5---MOTORS

I have worked with 3 types of motors viz. DC, Servo and Steppers.




DC MOTORS

As a beginner we mostly use DC motors. Stepper and servos are a little complicated. As we know DC motor has two leads. If we apply +ve to one lead and ground to the other, the motor will rotate in one direction, if we reverse the connection the motor will rotate in the opposite direction. If we keep both leads open or both leads ground it will not rotate (but some inertia will be there). If we apply +ve voltage to both leads then braking will occur. We can test this, first without applying any voltage we rotate the shaft of the motor, then apply ground on both lead and try to rotate the shaft. Both the cases feels the same. But if we apply +ve voltage (+12V) to both the leads and try rotating the shaft, we can feel the difference between the previous one. We have to apply more force to rotate the same rotation in the second connection. So we take this condition as braking.
The main things about a DC motor are Voltage rating, Current rating, Torque and Speed. It should be kept in mind that torque is inversely proportional to speed. So while building a robot we should keep in mind the problem statement and accordingly choose between torque and speed.


By looking at the motor datasheet can determine the output velocity and torque of our motor. But unfortunately for robots, motors commercially available do not normally have a desirable speed to torque ratio (the main exception being servos and high torque motors with built in gearboxes. In robotics, torque is preferrable than speed. With gears, we can exchange the high velocity with a better torque. This exchange happens with a very simple equation:

Torque_Old * Velocity_Old = Torque_New * Velocity_New

Torque_Old and Velocity_Old can be found simply by looking up the datasheet of our motor. Then what we need to do is put a desired torque or velocity on the right hand side of the equation.
So for example, suppose our motor outputs 3 lb-in torque at 2000rps according to the datasheet, but we only want 300rps. This is what equation will look like:

3 lb-in * 2000rps = Torque_New * 300rps

Thus our new torque will be 20 lb-in.
Now suppose, with the same motor, we need 5 lb-in. But suppose we also need 1500rps minimum velocity. How would we know if the motor is up to spec and can do this? Easy . . .

3 lb-in * 2000rps = 5 lb-in * Velocity_New

Velocity_New = 1200rps

We now have just determined that at 1200 rps the selected motor is not up to spec. Using the simple equation, we have just saved tons of money on a motor that would have never worked. Doing all the necessary equations before designing our robot will always save you tons of money and time.


The photograph of the DC motor given is a 12V DC motor with gearbox and extended shaft



There are 0.1uF Capacitors on motor terminals... these were put so as to make sure that no AC component goes into the motors.









STEPPER MOTORS

Stepper motor is an electromechanical device which converts electrical pulses into mechanical movements. The shaft of a stepper rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.
Lets see why steppers are used in robotics:

1. The rotational angle of the motor is proportional to the input pulse.

2. The motor has full torque at stand-still.

3. Precise positioning can be done. They have an accuracy of 3-5% of a step and this error is non cumulative from one step to the next.

4. It is very much reliable since there are no contact brush in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.


As the saying goes everything has its own merits and demerits…


Steppers are vulnerable to resonance if not properly controlled and it can also not be operated at extremely high speeds.


A stepper motor can be a good choice whenever controlled movement is required. They can be used in applications where we need to control rotation angle, speed, position and synchronism.


Stepper motors are current controlled motors unlike DC motors which are voltage controlled.





Servo functioning in easy words can be thought as moving a electromagnet near a piece of iron. The iron piece will move (get attracted) towards the electromagnet. The dots in the picture are the coils in the picture are the electromagnets which attract the arrow (which instigates the motor shaft movement).







This is one of the the steppers which i bought for 600 bucks each. They were high torque steppers and the suppliers took 100 out of the 600 for the special light weight aluminium wheel and the wheel grip (the blue colour in the picture)



BIPOLAR STEPPER



Stepper motors are available with 4 wires, 5 wires, 6 wires. Wires are not like a DC motor. 4 wire stepper motor is surely a bipolar motor, 5 wire motor will be surely a unipolar motor and 6 wire motor can be used as a unipolar motor as well as a bipolar motor.

First of all I have never used a bipolar stepper. Mine was a 6 wired motor but I used it in the unipolar mode. Things are a little more complex for bipolar permanent magnet stepping motors because these have no center taps on their windings. Therefore, to reverse the direction of the field produced by a motor winding, we need to reverse the current through the winding. We could use a double-pole double throw switch to do this electromechanically; the electronic equivalent of such a switch is called an H-bridge and is outlined below






It is worth noting that H-bridges are applicable not only to the control of bipolar stepping motors, but also to the control of DC motors, push-pull solenoids (those with permanent magnet plungers) and many other applications.
With 4 switches, the basic H-bridge offers 16 possible operating modes, 7 of which short out the power supply! The following operating modes are of interest:
Forward mode, switches A and D closed.
Reverse mode, switches B and C closed.

These are the usual operating modes, allowing current to flow from the supply, through the motor winding and onward to ground. The figure below illustrates forward mode:






Fast decay mode or coasting mode, all switches open.


Any current flowing through the motor winding will be working against the full supply voltage, plus two diode drops, so current will decay quickly. This mode provides little or no dynamic braking effect on the motor rotor, so the rotor will coast freely if all motor windings are powered in this mode. Figure below illustrates the current flow immediately after switching from forward running mode to fast decay mode.







Slow decay modes or dynamic braking modes.


In these modes, current may recirculate through the motor winding with minimum resistance. As a result, if current is flowing in a motor winding when one of these modes is entered, the current will decay slowly, and if the motor rotor is turning, it will induce a current that will act as a brake on the rotor. Figure below illustrates one of the many useful slow-decay modes, with switch D closed; if the motor winding has recently been in forward running mode, the state of switch B may be either open or closed:





The image below shows an example of an H-bridge








The main thing to keep in mind is that there should not be any short circuit of A-B or C-D.

The following operating modes are available