Ideally (in that magical land the place concept and follow are the identical factor), the velocity of a everlasting magnet DC motor is precisely proportional to the enter voltage: Pace = OkaysV, the place fixed Okays is particular to the motor in query. However since actual motors exist alongside us within the sensible world, their habits differs from this theoretical preferrred, largely as a result of actual motors embody resistance: Mechanical resistance (friction), and electrical (winding resistance = Rw). To provide the torque wanted to beat the previous, motors should draw present (I) and once they do, present passing by means of Rw decreases the efficient driving voltage (-IRw). Due to this fact, when a motor is loaded by inside and/or exterior friction, present draw will increase and efficient voltage decreases. So, it slows down, as predicted by:
Pace = Okays(V – IRw)
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
One technique to scale back (or get rid of) this impact and make motor velocity fixed, regardless of various frictional loading, is to actively cancel Rw by sensing present I and including a proportional compensating time period to V, thereby forcing:
Pace = Okays(V – IRw + IRw) = OkaysV
Determine 1 presents a easy, versatile circuit that does precisely that, plus the comfort of grounding one of many motor connections. Right here’s the way it works.
Determine 1 The Rw-cancelling motor drive.
Rail-to-rail op-amp A1 compares the velocity setpoint from potentiometer R5, variable from (0.99)Vdd to Vdd, to the motor drive voltage Vs that’s output by Q1 by way of the 100:1 R4/R3 voltage divider suggestions loop, and servos Q1 base present accordingly. Vs is thereby settable from a minimal of zero (R5 absolutely CCW), to a most of Vdd minus the sum of the saturation voltages of A1 and Q1 (absolutely CW). The usable limits of Vdd are 3V to 16V, corresponding with A1’s TLV271 datasheet rankings. The Vdd and Q1 needs to be chosen for compatibility with motor drive (Vs and present) necessities.
However what about Rw cancellation?
Motor present I, as mirrored in Q1’s collector present, is sensed by R1. An adequately correct estimate for R1, on condition that cancellation is in the end fine-tuned with R2, will be primarily based on a easy multimeter measurement of Rw whereas the motor shaft is held stationary. Typical R1 resistances (R1 = Rw / 50) will lie within the milliohm vary, making implementation of R1 with a easy circuit-board-trace meander possible. This can be a good factor as a result of a circuit board hint, being copper, can have a temperature coefficient much like the (likewise copper) motor wiring: ~ 3930ppm/oC, which can enhance the steadiness of Rw cancellation versus temperature, offered that motor and R1 are in comparable thermal environments. Notice that R1’s voltage drop attributable to its connection to Q1’s collector isn’t subtracted from max motor drive, however as an alternative makes due with the (in any other case wasted) distinction between Q1’s base and collector saturation voltages. Notice that the wattage score wanted for R1 = Vdd2/Rw/50 and a a number of of the IR1 voltage developed (as set by R2) is added to Vs by way of the +40dB R4/R3 suggestions loop.
Correct adjustment of R2 for Rw cancellation and fixed velocity impartial of friction will thereby consequence within the (magical?):
Okays(V – IRw + IRw) = OkaysV
R2 adjustment will be achieved in a wide range of methods. For instance, motor velocity will be quantitatively measured (e.g., with a cheap optical tachometer) the place R2 is adjusted for a relentless velocity whereas frictional load is diversified. Or it may be achieved subjectively (by ear, listening to the pitch of the spinning motor’s whine) for a equally correct consequence—in the event you can learn music.
The Determine 1 circuit can thus generate a dialed-in motor drive voltage and compensate it for winding resistance, thereby attaining, and holding an correct and secure set velocity. However that velocity is manually set by a pot wiper place. What if we’d like a extra automated supply of motor management?
Determine 2 solutions this query with a variant of Determine 1 that replaces potentiometer R5 with a cheap ripple-canceled PWM-DAC.
Determine 2 PWM DAC management of Rw-cancelling motor drive.
Gate U1a of triple-switch U1 regeneratively interprets the logic degree PWM enter at R7 right into a sq. wave output for gates U1b and U1c. Vdd voltages between 3 and 5V move on to U1 pin 8 (Vss) by way of R9, whereas Vdd > 5V causes shunt regulator U2 to kick in, limiting U1’s Vdd -Vss to a logic suitable 5V.
U1b generates an uninverted PWM waveform for low-pass averaging by the R5, C3, and R10 community and output of Vs, whereas U1c cranks out an AC-coupled inverted model appropriate for analog subtraction ripple cancelation by way of R6C4 per the scheme described on this design concept.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a methods. In all, a complete of 64 submissions have been accepted since his first contribution was revealed in 1974.
Associated Content material
