Cobalt blue tarantula

Lately I have pondered (in-between the bouts my extremely demanding and satisfying job) about how to model the physical layout of the robot.

I have proclaimed it to be a hexapod from the start. This idea seemed like the most logical at the time. 4 is too few and 8 is too many. Having an odd number of limbs leeds to more complex gaits. Also, keeping the number of movable parts per limb low would reduce the cost and complexity of each limb.

However now I am not so sure any more. I personally like spiders better than insects and spiders have 8 legs. Also, the number of moving parts per limb in a spider (and in insects too for that matter) is much higher than 3. Evolution is the best mechanism to achieve optimization there is, and it has been working for over 4 billion years to perfect the spiders we see today. It follows thusly that one would be unnecessarily stubborn/proud not to at least observe, analyse and learn from the way spiders are.

Anatomy of a spider, for study before deciding on the anatomy of a robot.

What's in a spider anyways?

So I decided  to go all in, and find the perfect spider species to use as a template for the design of the robot. I ended up with the cobalt blue tarantula as the best  candidate for the following reasons:

• It is strikingly beautiful both in color and proportions. I firmly believe that aesthetics play a bigger role in design than merely pleasing one's eye.
• It is not too specialized. Many spiders have specialized into niche markets that make them unfit as a model to a "general purpose" robot. It is a solitary hunter and so is my robot.
• It has an well balanced anatomy that would be relatively easy to replicate.
• It has a temper similar to that which I want to give the robot (shy but aggressive).

Finding the right brush-less Direct Current (BLDC) motor to use as generator

Since I got my Honda GX25 motor, the task of finding the right generator to make the most efficient and compact generator for the batteries in the robot has lingered somewhere in the back of my mind.

First I thought about using an car alternator for this, but have since found perhaps an even better alternative, namely high-end radio controlled helicopter motors. It turns out alternators weigh a lot and are clunky and heavy, while BLDC motors from modern RC helis are not.

So how would I go about finding the correct BLDC for my project? I got some help here and ended up with the following:

• Lead-acid batteries have optimal charging voltage of 13.8 to 14.4 volts.
• The GX25 motor has optimal RPM (RPM with highest torque) @ 6000 RPM.
• The GX25 motor has an effect of around 0.7kW.

With this as input finding a motor for our project is easily calculated as follows:

• Desired kv (rpmvolt) = 6000 rpm/14.4 volt ~= 417 kv
• Desired continuous effect is above 700 W. We will go with at least the double to put the least amount of strain on the generator.
• Desired continuous amp is 700/14.4 ~= 48 A

(disregarding all the voltage losses etc).

Next I just chuck these parameters into hobbyking's motor finder to get some alternatives, for example the Turnigy SK3 5055-530kv with the following specs:

Turns: 12TVoltage: 5~8S LipolyRPM/V: 430kvInternal resistance: 0.019 OhmMax Loading: 70AMax Power: 1750WShaft Dia: 6.0mmBolt holes: 25mmBolt thread: M4Weight: 378gMotor Plug: 4mm Bullet Connector

I must investigate this further to ensure that I don't go buying one of these motors that wont work because of some parameter I did not think of.

Big push for the generator project.

I decided to make a push for the GX25 generator project.

First I removed the clutch from the GX25. I will be crafting some kind of frame for the generator assembly and also a converter plate to go on the shaft of the motor.

Next I prepared to make the optical tachometer circuit for the ECU to judge motor speed precisely. I had some luck pointing my manual tachometer to  black/white tape on the makeshift plate mounted on shaft. Still TODO: I still need a way to pump initial fuel into cold engine (manual pump), and actuating the choke might be handy in cold weather. But after some new intel (se bottom) I think I have found a way to make the generator act as starter motor as well, eliminating the added complexity of a separate engagement mechanism and separately controlled starter motor. Even better, I get to keep the recoil starter for debugging purposes (I already put it back on). Fun fun.

Next I put an ad out where I proclaim that I want any excess generators or alternators that people may have lying around. This resulted in me getting in contact with a fellow who had a 140AMP 12VDC 2005 mod Volvo V75 alternator for cheap, which I bought.

Next, I put an ad out where I proclaim that I want any excess electircal motors, which has resulted in a few emails from people who want to get rid of motors. So far I didn't get any motors though.


Next, I went dumpster diving, or more correctly, electrical appliance recycling plant scavenging. I will keep the location secret because, oh-boy was there a lot to be found!

3.5 HP 180V  7.5 AMP 5200 RPM Threadmill motor

Ditto

Threadmill motor driver circuit

ACME Screw linear actuator from threadmill.

One of 2 washing machine motors.

And last but not least, I put a question on electronics stackexchange asking what motor I would be looking for to make my generator. I got an excellent answer that I am definitely going to pursue next. In the same fell swoop I asked a question about how to measure the torque vs. RPM curve for my motor collection. Turns out there is something called "prony brake".

EMCO F1P Step motor specs

After my last post I have learned alot about step motors. I have also manged to figure out the specs of the step motors found in the EMCO F1P, and as always the quality of this machine turns out to be stellar.

My sources are:

Article on oriental motor. Explains the inner workings of stepper motors beautifully.

Article on stepper motors courtesy Douglas Jones of the University of Iowa.


The step motors in the EMCO F1P have 10 wires coming out of them. The wires are connected in consecutive pairs, one pair per two oppositely facing windings inside the motor (10 windings). The motor has almost no resistance when turning the shaft in un-powered state. The stator inside the motor has 50 teeth.

From this, I have gathered that this is a bi-polar, uni-filar, 5-phase motor design with 50 * 10 = 500 steps and 360 / 500 = 0.72° rotation per step.

This kind of motor is less common, and requires a more complex/expensive driving circuit. It offers higher precision, less vibration and noise and more torque at mid to high speeds than the more traditional 2-phase designs. Also there are more ways in which the driving circuit can drive it.

This has made me determined to keep the motors and find a new controller for them during my retrofit-project.

Anatomy of a step motor

I decided in the beginning of 2015 to stop spending time on my EMCO F1P CNC machines instead of the main goal of the proejct. Ironically that made me spend more time on the CNC,because I need to get them out of the way quickly.

Long story short, I want to convert one to use a new control board but without sacrificing the step motors. So I have now carefully removed one step motor (Z-axiz) and opened it up to see what's inside it.

Based on what I have gathered from searching around and posting a question on stack exchangel, this motor is most likely a bipolar motor with 10 windings.

Some pictures:

10 coils

Coil wiring

5 Screws

Removed connector housing.

10 wires + ground.

Motor removed from belt

Belt drive with tacho generator

Heat-sink with thermal paste.