needed to get the base frame of the servo arm aligned in parallel to the table below the bot.
Figure 12 shows the Y/Z plain.
To get the angle of the square tube in that plane, we need to know the distance between cord mount and pivot axis of the bearing on top (BarLength). Alpha is zero in the point of origin. If the WinchBot moves along the Y or Z axis, we get:
alpha = atan(Y₁ / (BarLength - Z))
Servo number 1 must compensate that rotation by turning for alpha degrees.

Figure 13:
The frame of the WinchBot forms an equilateral triangle. The distance between the deviation point of Motor 1 and the center point of the triangle is needed. Equilateral triangles are also equiangular, thus we get 60° for all angles.
Half that angle is 30° on top of the light grey triangle, by what we get for the distance:
Y_M1 = cos(30°) * (BaseLength/2)
Z_M1 equals the length of the square tube in the point of origin (BarLength). Now we need to get the error D_Y and D_Z caused by the fact that the cables are running to a ring with radius "CordMount" on bottom of the square tube (see figure 12). For that error we get:
D_Y = cos(alpha) * CordMount
and
D_Z = sin(alpha) * CordMount

Finally we get the cable length at X_L1/Y_L1/Z_L1 using the Pythagorean theorem:
L₁ = √ ( (Y_M1 - Y₁ - D_Y)² + (Z_M1 - Z₁ - D_Z)² )

Electronics

Figure 14:
5 servos, 3 geared DC motors, 3 H bridges, 6 optical sensors and an Arduino Uno are needed to operate the WinchBot.
The servos are connected through shielded cables. Connect the shield on the upper end to ground of the Arduino.

Software / Downloads

The software runs on Linux computers from the command line. You can get the package, including the installation instruction and all plans and drawings in the column Download.

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