Certificates  of participation of Mind-Sparks Robotics are avialable at Robot Study Circle ,Robotics And Automation Lab.

The most important (and the most neglected ) part of any robot design is the power supply. Many a times you see good robots failing at the time of competition due to poor power supply design.

Here we try to present a few issues encountered while designing the power supply and some tips to aid you….

Lets divide our discussion in two parts:

Group 1: Wired

Group 2: Robots with onboard supply/ Wireless robots

1] Wired robots:

In this case efficiency isn’t major issue for power supply. Just check for 230V socket, have a good DC supply (2A 0-32volts ideally) for your motors, if you have any circuit on robots regulate using linear IC regulators like 7805 and a higher value capacitor at its i/p and o/p.

If there is no cct. onboard and the robot is a simple machine, control current to left and right motors by using rocker switches so you can have turning (wheelchair drive) and straight line motion using just with two switches. The connections for the rocker switch is shown in the fig.

Checkpoints:

· Check if your power supply is enough to supply required current; even at critical times like when motors suddenly take more current due to change in load (ramps/switching directions/dragging weights etc).

· Always regulate supply to circuit (linear regulator and large capacitor at i/p and o/p of regulator) as long wires can ruin regulated feature of your dc supply.

· Before starting your supply please keep all knobs on lowest voltage (usually to left) else just before competition you will enjoy “hot” circuits and probably burnt.

· Always keep supply current rating twice or more than your motor ratings as a factor of safety, else as motors takes sudden current at some moments it may stop working your circuit, turn off LEDs , reset microcontroller and slow down due to itself.

2. Autonomous Robots:

This section includes: true intelligent robots with onboard supply, wired/wireless robots requiring supply onboard

Check Points:

· First of all this is tradeoff between weight and power. Considerations are not less than fuel-weight considerations in F1 race.

· Always try to separate the supplies for part of the circuit requiring precise voltage (microcontroller etc) and those that dont require it (motors etc). This to ensure that the logic circuit isnt affected by the supply changes due to current surge in motor etc.

· Search market for optimum battery or combination of batteries with respect to type, make, price, number of batteries to be used and weight.

· Most important rating is AHr or mAHr rating which tells how much supply it can provide for how much time. This decides your run time of robots or more important whether it will move from its place or not at all (a problem usually faced when having a heavy chassis and a weak source).

· If circuit needs specific voltage like 5V, you can try for linear regulator (check 7805 datasheet).

· But remember linear regulator is somewhat inefficient. It uses more power than supplied to your circuit in heating itself, so proper heat sink is needed (esp. when the i/p voltage is greater than 9 volts).

· One option to reduce this wastage is reducing headroom voltage ( i/p to your regulator) and keeping just above its minimum requirement ( 5+2 Volts ) but this is not recommended as voltage falls when batteries drain and it may brownout ( stop functioning) the regulator. Also usually same batteries are used for driving motor and motor needs more voltage like 12 or 15V as a general case.

· One option is to use SMPS (or buck-boost or switching regulator). This basically uses high speed switching to change voltage levels.

· BUCK = step down. To reduce 12V battery i/p to say 5V.also you get increased current capacities even like 1A or more than that (3A,20W ICs are available but simple circuits wont take this much, at least in our case)

· BOOST = steps up i/p voltage. This can be used to drive high voltage hardware but just check for maximum current as usually current o/p is low but once again above what we need usually.

· These regulators are much efficient like 80 + %. But design issues are much more. Basically they use energy storage elements like inductor and capacitors. Also their high switching shouldn’t affect rest of circuit. Also ICs, coils needed and FETs as they usually use are not available here in Pune easily at low cost. Better to use ISR switching modules which have integrated switch (FET) and on-module coils too.

· If this scares you a lot then the old friend 78XX is always there. Remember 78xx have a current rating and if you are going even near to them better to use circuits taught in classroom to increase current capabilities. Also prefer use of heat sink and recommended capacitors. 78XX is closed loop system and you can’t make it unstable by adding wrong capacitor as dominant zero.

Example: let us see how much 7805 is efficient (rather inefficient) for 12V i/p, 5V o/p and 100mA load current;

Sol: power consumed by circuit = 100mA * 5V = 500mW

Power consumed by regulator IC= 100mA * (12-5) = 700mW

So efficiency = 500/1200 = 41.67%

But it’s constructed in few minutes and no inductor, FET and other issues…

So it’s your decision…

I think all these stuff is not at all sufficient for a good supply design.

