Catch diodes

The basic principle is very simple: while the bridge is on, two of the four switching elements will carry the current, the diodes have no role. However once the bridge is turned off the switches will not conduct current any more. As discussed earlier, by far the most common load for an H-bridge is an electric DC motor, which is an inductive load. What this means is that during the on-time the motor will build an electromagnetic field inside it. When the switch is turned off, that field has to collapse, and until that happens, current must still flow through the windings. That current cannot flow through the switches since they are off, but it will find a way. The catch diodes, are in the design to provide a low-resistance path for that collapse current and thus keep the voltage on the motor terminals within a reasonable range.

Voltage Spikes in H-bridges

Most of H-bridges controller uses Pulse Width Modulation (PWM) to control how much current flows through the motor. It attempts to limit current "rushes" by ramping the current from zero (motor not operating) to 100%, then on the change of direction from 100% to 100% the other direction.

The interesting bit comes when the controller is asked to stop the motor. This occurs when it receives a signal indicating zero speed.



When the controller is commanded to stop, it first slews back from what it is currently doing to 0%, then it slews from 0% to 100% "braking." Braking is accomplished by turning on both lower switches. (sw2,sw4)

This creates a current path between the terminals of the motor, and if the motor is turning the motor will generate a voltage resists the motors turning direction. (This is called back EMF in the motor tutorial). This back EMF then slows the motor down until the motor stops.
Braking, like going forward and backward, starts out with a 0% duty cycle PWM wave and then slews up to 100% duty cycle. This starts braking softly and then increases up to a full brake condition. The current flow is shown in the diagram below.


The hypothesis was that by shorting the motor, I induced a change in the current flowing through the motor, this change in current was reflected as a voltage potential across the motor terminals and was governed by the equation


The equation is the classic description of voltage behavior across a coil due to a change in current.

The perplexing part to me was how could the voltage be greater than the input voltage? The theory is that by rapidly changing the current I induced a higher voltage spike than simply the back EMF voltage.

Back-EMF

Back-EMF refers to using the voltage generated by a spinning motor (EMF) to conclude the speed of the motor's rotation. This can be used in motion control algorithms to modulate the velocity or to compute the angular distance the motor has traveled over time. This article attempts to describe this form of motion control feedback in more detail.

Typically a motor takes power in the form of voltage and current and converts the energy into mechanical energy in the form of rotation. With a generator, this process is simply reversed. A generator takes mechanical energy and converts it into both electrical energy with a voltage and current. Most motors can be generators by just spinning the motor and looking for a voltage/current on the motor windings.



http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html


The DC motor that is driven, however is also a dynamo, so in theory it can be used to measure its own speed. All we need to do for that is to measure the Back-EMF voltage, which we denoted with Vg previously and called generator-voltage.

Operation H-Bridges

The following table summarises operation.
You can turn motor to move "left" and "right" direction by operate H-Bridge switches and you can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motors terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit.

H-bridge

An H-bridge is an electronic circuit which enables DC electric motors to be run forwards or backwards. These circuits are often used in robotics. H-bridges are available as integrated circuits, or can be built from discrete components.



To power the motor ,you turn on two switches thet are diagonally opposed. In the picture to the right, left imageine that the high side left(s1) and low side right(s2) switehes are turned on. The current flow is shown in green. The motor begins to turn in a "positive" direction. So that right imageine,you turn on the high side right(s3) and low side left (s2) switches.The current flows the other direction through the motor and the motor turns in the opposite direction.