FVCMotorFeedbackControl110705

FVCMotorFeedbackControl110705 - i FVC Speed Control...

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Unformatted text preview: i FVC Speed Control Contents Feedback Control of DC Motor Speed ................................................................. Introduction ........................................................................................................ Generator Based Speed Controller ....................................................... FVC based speed controller ...................................................................... M.D. Fox 11 ê 08 ê 05 1 1 4 4 1 Droop in speed at full load from no load ....................................................... 4 General ........................................................................................................... 5 7 Results ................................................................................................................. Discussion and Conclusions ..................................................................... ©2005 FOX Research, Inc. All rights reserved. Feedback Control of DC Motor Speed Introduction M. D. Fox 10 ê 28 ê 05 Update 11 ê 08 ê 05 Here we look at two approaches to speed control using a feedback speed controller. The first approach utilizes a generator to output a DC voltage that is used to control a feedback loop for stabilizing the speed of a motor. The problem is that this approach really requires three machines for the experiment. The second approach utilizes the RPG on the motor which is attached to a frequency to voltage converter to stabilize the motor speed. While both approaches work, the second is favored because it requires only two machines. Generator Based Speed Controller The theory for this type of controller is explained in detail in other sets of notes. Due to a poor mechanical coupling, the motor speed was somewhat irregular, and as a result we used the cursor function on the scope to estimate speed. 2 FeedbackPWMSpeedControl110805-`.nb Figure 1 Experimental setup. Full Load. Top view showing circuitry. FeedbackPWMSpeedControl110805-`.nb 3 Figure 3 Load Off 4 FeedbackPWMSpeedControl110805-`.nb Load on .426 A. [IRF 3205] à Droop in speed at full load from no load 5.43 - 5.10 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄ 100 ÄÄÄÄÄÄÄÄ Ä 5.43 6.07735 FVC based speed controller à General One of the outputs from the RPG is fed through a high pass and then an Op Amp LPF [low pass filter], which can also be considered a leaky integrator. The output of this Op Amp will then be a DC voltage proportional to the speed of the motor. This is then fed into the LF356 in the same way as the generator output [see Fig. 4 below]. FeedbackPWMSpeedControl110805-`.nb 5 RF = 33 kΩ 4.7 kΩ 1 kΩ 1 nF RPG 10 kΩ 100 kΩ + .1 m F Vi 4.7 k + Vo Figure 4 Circuit diagram for frequency to voltage converter for closed loop motor speed control. Op Amp is TL082. Figure 5 Typical waveform for VFC based speed controller. Channel 1 is feedback from output of VFC, Channel 2 is alternate RPG output. Results The raw data for the closed and open loop configurations are shown in Tables 1 abd 2 respectively. The speed was adjusted down to a level where there was plenty of additional voltage available for speed control without having to worry about the supply saturating. A resistance box was used to apply varying loads up to a maximum of about .21 Ampere. 6 FeedbackPWMSpeedControl110805-`.nb The results in the form of plots of speed as a function of load [current into the load machine is proportional to machine torque], are shown in Fig. 7, below. The open loop design [in red ] exhibits much higher speed droop with load than the closed loop design. R,W 10100 100 90 80 70 60 50 40 30 20 Iin,mA 0. 0.08 0.09 0.1 0.11 0.12 0.14 0.17 0.21 0.27 Table 1 Raw data for closed loop system. 1 2 3 4 5 6 7 8 9 10 f ,kHz 7.921 7.783 7.776 7.774 7.752 7.739 7.717 7.678 7.575 7.429 1 2 3 4 5 6 7 8 9 10 R,W 10100 100 90 80 70 60 50 40 30 20 Iin,mA 0. 0.08 0.08 0.09 0.1 0.11 0.12 0.14 0.17 0.21 Table 2 f ,kHz 8.04 7.319 7.249 7.158 7.027 6.826 6.71 6.462 6.186 5.765 Raw data for open loop system. FeedbackPWMSpeedControl110805-`.nb 7 f,kHz 8 7 6 5 4 3 2 1 0.05 0.1 0.15 0.2 0.25 I, A Figure 7 Motor speed as a function of load current [proportional to load torque], for Open Loop [Red] and closed loop [Black] designs. At peak load, which we will define as ~.2 A the open loop design speed was down 28%, while the closed loop system was down only 4.3%. Solid lines are linear regressions. Discussion and Conclusions Here we compare two types of speed control design. The RPG based design appears to have a number of advantages. First, it does not require separate machines as sensor and load, thus allowing a standard two machine setup to provide both loading and speed control. Second, the operation was much smoother, allowing the use of standard scope frequency automeasurements for frequency rather than the more cumbersome cursor approach. Finally it appeared to provide somewhat better speed stabilization, with a full load droop of only 4.3% compared to the open loop speed droop of 28%. Overall, I plan to use the RPG based design in future 214 laboratories. ...
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