MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL DESCRIPTION The M51971 is a semiconductor integrated circuit designed to control the motor rotating speed. The built-in FG amplifier with high gain enables to use a wide range of rotating speed detector (FG detector). Use of less external parts enables DC motors to be controlled with high precision. PIN CONFIGURATION (TOP VIEW) Non-inverted input 1 Inverted input 2 Amplifier output 3 ●Wide range of supply voltage • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 4 – 17.5V ●Variation coefficient of supply voltage • • • • • ±0.005%/V (standard) ●Load variation coefficient • • • • • ±0.01% (standard, full load range) ●Temperature coefficient of rotating speed • • • • 7ppm/˚C (standard) ●Built-in high performance FG amplifier 4 5 Stabilized voltage 6 GND 7 Integration capacitance 8 Output 9 APPLICATION M51971L Schmitt input Time constant FEATURES Power supply 10 Motor rotating speed control in floppy disk driver, player, tape recorder, car stereo, etc. Outline 10P5 RECOMMENDED OPERATING CONDITIONS Non-inverted input 1 Inverted input 2 Amplifier output 3 Schmitt input 4 Time constant 5 10 Power supply M51971FP Supply voltage range • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 4 – 17.5V Rated supply voltage • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 9V Input voltage range at pin 1 • • • • • • • • • • • • • • • • • -0.4 – Vcc Note 1 Input voltage range at pin 4 • • • • • • • • • • • • • • • • • • • • • • • • • • -0.4 – Vcc Highest setup tacho-generator frequency • • • • • • • • • • • • • • • • 2.5kHz Minimum trigger pulse width (input pulse at pin 4 ) • • • • • • • • • • 40µs Note 2 Note 1: The linear operation range is -0.4 to +0.4V. Note 2: This condition applies to both periods: from pulse rising to pulse falling and pulse falling to pulse rising. 9 Output 8 Integration capacitance 7 GND 6 Stabilized voltage Outline 10P2-C BLOCK DIAGRAM Amplifier Schmitt output intput 3 VLS Non-inverted input 4 Integration capacitance 5 8 Schmitt comparator Operational amplifier Buffer amplifier 1 Timer 1.9V Inverted input Time constant 2 VLS Power supply 10 1.9V Stabilized supply voltage 7 6 GND Stabilized voltage Constant current control Over-shoot prevention circuit 9 Output MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL ABSOLUTE MAXIMUM RATINGS (Ta=25°C unless otherwise noted) Symbol Parameter Vcc V1 I3 I6 V4 I9 Supply voltage Apply voltage at pin 1 Source current at pin 3 Source current at pin 6 Apply voltage at pin 4 Source current at pin 9 PdF Power dissipation KθF Thermal derating Topr Tstg Operating temperature Storage temperature Conditions Ta≥25˚C Ratings Unit 18 -3 – Vcc -5 -5 0 – Vcc -20 880 (M51971L) 450 (M51971FP) 8.8 (M51971L) 4.5 (M51971FP) -20 – +75 -40 – +125 V V mA mA V mA mW mW / °C °C °C ELECTRICAL CHARACTERISTICS (Ta=25°C, Vcc=9V unless otherwise noted) Symbol VCC ICC VS I1 I2 V 1 LS AV I4 V 4 TH V 4 HY V5 S Tτ I8C rCD R9 V 9 max V 9 min VBO Parameter Supply voltage range Circuit current Stabilized supply voltage Input current at pin 1 Input current at pin 2 Level shift voltage at pin 1 FG amplifier voltage gain Input current at pin 4 Threshold voltage at pin 4 Hysteresis width at pin 4 Saturation voltage at pin 5 One-shot pulse width Charging current at pin 8 Ratio of charging to discharging current at pin Output protection resistance at pin 9 Maximum voltage at pin 9 Minimum voltage at pin 9 Buffer amplifier offset voltage Test conditions 8 Min. 4.0 Voltage at pin 6 2.44 V 1 = 0V -3.0 V 1 = 0V -180 V 1 = 0V 1.51 V 1 =0.2mVrms, f=500Hz, External set gain=60dB 54 V 4 = 2.5V Uses level shift voltage at pin 1 as the reference. 