When the current to the inductor is suddenly switched off, e.g., by switching off a driving transistor, Equation 20.15 indicates that there will be a large transient voltage build-up acr
Trang 1storage elements, where the energy is stored in the induced magnetic field The voltage across an ideal
inductor V I (t) is
(20.15)
where i I (t) is the current going through the inductor and L is the inductance When the current to the
inductor is suddenly switched off, e.g., by switching off a driving transistor, Equation (20.15) indicates that there will be a large transient voltage build-up across the inductor If not properly suppressed this
transient voltage can shorten or even damage the driving transistor This is sometimes called inductor kickback.
A simple method of reducing the instantaneous switching voltage surge is to create a loop for the excess energy to flow This can be done by placing diodes in parallel with the load, see Fig 20.54 Figure 20.54 illustrates two methods of using flyback or free-wheeling diodes to suppress switching voltage
surge when driving inductive loads
Open-Collector Output
For some digital devices, the output stage (pin) is simply the collector of a transistor This is called an
open-collector output, see Fig 20.55 Since the output of the device is only the collector of a transistor,
it has no output drive capacity The output value can be measured through a pull-up resistor, see Fig 20.55 Open-collector output is convenient for driving electromechanical devices if the output transistor can sink adequate current, see Fig 20.57
Isolation
Recall that the power amplification/modulation part of an electromechanical actuator contains both low-and high-energy signals, see Fig 20.2 For safety and reliability reasons, it is desired to prevent transients
or noise spikes in the high power side of the system from the signal processing (low power) side of the
circuit Mechanical relay is one option Optoisolators or optocouplers use light to couple the high and low
FIGURE 20.54 Using diodes to reduce swithcing voltage when driving inductive loads
FIGURE 20.55 Open-collector output
V I( )t L d
dt
-i I( )t
⋅
=
Trang 2storage elements, where the energy is stored in the induced magnetic field The voltage across an ideal
inductor V I (t) is
(20.15)
where i I (t) is the current going through the inductor and L is the inductance When the current to the
inductor is suddenly switched off, e.g., by switching off a driving transistor, Equation (20.15) indicates that there will be a large transient voltage build-up across the inductor If not properly suppressed this
transient voltage can shorten or even damage the driving transistor This is sometimes called inductor kickback.
A simple method of reducing the instantaneous switching voltage surge is to create a loop for the excess energy to flow This can be done by placing diodes in parallel with the load, see Fig 20.54 Figure 20.54 illustrates two methods of using flyback or free-wheeling diodes to suppress switching voltage
surge when driving inductive loads
Open-Collector Output
For some digital devices, the output stage (pin) is simply the collector of a transistor This is called an
open-collector output, see Fig 20.55 Since the output of the device is only the collector of a transistor,
it has no output drive capacity The output value can be measured through a pull-up resistor, see Fig 20.55 Open-collector output is convenient for driving electromechanical devices if the output transistor can sink adequate current, see Fig 20.57
Isolation
Recall that the power amplification/modulation part of an electromechanical actuator contains both low-and high-energy signals, see Fig 20.2 For safety and reliability reasons, it is desired to prevent transients
or noise spikes in the high power side of the system from the signal processing (low power) side of the
circuit Mechanical relay is one option Optoisolators or optocouplers use light to couple the high and low
FIGURE 20.54 Using diodes to reduce swithcing voltage when driving inductive loads
FIGURE 20.55 Open-collector output
V I( )t L d
dt
-i I( )t
⋅
=
Trang 3
energy side of the device Typically, an LED source is combined with either a phototransistor or photo thyristor, see Fig 20.35 In addition to signal isolation, optoisolators also help to reduce ground loop issues between the logic and power side of the circuit
Grounding
It is important to provide common ground among the different devices For electromechanical actuators, the high energy side is often switching at high frequency; if the ground point of the high energy side of the circuit is directly connected to the ground of the low energy side of the circuit, switching noise may propagate through the ground wire and negatively affect the operation of the low energy side of the system It is recommended that separate common grounds are established for the high and low energy side and the two grounds are then connected at the power supply In addition, an adequate-sized ground plane needs to be provided to minimize the possibility of differences among grounding points
20.2 Electrical Machines
C J Fraser
The utilization of electric motors as the power source in a mechatronic application is substantial Electric motors, therefore, often feature as the prime mover in a variety of driven systems It is usually the mechanical features of the application that determines the type of electric motor to be employed The torque–speed characteristics of the motor and the driven system are therefore very important It is perhaps then a paradox that while the torque–speed characteristics of the motor are readily available from the supplier, the torque–speed characteristics of the driven system are often quite obscure
The dc Motor
All conventional electric motors consist of a stationary element and a rotating element, which are separated
by an air gap In dc motors, the stationary element consists of salient “poles,” which are constructed of laminated assemblies with coils wound round them to produce a magnetic field The function of the laminations is to reduce the losses incurred by eddy currents The rotating element is traditionally called the “armature” and this consists of a series of coils located between slots around the periphery of the armature The armature is also fabricated in laminations, which are usually keyed onto a location shaft
A very simple form of dc motor is illustrated in Fig 20.56 The single coil is located between the opposite poles of a simple magnet When the coil is aligned in the vertical plane, the conventional flow of electrons is from the positive terminal to the negative terminal The supply is through the brushes, which make contact with the commutator segments From Faraday’s laws of electromagnetic induction, the “left-hand rule,” the upper part of the coil will experience a force acting from right to left The lower section will be subject to a force in the opposite direction Since the
FIGURE 20.56 Single-coil, 2-pole dc motor.
Magnet
N +ve
-ve
Brush
Coil
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