Always Use factor of safety in your design.

This discussion is just for sending power from battery to your robots…its further processing and conditioning also has some issues not discussed here.

I have written this according to my belief and everybody can have these differently so I cannot guarante mine to be as the best one.

Feel free to contact, thank you,

Shriram Devi

T.Y. E&TC,COEP

A line-follower is an autonomous vehicle which follows a line drawn on a contrast surface, say white line on a black surface.
For many TRUE robotics competitions, which require an autonomous vehicle, a line-follower is a pre-requisite. And unbelievably, a line-follower can be constructed without using any microprocessor/controller and is a mere game of two sensors, a comparator IC, and DC motors to move your vehicle!!

Let’s see how the simplest of line followers can be constructed…

Working Principles:
__________________

A comparator can always be connected in inverting or non-inverting mode.
One i/p to a comparator is fixed, called ‘reference’ and the other i/p is dynamic.
Depending upon the mode of operation (inv/n-inv) the o/p toggles between HIGH and LOW states with the changes in i/p. As we are not using any ‘intelligent’ device such as a uC, the mode of operation of comparator depends upon the colors of the line to be followed and the surrounding surface. For example, to trace a white line on a black surface, the comparator is connected in non-inv mode. To trace a black line on a white surface, the mode of operation will be inverting!

Now about the sensors…Here we will use simple optical sensors. Generally, the problem-statement of the competition states to follow a white line on a black surface (or the other way round!). The light emitted by LED will be reflected from white color and will be absorbed by black surface. The problem statement specifies (well, it has to) the thickness of the line. In the construction of our vehicle, we ensure that the distance between the two sensor-pairs is slightly greater than the thickness of the line. The o/p of the photodiode is fed to the comparator.

Now we’ll drive the motors…Generally the dc motors are driven at 12V
and max 150mA consumption. These motors can be driven by the comparator IC through transistors used as switches! Just a trick — the o/p of the LEFT comparator (the one connected to LEFT sensor) drive the RIGHT motor and vice versa — and you are done with it!!

Here’s the working, be sure to get it— We assume that the
line-to-be-followed is between the two sensor-pairs. We also assume to follow a white line on a black surface. When the line is under LEFT sensor, the LEFT comparator will trip, giving HIGH o/p. This, in turn, drives RIGHT motor. At the same time, the RIGHT sensor will not sense any line under it and RIGHT comparator will be untripped, LOW o/p. Thus, as per our cross-connection, LEFT motor will be idle. The net effect of this operation is — the vehicle takes a LEFT TURN. As the distance between sensors and the thickness of the line differ slightly, the RIGHT sensor will be tripped (untripping the LEFT one) soon after the prev LEFT turn and the same cross-action will be repeated!!! The net result of these operations will be—the vehicle follows the line, slightly
in a zig-zag manner, but doesn’t leave the line! Well, that’s after all a line follower!!

Few hints for the actual construction—

• The o/p of comparator is connected to one end of dc motor, the other end is kept grounded.

• O/p of L339 is open-collector, may need an external pull-up resistor for proper working of transistor.

• Make sure to match current capacities of ICs, transistors and motors used. Refer datasheets widely available on internet (and on the RSC CD)

• The sensitivity can be changed by varying the ref voltage to comparators, which is necessarily identical to both comparators.

• Make sure to shield the sensors of ambient light as it will cause false triggering.

Once we construct a ‘white line follower on a black surface’ the same can be
used as ‘black line follower on white surface’ simply by connecting the other terminals of dc motors (those not connected to comparator) to +ve of supply!

Limitations of our vehicle:
__________________________

1. As it uses only two sensor-pairs, the resolution is low. It will follow a zig-zag
path about the line instead of strictly following it.
2. The ready-to-go vehicle will only love smooth curves and will necessarily fail at
sharp turns and 90deg turns.
3. As there is no ‘intelligence’, if ‘blanked out’ it will do nothing to ’search’
the line.

A small investment in power supply/batteries will make your life easier in the world of robotics

-Amey S. Deshpande

An interesting article on how one can look beyond one’s curriculum and stand out as an engineer. Read here.