0 20 Rτ = 75kΩ Rτ = 75kΩ, Cτ = 4700pF 375 V 8 = 1V -260 V 8 = 1V -14.5 I 9 = -20mA 65 2.9 V 8 = 1V, V 8 - V 9 0 Limits Typ. 3.2 2.71 -0.5 -30 1.89 59 0.4 16 37 3 395 -190 -11.6 100 3.2 50 100 Max. 17.5 6.0 2.98 Unit 2.27 64 2.0 40 55 20 415 -140 -9.0 150 V mA V µA nA V dB µA mV mV mV µsec µA – Ω 200 200 V mV mV MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL TYPICAL CHARACTERISTICS Thermal Derating (Maximum Rating) Rotating speed–Supply voltage characteristics 1000 3005 M51971L No load Rotating speed N (rpm) Power Dissipation Pd (mW) 800 600 M51971FP 400 3000 200 0 0 25 50 75 100 2995 125 0 4 Ambient temperature T a (°C) 8 Rotating speed–Motor torque characteristics 20 3005 VCC=9V VCC=9V No load Rτ, Cτ Outside constant temperature bath Rotating speed N (rpm) Rotating speed N (rpm) 16 Rotating speed–Ambient temperature characteristics 3005 3000 2995 0 50 100 3000 2995 -50 Torque T (g-cm) -25 0 25 50 75 100 Ambient temperature T a (°C) Circuit current–Supply voltage characteristics Circuit current–Ambient temperature characteristics 5 5 4 4 Circuit current ICC (mA) Circuit current ICC (mA) 12 Supply voltage VCC (V) 3 2 1 3 2 1 0 0 4 8 12 Supply voltage VCC (V) 16 20 0 -50 -25 0 25 50 Ambient temperature T a (˚C) 75 100 MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL FG amplifier open loop voltage gain, phase transition characteristics Revel shift voltage at pin 1 – Input voltage characteristics at pin 1 2.5 (V) 100 20 -90 Phase -120 0 -20 10 100 1k 10k 100k Level shift voltage at pin 1 V 1 40 Phase φ (degree) Voltage gain AV (dB) Voltage gain 60 VCC=9V LS VCC=9V 80 -150 1M 2.0 1.5 -0.6 -0.4 -0.2 Frequency FIN (Hz) 0 0.2 0.4 0.6 0.8 1.0 Input voltage at pin 1 V 1 (V) Level shift voltage at pin 1 – Ambient temperature characteristics Voltage at pin 3 – Output current characteristics of pin 3 4 VCC=9V V 1 =0V 2.5 Voltage at pin 3 V 3 (V) Level shift voltage at pin 1 V 1 LS (V) 3.0 2.0 1.5 1.0 3 2 1 0.5 0 -50 -25 0 25 50 75 0 -15 100 Ambient temperature T a (°C) -5 0 5 3 10 15 (mA) Saturetion voltage at pin 5 –Sink current characteristics at pin 5 50 40 VCC=9V (mV) 40 S 30 Saturation voltage at pin 5 V 5 ON level 20 10 0 -10 OFF level -20 -30 -40 -50 -50 -10 Output current at pin 3 I Threshold voltage at pin 4 – Ambient temperature characteristics Threshold voltage at pin 4 (mV) 1.2 1.4 30 20 10 0 -25 0 25 50 Ambient temperature T a (°C) 75 100 0 0.2 0.4 0.6 0.8 1.0 Sink current at pin 5 I 5 (mA) 1.2 MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Stabilized voltage–Supply voltage characteristics Stabilized voltage–ambient temperature characteristics 5 3.0 4 Stabilized voltage VS (V) Stabilized voltage VS (V) VCC=9V 2.8 2.6 2.4 3 2 2 2.2 2.0 0 4 8 12 16 20 1 24 -50 -25 Supply voltage VCC (V) 0 25 50 75 Ambient temperature T a (°C) Stabilized voltage–Source current characteristics of pin 6 Voltage at pin 8 – Input signal frequency characteristics 2.8 4 VCC=9V Voltage at pin 8 V 8 (V) Stabilized voltage VS (V) VCC=9V 2.7 0 1 2 3 Source current at pin 6 I 2 6 0 401.5 5 4 402.0 402.5 403.0 Pin 8 – input signal frequency fIN (Hz) (mA) Charging current at pin 8 – Ambient temperature characteristics Discharging current at pin 8 – Ambient temperature characteristics 30 -300 (µA) VCC=9V VCC=9V 25 Discharging current at pin 8 I 8 d -250 c (µA) 3 1 2.6 Charging current at pin 8 I 8 100 -200 -150 -100 -50 0 -50 -25 0 25 50 75 Ambient temperature T a (°C) 100 20 15 10 5 0 -50 -25 0 25 50 Ambient temperature T a (°C) 75 100 MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Output voltage range at pin voltage characteristics – Supply 9 Buffer amplifier offset voltage – Voltage characteristics at pin 8 160 3 2 Ta = -20°C Ta = 25°C 1 Ta = 75°C Buffer amplifier offset voltage VBO (mV) Output voltage range at pin 9 V 9 (V) 4 VCC=9V 120 80 40 0 –40 0 4 8 12 16 0 1 100 90 4 How to determine Rτ and Cτ These constants