Wednesday, October 21, 2015

How Do I Robot?! Part 2 + Meet The Builders #2

Building on the last instalment today we dive into control systems and motor controllers!
Sorry for the delay, this is the 3rd time I've had to re-write the article.

But first don't forget to checkout
Not only is it full of info for new builders but is a virtual meeting place for a majority of combat robot builders the world over!

Last week we told you about the 4 components needed for any robot.

1: Motors
2: Batteries
3: Control system
4: Motor controllers

With these 4 basic components you can build robot. Without them you will certainly have issues.
We asked Idea Factory Labs a rookie team to take part in our second Meet The Builders Instalment.

Team Idea Factory Labs 11pm Sept 18th
Cameo by ShaunZ of Team Orbital Spin in the back!

Idea Factory Labs is a new team who had plans to build and enter a 30lb sumo robot for Event #1 at Maker Expo. Unfortunately they lacked one of the 4 critical parts and try as we might we could not get them online for Sept 19th. 

Here's their take on the success and failures of their first 30lb robot Revered Rust Bucket. 

As Andrew said, time is always a factor but without the basic parts its difficult to make progress.

So here's what you need to know.

- Control System -

When it comes to sumo bots you have two options
-Radio Control

What you decide here may effect what motor controllers you need.

Lets talk about tether control.
What is involved:
1) Control box
The drive uses a control box with a number of analog switches on it
2) Signal wires
Low voltage signal wires run from the control box into the robot. (This means you have a physical connection with the robot)
3) Motor controllers ESC/Relay (more on this later)

Here is a handy intractable on a simple tether controlled robot.

Items 1-2 are fairly simple so lets look at Item 3, motor controllers regarding tether control.
Depending on your operating voltage you may be able to use simple switched attached directly to the robots motors like in the above instruct-able. However it is more likely you will need to use relays.

Relays are easy to use electrical switches, by applying a low voltage and current say 5v and 500ma you can turn a relay on (or off). In doing so the relay then allows a higher voltage/amperage to pass through its circuit, say 24v 10amp.

Using a relay allows you to use safer power levels in your control box and keep the high power stuff in the robot.

Why can't I just run 24v and 20amps to my control box with switches?
Because its against the rules!

Automotive relays can be had for under $5 each and operate on 12v.

Standard relay: Will drive a motor in 1 direction
For forward and backward control you will require 2 relays.

Reversing relay: Will drive a motor in 2 directions
For forward and backward control only 1 of these is required.

Here is a quick list of single and double through relays.
-12v Reversing Relay
-24v Reversing Relay
-12v Standard auto relay

Here is a little diagram showing how relays work!

Imagine the light bulbs as motors and you can see how this might work in a robot.
Its worth mentioning however you would have to remove one of the light circuits. You will notice with this circuit one light is always on. This means if you replace the lights with motors then 1 motor will always be running.

Now lets tackle Radio Control

What is involved:
1) Radio transmitter
2) Radio receiver
3) Motor controllers ESC/RC Relay (more on this later)

RC transmitter and receiver

For purposes of this discussion we are going to assume we are using off the shelf components and not custom built radio systems.

First thing is you need a radio system. Stick to the following frequencies:
-FM 75mhz
-PCM 75mhz

Other frequencies such as AM or FM 72mhz are not suitable for ground based robots or unsafe due to  interference issues. Also avoid simple RC toy controllers.

A complete starter radio system can be had for less then $50

Your transmitter sends out a radio frequency which is then picked up by the receiver. The receiver then turns that signal into an electrical pulse (RC PWM). An electronic speed controller, ESC can then use that RC PWM signal to control a motor.

Here is a little setup picture of a basic RC setup.

- Motor Controllers -

Now its the big and more confusing topic. How do I drive my motors off an RC rig?!

Use an electronic speed controller (ESC) or an RC Relay

An ESC is a little circuit which interprets the RC PWM from the receiver and uses it to drive a motor.
An ESC allows you to run a motor from 0% to 100% and everywhere in between for proportional control.

When choosing an ESC consider the following;
-Motor Voltage
-Motor amperage

Lets assume we're using this motor:

According to the specification sheet this is a 12v motor with a stall current of 23amps.

The stall current is theoretically the most current this motor can draw, so our ESC should be able to handle a minimum of 23amps (lower and we risk damaging the ESC).

Here is a list of popular 30lb combat robot ESC's which would work in this instance
-BotBitz 30amp
-Talon SRX
-Roboclaw 2x30
-Marine Controller
-Scorpion XXL

A single channel ESC like the Talon SRX or BotBitz 30amp will drive 1 motor forward and backward.
A 2 channel ESC like the Ragebridge or Roboclaw 2x30 will drive 2 motors forward or backward independently.

For instance I use a Roboteq AX1500 2 channel ESC in my 120lb robot. This allows me to use 1 circuit board (AX1500) and run each set of drive motors of my robot independently.
Being able to run motors independently is important for a robot with tank style steering.

What is tank style steering?
The robot below has 2 wheels and 2 motors, one on each side of the robot. By altering the direction of these wheels the robot can drive forward/backward or turn.

What is an RC Relay?

An RC relay is kind of like a dumb ESC, it has a small circuit onboard which can read the RC PWM signal from the RC. When the signal from a transmitter is given the circuit in the RC Relay will open or close said relay.

Here is an example of a motor control circuit with an RC Relay.

Just remember, when relays are turned on they give the motor 100% power. This might make it difficult to drive.

The circuit above will allow you to remotely drive a motor in a single direction. You will need another RC relay wired slightly differently (with diodes) to drive the motor in the opposite direction.

Here is a list of RC Relays:
-Battle Switch
-Double Battle Switch (useful for running forward/backward)
-TD Relay (useful for running forward/backward)
-Pololu Relay

This should be more then enough to get to get you moving!

If you have more questions feel free to email, tweet or join our Facebook Group!

Until next time!

-Ravi B
All Black Robotics

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