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mpthompson
Joined: 02 Jan 2006
Posts: 650
Location: San Carlos, CA
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 Posted: Fri Jun 30, 2006 5:22 pm Post subject: Sensorless Fet element DC motor driver |
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I came across this Sensorless Fet element DC motor driver project that uses an ATmega16 to provide a relativley cheap solution to implementing low or high power servo and drive systems using conventional DC motors. The board is clearly intended to drive things other than RC servos, but he seems to do some clever things to coordinate current and voltage measurements that may be worth looking into. Below is a description of the PWM drive pattern with coordinated ADC measurements.
| Quote: | Sensorless control and PWM drive pattern
The bridge driver software I have written for this hardware generates PWM at 8.9Khz when running with an 18.4Mhz clock, or 10.7 khz when running with a 22.1184Mhz clock , is thus able to drive low inductance motors efficiently. The drive feedback voltage ADC conversions are auto triggered and timed to provide voltage and current samples from each bridge in turn. Current is sampled at the end of the drive pulse when it will have reached it's peak value, voltage is sampled before the beginning of the pulse to get most accurate generator voltage at this point. This ADC timing relies on the repedative nature of the curent pulses to produce a type of oversampling ADC effect (equal to having a 150khz ADC), this minimises processing requirements and allows the use of the AVR's internal 10 bit 15khz ADC speed especially considering the drive frequency. The current and voltage ADC samples once gathered are processed by a feedback routine, the driver under development uses this information to control the FET bridges duty cycles, switchs automatically between regenerative breaking and drive and changes directions to control the motor speed and torque. |
Considering our discussions of back EMF, current and voltage measurement I thought I would point this project out. Now if we could squeeze this circuit into a 12mm by 12mm of board space  .
-Mike |
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ginge
Site Admin
Joined: 14 Jan 2006
Posts: 1029
Location: Manchester, UK
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| Posted: Fri Jun 30, 2006 6:02 pm Post subject: |
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Nice find.
Although I am not quite sure if this is how it really works... My interpretation is that it is a current controller of a fixed speed DC motor. Sampling the current, and voltage lets you get a figure for power (P=IV). The controller then maintains a state where P is constant by throttling PWM, freewheeling, or pulling half of the H-bridge to ground.
If this is the case, then there is no reason why that can not be used along with any other input available. An average of integrated speed gained from the pot position, mixed with I and V would certainly help speed concerns.
I do like the way it samples the current and voltage for each bridge at the most optimum time, it's a good idea that should be researched further.
Just one thing, can anyone explain this:-
| Quote: | | This ADC timing relies on the repedative nature of the curent pulses to produce a type of oversampling ADC effect (equal to having a 150khz ADC)... |
Barry[/quote] |
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