determine the motor rotating speed. If the motor rotating speed and the number of poles of tacho-generator are assumed to be N and P, respectively, the following relational expression is generally established. According to the required rotating speed, select the constant in such a way that Rτ can be put in the range of 10kΩ – 500kΩ. When using a high resistance, take care for leak current that may flow on the surface of the printed circuit board. NP ≈ 1 1.20 • Rτ • Cτ Tacho-generator output frequency – Connection resistance characteristics at pin 10 10000 7000 5000 3000 τ= C 2000 F 2µ 02 7µ F 0. 1µ 300 F 04 500 F 1µ 0. 700 p 00 47 0 0. 1000 0. F Tacho-generator output frequency NP (Hz) 0% Upper side : Motor speed (FV conversion waveform of tachogenerator frequency) Lower side : Supply voltage Horizontal axis : 20 ms/div Time constant of motor ≈ 100 ms 3 Voltage at pin 8 V 8 (V) Supply voltage VCC (V) Application Characteristics Example 2 200 10 20 30 50 70 100 200 300 500 1000 Connection resistance Rτ (kΩ) at pin 1 Rτ (kΩ) 1 MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Brief Description on M51971 Operation Block Description To pin 6 Amplifier output VLS Schmitt circuit 1.9V Operational amplifier Non-inverted input 1 7.5k comparator Rτ 4 3 FG amplifier Timer Schmitt input A C 5 B Q1 A’ Logic for timer Cτ Inverted input 2 15k VLS D1 1.9V D2 E Over-shoot prevention signal D Timer output 10 Over-shoot prevention circuit Power supply 206µA I1 G Q3 Logic for over-shoot prevention Q2 Stabilized power supply 7 GND M 16µA Buffer I2 H 9 6 8 I Stabilized voltage OP Constant current control RF 100Ω CF1 CF2 FG amplifier The FG amplifier consists of an operational amplifier, revel shift circuit and diode for waveform clip. When a DC block capacitor is connected to pin 2 , output DC voltage at pin 3 becomes higher than DC voltage at pin 1 by VLS (≈1.9V≈3VBE). AC signals centering around the GND can be therefore amplified easily. The clipper diode limits the output signal amplitude to ±0.7V (VBE) max. and rapidly charges DC block capacitor with power supply turned ON. Schmitt circuit The Schmitt circuit is a comparator with histeresis, and has ON level of VLS + 20mV and OFF level of VLS - 20mV. Timer The timer generates basic time necessary for controlling the speed. This timer is a one-shot circuit triggered with input signals and generates pulse of 1.1 Cτ Rτ in pulse width. (≈190µA) for the period without one-shot pulse and generates sink current of I2 (≈16µA) for the period with one-shot pulse. The ratio of I1 to I2 is characteristic to the IC. The frequency of the tacho-generator to be set is determined by the one-short pulse width and this current ratio (I1 / I2 ≈12.6). TG = Tτ x I1 I1–I2 ≈ 1.09 x Tτ Where: TG : Tacho-generator signal frequency (set value) Tτ : One-short pulse width Over-shoot prevention circuit The over-shoot prevention circuit operates when over-shoot is large in particular, e.g. the motor is suddenly released from lock status. Q3 is set to ON for the period of one-short pulse width (Tτ) when the signal period of the tacho-generator in a motor is shorter than the one-shot pulse. Generally, electric charge of CF1 is discharged for this period due to RF • CF2 >>Tτ. Buffer amplifier Constant current control circuit The constant current control circuit is controlled with output of timer circuit. The circuit generates, at pin 8 , source current of I1 – I2 The buffer circuit is a voltage follower circuit using an operational amplifier. The input current is very small (10nA max.) and the circuit can drive the output current of 20mA. MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Input/Output Circuit Drawing To pin 10 To pin 10 I To pin 6 2 Control signal 3 8 100 200 To pin 6 Control signal 2k 