- Created on Sunday, 23 June 2013 16:13
- Last Updated on Friday, 02 August 2013 06:43
Have you ever watched an episode of the TV programme called Robot Wars where in the middle of what can only be described as an engaging robot punch-up one of the robots suddenly stops working. This enevitably prompts the controllers to frantically punch buttons on the remote control with bemused looks on their faces. It also prompts you to mouth, politely, at the controllers, "You muppets!"
Well, if may have left you, like me, wondering at the time what they may have done wrong. I've always thought that they've probably just knocked together a micro-controller, a sensor or two and some motor controllers without applying any particular attention to things like stall currents, torque etc. However, I could be wrong and maybe the robot just thought "Hang-on this is a hard way to earn a living" and decided to hold a spontaneous protest, which seems to be not to uncommon nowadays.
When you do get it right one of the more impressive things to demonstrate when designing a new robot is to get it to move and to do that you need a robot chassis. However, given the myriad of options available, choosing the right chassis can be quite difficiult.
Robotics and the DE0 Nano Development Kit
As the title suggests this review is about determining the suitability of a 4WD robot chassis for use in FPGA projects in general and the DE0 Nano Development and Education Kit in particular. A suitable robot chassis used in conjunction with Altera's Cyclone IV FPGA, on the DE0 Nano, will form the basis of an upcoming Project Series documenting our experiences in developing algorithms in FPGAs for use in robotics.
To avoid some of the errors witnessed in robot wars we intend to start our experimentation on using FPGAs in robotics from scratch. We wiil be, for example, documenting the development of a 4WD motor controller attached to the FPGA and performing real-time data acquisition using the DE0 Nano's ADC, the ADC128S022.
Hence, the choice of chassis is not of particularly importance, in the first instance. What is important is to find a chassis big enough to mount the DE0 Nano on to and all the associated circuitry attached to it, which will, typically, be sensors, motors controllers etc.
When our knowledge in the subject area of robotics is combined with our FPGA expertise we intend to design bigger and larger robots, which will require us to purchase larger chassis (Well done, spelt the same way pronounced differently, Chas-ee = singular, chas-eez = plural - Ed).
However, before then, we need to progress our robotics from an intermediate level to an advanced one, and integrate that knowledge with the inherent parallelism found in FPGAs at the logic element, or logic block level. Hence, the project should provide the opportunity to design complex parallel algorithms without relying on a real-time operating system, which micro-controller designs tend to do.
A 4WD Robot Chassis
The chassis chosen for our project is the appropriately named "4WD Smart Car Kits Chassis Mobile Platform 4 Drive Special Price Gift", which would suggest it has been titled to appeal to as wide an audience as possible! There are quite a large number of these and similarly titled kits available in the £20 - £40 price range. Most of them appear to be slanted towards robot design using the Arduino and the various Arduino shields used to mount analog and digital sensors.
Figure 2: The car chassis measures about 290 mm although the advertised length is 270 mm.
One of the significant differences between the various kits is the type of material used to make the chassis. That is, whether the car chassis is made of acrylic, aluminium or in the case of the one we purchased stiffened cardboard. The stiffened cardboard car chassis option seems well suited to mounting the DE0 Nano, as drilling it should be easier compared to if we had purchased an aluminium one. However, we have decided to spray paint the chassis black afterwards to give it that more menacing look. Also, we could decide to mount the DE0 Nano and the rest of the circuitry on an acrylic sheet first, before mounting the sheet to the car chassis.
This kit cost us around £20 and we got what we paid for, a four wheel drive chassis measuring approximately 270 mm x 140 mm x 40 mm with a carrying capacity of about 1Kg. The four wheel drive mechanisms consists of 4 gear motors with a gear motor rotation speed of 125 revolution per minute at 6V. It also includes four L-shaped fasteners, including screws and nuts, four long spacers, used as separators between the two car chassis pieces, some black and red wires and a 4 x AA battery compartment.
Figure 3: Car chassis and wheels compared to the DE0 Nano Development Kit (blue PCB) and the 2 x 13 Way Mixed Signal Dilettante Board (Green PCB).
As we've experienced before, when purchasing items directly from China, the minimum of documentation was expected to accompany the purchased product. In this case we were not disappointed as, if there was any documentation, it could only have been etched into the only other item accompanying the kit, which was the bubble wrap. Thankfully the bubble wrap performed its duty diligently and the kit arrived in pristine condition. The item's delivery time was quite impressive, just a few days after ordering.
The annoying thing about this kit's lack of documentation is the lack of information on the characteristics of the motors or gear boxes included with the kit. What is the reduction ratio? Torque? Weight? Stall current? and Size? The only good thing about the motors is that they look like the standard FF-030 sized motors, which should make them easily replaceable. As the characteristics of the motor and gear box is essential for the type of analysis we wish to perform, we intend to change the motors and gear boxes for ones with known characteristics when necessary.
Figure 4: The diameter of the wheels is about 64 mm while the width of the motor and wheels is about 52 mm.
Our aim in purchasing this kit is to use the DE0 Nano's FPGA as an advanced robot controller. Although the kit seems ideal for this purpose, now, we can't help but feel once we understand more of our exact requirements it may become redundant. If you need to purchase a robot chassis that is cheap without caring about the characteristics of the included motor, gear box or the type of rubber the tyres on the wheels are made from, then this kit could be for you. However, for the more discerning robot builders this type of kit may be too basic for your needs.