ALTERNATIVE APPROACH TO USE OF PULSE WIDTH MODULATION

Size: px

Start display at page:

Download "ALTERNATIVE APPROACH TO USE OF PULSE WIDTH MODULATION"

Caren Jacobs
6 years ago
Views:

1 ALTERNATIVE APPROACH TO USE OF PULSE WIDTH MODULATION Djuro G. Zrilic New Mexico Highlands University Engineering Department Las Vegas, NM Abstract-There are many requirements placed on the new generation of micro and macro robots, such as communication between robots, allowing coordinated actions, cognitive capabilities, smart behavior, etc. A pulse-width modulation (PWM) is widely used in different areas of control systems, such as robotics, industrial process control, power control systems, etc. A PWM circuit converts a DC voltage into a series of pulses, so that the pulse duration is directly proportional to the value of DC voltage. In spite of wide use of PWM, there are disadvantages of using PWM such as the possibility of generating radio-frequency interferences (RFI). In addition, when control by frequency modulation (FM) is desired the ON-period of PWM pulse is kept constant, but the frequency is varied in order to bring regulation. It is difficult to make the ON-period below a certain time duration. When the limit is reached, control by a PWM becomes impossible. The value of a minimum ONperiod depends on the transistor switch. Due to the varying pulse width, PWM is unusable for time division multiplexing (TDM). Usually these problems are encountered when a 555 IC timer is employed as PWM. In this paper we propose two alternative methods for PWM. Key words: pulse-width modulation, switch-mode power amplifier, delta-sigma modulation PROBLEM STATEMENT There are many applications of PWM. One particular application of PWM that is common to industrial robotics, industrial process control, power control systems, etc. is in switch-mode servo amplifiers. Here we will state the problem and briefly describe its operation as stated in reference [1]. DC loads, such as servomotors, relays, solenoids, lamps, heaters, and proportional valves, require currents of several hundreds milliamperes to several amperes. However, real industrial loads may require hundreds of amperes at hundreds of volts AC. The simplest way to apply significant power to a load is with a highpower operational amplifier. The power interface must meet several criteria. First, input is taken from the low-voltage, low-current output of the controller but the higher-power output must accurately track the input. Second, since the controller output often switches 120V AC, or even more, there must be insulation between the signal's common and the power line's neutral. Switching such a large amount of power certainly can produce significant electric noise. This noise must be reduced to a tolerable level. Finally, the power interface must be highly efficient. Any power dissipated by the power interface is normally converted to heat, and heat removal becomes a real problem. Operation of power amplifier in linear mode does impart minimum distortion to the signal. However, efficiency is very poor, often below 50%. By using high frequency switching techniques, amplifiers are being built which are much smaller, lighter, cooler, and consume (waste) considerable less power than the

2 traditional linear amplifiers. Switch-mode amplification is based on the principle of pulsewidth modulation. The average value of any wave is determined by the area between the wave and ground. For rectangular (switching) waveform this average value is Vdc = (Von/Vperiod)Vpeak (1) Changing the pulse width (Ton) while keeping the period (Tperiod) constant allows control of the signal's average value (Vdc). This is the technique used in switch-mode amplifiers to control the output voltage, compensating for changes in the load current or power supply (unregulated) voltages. The block diagram of a simple switch-mode amplifier is shown in Figure 1. Figure 1 The block diagram of a simple switch-mode servo amplifier During the design of a switch-mode amplifier, special attention must be paid to all parts depicted in Figure 1. In the majority of applications a 555 IC timer is used as PWM [2]. Unfortunately, this IC circuit operates in open loop configuration and it is prone to impulse noise generation. An example of this is shown in Figure 2, which shows a simplified model of PWM with associated waveforms contaminated with noise. Signal A represents the reference ramp (sawtooth) waveform, signal B is noisy regulating voltage, and signal C is output of a comparator circuit. Figure 2 Principles of PWM with associated waveforms

3 One major reason for using a switch servo amplifier is increased efficiency. Ideally, the switch should dissipate no power. However, when saturated there is a definite voltage drop across the transistor, a none zero ON resistance. If a PWM pulse stream consists of undesired pulse spikes (high frequency noise) much power is dissipated during rises and falls of these spikes. To eliminate unwanted pulse spikes in PWM stream we propose two different closed loop PWM configuration. ALTERNATIVE METHODS OF PWM A sampled converter amplifier, controlled by PWM, is shown in Figure 3. The operation of this system is identical to the operation of the system in Figure 1 [1]. For small robot applications transistor is supplied with on-board batteries. The output of the transistor has variable pulse width (Ton), which is rectified and then filtered to get nearly DC voltage Figure 3 Sampled converter amplifier proportional to the average value of the pulse-width wave. Should some variation in the load cause the output to try to increase, the PWM will sense this and reduce the pulse width to the switching transistor and thus, lower the output voltage to the load. Conversely, an increase in voltage at the PWM input will cause the PWM to increase a pulse width out of the switch transistor. This causes voltage to rise to the load. Should some impulse noise appear at the output of the comparator circuit, it will be eliminated by the sampled quantizer. In addition, one of benefits of a negative feedback is in suppression of undesired noise components. Figure 4 shows the outputs of PWM when the same level of the noise is added as in Figure 1. Signal A presents sawtooth reference. Signal B is noisy regulating input to the PWM, and signals D and Q are output of the comparator and D flip-flop respectively.

4 Figure 4 Output Q of PWM from Figure 3 Another possible alternative for noise free PWM uses principles of delta-sigma modulation (DSM). In addition to standard configuration of DSM this solution employs reference ramp generator and double feedback. Figure 5 shows system of sampled amplifier with embedded DSM. Figure 5 Sampled amplifier with embedded DSM Figure 6 shows output of DSM when the same level of noise is added as in Figure 2. We can see that high frequency spikes are eliminated (signal Q). Again, signals A, B, D, and Q represent the respective signals from Figure 5.

5 Figure 6 Output of DSM from Figure 5 CONCLUSION Pulse-width modulation has many applications. The majority of applications use a 555 IC timer as pulse-width modulator. Here we proposed two alternative solutions based on principle of negative feedback and sampled quantizer, and delta-sigma modulation. The advantage of these alternative approaches over the 555 IC timer is in elimination of a pulse spikes in PWM stream and possibility of multiplexing numerous PWM signals. Although proposed alternatives contain the basic control element (PWM) for switching regular design, practical applications require other functions which have to be implemented. It is important to mention of existence of regulatory PWM IC such as SGS 1525A and its complementary output version, the SGS 1527A. This IC circuits have capability of adding more power supply elements to the control IC, as well as optimize the interfacing of high current power devices [3]. ACKNOWLEDGEMENT The author wishes to acknowledge the support of US Department of Education under grant P120A , and to thank Ron Denny, undergraduate student at University of New Mexico, for help in preparation of this paper. REFERENCES [1] Michael Jacob, Industrial Control Electronics, Prentice Hall, 1988, pp , ISBN [2] Tony Meacock, An Inexpensive Pulse Width Modulated speed Control for 12 DC Motors, Electronics World, November 2005, pp.50. [3] SGS-Thomson Microelectronics, AN250/118, pp.1/13-13/13.

Learn about the use, operation and limitations of thyristors, particularly triacs, in power control

Exotic Triacs: The Gate to Power Control Learn about the use, operation and limitations of thyristors, particularly triacs, in power control D. Mohan Kumar Modern power control systems use electronic devices