1 To pin 10 To pin 10 To pin 10 To pin 10 6 I 1k To pin 6 7.5k 2k 1k 100Ω To the next stage 4 5 9 15k I I 3k Timing Chart I. In normal operation II. Normal operation to rapid discharging operation A, A’ A, A’ B Approx 10µS B Approx 10µS C C D D E E F F G G H, I Tτ TG TG 1.09Tτ H, I MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Application Circuit Examples I. When the output impedance of the tachogenerator is low; RS Rf RNF Rτ CNF VCC 3 4 5 6 M 10 M51971L M51971FP 1 RNF 3 9 4 VCC 5 6 1 9 8 7 RF CF2 CF1 M 10 M51971L M51971FP 2 8 7 Rτ CNF Cτ Cτ 2 R1 G CS G III. When the signal amplitude of the tachogenerator is large; RF CF2 CF1 M : Motor G : Tacho-generator CS : Coupling capacitor for AC amplification RS, Rf : FG amplifier gain set resistance RNF, CNF : Filter for noise removal Rτ, Cτ : Time constant for motor speed setup CF1, CF2, RF : Phase compensation capacitance and resistance to stabilize integration and speed control systems Notes: 1. The signal amplitude of the tacho-generator for set motor rotating speed must be set to 1 mVP-P or more. In the above three examples, the portion over Vf (0.7V) of the output waveform at pin 3 is clipped in the built-in waveform clip diode. IV. When the input waveform is pulse shape Input pulse signal Rτ VCC Cτ 2. FG amplifier gain ≈ 1+ω2GCS2(RS+Rf)2 1+ω2GCS2RS2 ωG : Angle frequency of tacho-generator signal 4 3. The CS, RS, RNF and CNF values are desirable to be selected as follows: (Values omitted) 5 6 M51971L M51971FP 3 2 1 9 8 7 RF CF2 CF1 CS≤4.7µF 2 1 ωG ≥ CSRS ≥ ωG M 10 Note: The threshold voltage at pin 4 to GND is approx. 1.9V. 1 RNF • CNF ≤ ωG II. When the output impedance of the tachogenerator is high and the signal amplitude is small; V. When turning the motor ON/OFF with control signals; Rτ VCC Cτ RS Rf RNF CNF Rτ CS VCC Cτ 2 3 4 5 5 6 G 1 Control signal 9 CF1 CF1 RF CF2 8 7 RF CF2 9 8 7 M51971L M51971FP M 10 M51971L M51971FP M 10 6 Q1 R1 When Q1 is set to ON : Stops the motor. When Q1 is set to OFF : Controls the motor rotating speed. MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL VI. To switch the set rotating speed in stages with control signals VIII. To limit drive current to the motor 1) Control signal 1 Rτ Control signal 2 VCC Cτ 6 5 Rτ1 M51971L M51971FP Rτ2 6 R1 M51971L M51971FP Rτ3 IM M 10 200Ω 9 Q1 5 7 Cτ Q2 8 7 RF CF2 CF1 RSC 0.7V VBE2 IMmax = RSC ~ ~ RSC VII. Limiting output current at pin 9 to prevent the IC from heating 2) To reduce power loss due to a current limiting resistance Rτ VCC Rτ Cτ 5 6 VCC M 10 Cτ RSC M51971L M51971FP 9 5 Q1 R1 8 7 9 max = V 9 max :V R 9 + RSC 9 max ~ 3.2V, R ~ 9 ~ 100Ω ~ Q2 Rτ Input/output transmission characteristics R’τ R’τ > 2Rτ 10 6 5 Output M51971L M51971FP 7 Output voltage Cτ 4 9 Schmitt circuit 8 R3 ) / RSC R2 + R3 0.7V x R2 ~ ~ (R2 + R3) • RSC IX. Frequency comparator VCC R3 IMmax = (VBE2 – VBE1 x (See the Electrical Characteristics and Typical Characteristics.) Frequency input Q1 RF CF2 CF1 I 200Ω 9 R2 RF CF2 CF1 IM M 10 M51971L M51971FP 8 7 6 fTH2 fTH1 Input frequency CF Note: The selected Hysteresis of the Schmitt circuit must be more than or equal to the ripple current at pin 8 (to prevent chattering). ~ fTH1 ~ ~ fTH2 ~ 1 1.20 x Rτ • Cτ 1 1.09 x Rτ // R’τ x Cτ x In 3(Rτ + R’τ) R’τ – 2Rτ RSC MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL Hint for designing a stabilized speed control system (Method for determining the filter constants (CF1, CF2 and RF) at pin 8 ) 8 must be determined to satisfy the log GM (jω) The filter constants at pin system stability. 