- Introduces essential concepts of circuit & switching theory
- Its modular design allows it to be integrated within any power-requiring project
- Uses a single 1.5v, AA battery but it can be expanded to be used with any size or number
HOW IT WORKS
The idea behind the Control Bar is to take advantage of a rotary action to connect a different voltage polarity to the motor it controls, when set to different positions. It consists of a rotor where the cables feeding the load are attached through screws (these being electrically conductive). The rotor is fixed to a pole inside the casing around which it is free to rotate. An inner ring provided with two aluminium sheet “C- shaped” strip is also fixed to the casing by means of two screws, one wiring its contacting strip to the positive terminal of the battery and the other to the negative. Each strip ends in two flaps that act as sprung-stops for the rotor screws, contacting when the rotor is given a CW motion. When this happens, the circuit is closed and the motor is fed with the battery voltage and power required to perform its duty! If the motion of the rotor is, however, in the opposite direction (CCW), the circuit is closed with the inverted voltage polarity, forcing the motor to then rotate in the opposite direction: full control! A cover or cap (04-CTLBAR) for the casing helps to protect the flapped terminals and prevent any of the parts from falling out of position.
The battery container is split into two parts: a holder and an axial spring. This makes it easier to 3D Print and assemble. Both parts are also provided with aluminium strips that work as electric terminals. The axial spring fits into the holder by means of a “dovetail” sliding fit. Its function is to guarantee the battery contact with the terminals at all times. Also note that the casing attaches to the holder through a dovetail fit at the bottom in case you need casing and battery together.
Most of the effort to assemble this project comes in the fitting of the aluminium strips (refer always to the drawings for assembly). These are made out of kit component K1-AS and by carefully cutting with scissors the strips to size (see sheet 3 of the drawing for the dimensions for each strip). Once obtained, the strips need to be inserted into the slots at the inner ring, the holder and the axial spring using long nose pliers , then folding the strips as needed to obtain the desired shape. Notice that the battery holder has a 3mm hole at the terminal end where a screw can be inserted. This is to create a small protrusion in the aluminium strip to improve contact with the battery. Also, the axial spring has a hole for a screw to be inserted and used as a jack in case it becomes difficult to remove from the holder. DO EXERCISE EXTREME CAUTION WHEN CUTTING K1-AS. THIS MAY RESULT IN SHARP EDGES.
All that is left, is the action of wiring everything up. For this, you’ll need to use K1-WR, K1-WB (wires) and K1-TS (electrical terminal). If you don’t have a “Crimping Tool”, you can hammer-down the terminal’s plastic barrel to trap the cable edge within. If your dovetails don’t seem to fit due to material shrinkage (especially when using ABS) file down lightly any of the surfaces until you reach the right conditions for assembly.
A diagram of the circuit easily reveals how voltage polarity is controlled. The battery is represented by the voltage source wired to the Control Bar. The motor, or any load, are represented by a square box with terminals T1 and T2. Notice that the direction of current I is always from the positive terminal of the battery to the negative. When the rotor at the control bar is rotated in the CW position, the circuit is closed connecting T1 to the positive side of the battery and T2 to the negative. But when the control bar is rotated CCW to the opposite contacting position, T1 is now connected to the negative terminal of the battery and T2 to the positive, thus changing the polarity on the load. The rotation of direct current (DC) motors is easy to control by changing polarity. To understand this, you need to brush upon some basic electromagnetic principles. Explained in simple terms, a force is created over any conductor crossing a magnetic field when an electric current is made to flow through the conductor. The magnitude of the force is given by the product of the magnetic field intensity and the amount of circulating current and its direction is always perpendicular to both: the direction of the magnetic field and the direction of the current. This is best represented by vectorial equation:
F = force [N], I = electric current [A], L = conductor length vector [m], B = magnetic flux density vector [wb/m2]
With the help of something called a “commutator”, this same principle is applied to make a DC motor spin. The magnetic field is created by the two magnets sitting inside the casing. Multiple copper conductors forming a loop are then installed into a rotor. This figure is a simple representation. When the battery is connected, a current flows through the rotor coil from R1 to R2 creating a force over the conductors in the illustrated direction, thus causing a rotation. As the rotor reaches half a turn, it briefly disconnects from the “commutator” terminals but inertia keeps it spinning until it reconnects. This is done to guarantee that the force over the conductors is always applied in the same direction and rotation is kept the same (note that after half a turn, current now flows in the opposite direction, from R2 to R1, alternating every half a turn. This is where the “commutator” name comes from). If voltage polarity is reversed at terminals T1 and T2, then the direction of the current, created force and rotation are also reversed and this is what we do with our Control Bar.
What would happen if the magnets were in the rotor and the conductors seating statically in the casing? Could you argue that a commutator would not be needed then? If so, would this still work with DC or would it have to be AC? If you would like to find out more, try searching the web for “brushless permanent magnet motors and generators”.
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