1. Transfer Function of the Motor Speed Control System ωM log ω Approximate motor transfer function Control circuit - GC (S) Motor GM (S) 3. Transfer Function of Control Circuit Using the M51971 Motor speed control system The motor speed control system is a negative feedback system including a control circuit and a motor. As the condition necessary for stable negative feedback, the phase must be generally 180˚ or less in the frequency area where the gain of open-loop transfer function (GC(S) • GM(S)) is 1 or more. If input information is assumed to be given continuously (the tachogenerator frequency is assumed to be infinitely high), the transfer function from the input at pin 4 to the output at pin 9 is as follows: GC(M51971)(S) ≡ = Tτ ( I 8 C + I 8 d ) 2. Transfer Function of Motor ∆Tg = KT • ∆la = (SJ+D) • ∆ωv • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (1) Where: Tg : Torque generated in the motor KT : Proportional constant between the torque generated in the motor and the armature current J : Inertia moment of Motor and load D : Coefficient of viscosity friction If the number of poles in the tacho-generator is assumed to be P, the relation of ω = P • ωv exists between tacho-generator angular frequency ω and motor angular velocity ωv and, therefore, the motor transfer function (transfer function including motor and tacho-generator) GM(S) takes a single-pole transfer function as follows: GM(S) = ∆ω P • KT = ∆la D • (1+S • J ) D = •••••••••••••••••••••••••••••• (2) •••••••••••••••••••••••••••••••••• (3) ••••••••••••••••••••••••••••••••••••••••• (4) •••••••••••••••••••••••••••••••••••••••••••• (5) KM 1 + ωSM Where: KM = P • KT D ωM = D J Where : CF1 + CF2 x 1 + S/ωF1 S(1 + S/ωF2) ••••••••••• (6) ~ 1.10 x Rτ x Cτ Tτ : Timer pulse width ~ l 8 C : Charging current at pin 8 l 8 d : Discharging current at pin ωF1 ≡ 1 RF • CF2 ωF2 ≡ CF1 + CF2 RF • CF1 • CF2 8 If the gain of the circuit connected to the back of pin 9 of the M51971 is assumed to be KCP, transfer function GC(S) for the entire circuit is as follows: GC(S) = KCP x Tτ ( I 8 C + I 8 d ) CF1 + CF2 x 1 + S/ωF1 S(1 + S/ωF2) ••••••••••••• log GC (jω) If the motor armature current and angular velocity are assumed to be la and ωv, respectively, the following equation is established. ∆ (output voltage at pin 9 ) ∆ (input frequency at pin 4 ) ωF2 ωF1 log ω Approximate transfer function of control circuit (7) MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL 4. Necessity for stable control Stable control requires the gain of GC(S) • GM(S) to be the phase characteristics of 180˚ or less in a frequency area of 1 or more. The relation of the phase and the gain is determined according to the Baud’s theorem when all poles and zero points of the transfer function are placed at the left side of the complex sphere. If GC(jω) • GM(jω) follows the Baud’s theorem, in a frequency area of | GC(jω) • GM(jω) | ≥ 1 the inclination of gain of GC(jω) • GM(jω) must be -12dB/oct or more for stable control. For the reason above, when the circuit constant is selected to achieve ωF1 ≈ ωM, and the inclination of the gain of each of GC(jω) and GM(jω) is -6dB/oct, that is, the following formula must be established with respect to the frequency of ωF2 where the inclination of the gain of GC(jω) • GM(jω) begins to be -12dB/oct. | GC(jωF2) • GM(jωF2) | < 1 ••••••••••••••••••••••••••••• (8) To make a precise control, the gain of open-loop transfer function must be large in the entire area of frequency. The variation of the motor rotating speed attenuates due to disturbance at an inclination of -6dB/oct with the frequency of ωM or more. The capability of rotating speed control in the frequency area from ωF1 to ωF2 is determined by the gain of open-loop transfer function at ωF1(≈ωM). The following formula is established with | GC(jωF2) • GM(jωF2) | < 1 and when the inclination of the gain of GC(jω) • GM(jω) is almost equal to -6dB/oct with the frequency of ωF2 or less. | GC(jωM) • GM(jωM) | < ωF2 ≈ ωF2 • • • • • • • • • • • • • • • • • • • • • • (9) ωF1 ωM Improvement of control precision in the frequency area from ωF1 to ωF2 requires the following conditions. ωF1 ≈ ωM ωF2 ωF1 >> 1 •••••••••••••••••••••••••••••••••••••••••••••••••••• (10) ••••••••••••••••••••••••••••••••••••••••••••••••••••• (11) The KCP or CF1 + CF2 value must be set to satisfy formulae (4) and (5). 5. Influence on the Stability of Tacho-generator Frequency The control system that is controlled with tacho-generator frequency, i.e. period, is a kind of sample hold system controlled with discrete information in the time axis. Addition of extra phase delay to sample hold operation makes the system more unstable. More precise transfer function H*(jω) (GC*(jω) • GM*(jω)) taking the above operation into account is as follows, when H(jω)(GC(jω) • GM(jω)) is assumed to be the transfer function where this operation is not taken into account: –j sinπ(ω/ωG) H*(jω) = e π(ω/ωG) 2πω ∞ ωG ΣH(jω + jnωG) n=–∞ •••••••••••••• (12) Where: ωG: Set value of tacho-generator frequency That is, extra phase delay of 2πω/ωG (radian) must be taken into account. That is, if the angular frequency satisfying | GC*(jω) • GM*(jω) | = 1 is assumed to be ωodB, the following relation must be established. ωG > 4 • ωodB • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (13) When this determines ωG, the possible gain of open-loop transfer function with ωM can be obtained. ωG | GC(jωM) • GM(jωM) | < 0.357 x ωM • • • • • • • • • • • • • • • • • • • • (14) This formula (14) must be satisfied in the control system using the frequency of the tacho-generator regardless of the control system and indicates that the upper limit value of the control gain with ωM is inevitably determined when the motor and tacho-generator are determined. Improvement of the control precision in the rotating speed requires | Gc(jωM) • GM(jωM) | >> 1. The following formula must be therefore established. ωG 0.357 • ωM >> 1 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (15) 6. Conclusion According to the theoretical consideration above, the design of speed control system making the best use of the characteristics of the motor is described as follows: (1) ωF1 ≡ 1 ≈ ωM • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (16) RF • CF2 If ωM sharply changes with motor load changed, a circuit constant is desirable to be set around minimum ωM. (2) ωF2 ≡ CF1 + CF2 ≥ 1 ωG 4 RF • CF1 • CF2 •••••••••••••••••••••• (17) As CF1 is smaller, influence by ωF2 becomes smaller, but the peak-to-peak value of the output pin waveform becomes larger and the drive waveform becomes closer to pulse shape. In most of design cases, both sides are therefore desirable to be equal. (3) Selection of gain constant Keeping the relation satisfying formulae (16) and (17) above, obtain a value for stable control by changing the KCP or CF1+CF2 value. If the motor set speed is divided into several stages, stage of lower speed is less stable. In this case, experiment must be made at lower speeds. MITSUBISHI <CONTROL / DRIVER IC> M51971L/FP MOTOR SPEED CONTROL How to find rough value of motor transfer function Motor speed Motor drive current Tacho-generator frequency ω (1) Finding KM (2) Finding ωM Though ωM is found by measuring the motor frequency response, this method generally takes a lot of time and labor. Measurement of step response can find rough values easily. 63% τM t KM = ∆ω ∆Ia = 2π ∆f ∆Ia Motor drive current Ia Plot the relation between the motor drive current and tachogenerator frequency to obtain the inclination. Supply step-shape current to the motor in static status, measure time τM until the motor speed reaches 63% of the final speed and then find ωM by the following formula. 1 ωM = τM ••••••••••••••••••••••••••••••••••••••••••••••••• (18)