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Motor Basic – Key terms for motor control Zhang Chen Jian MAT, MDBU, Sept 2010 DC motor behavior Basing on DCM model, analyze potential control : • • • • Speed Torque Current Direction Brush DC Motor Speed-Torque Curves Increasing Voltage Let’s assume you have a constant torque load, as shown by the red vertical line below. As you steadily increase the voltage on the motor terminals, the speed will go up proportionally. Conversely, if you hold the speed constant as indicated by the blue line, and steadily increase the motor’s current, the torque will go up. So current controls the motor’s torque, but voltage controls the motor’s speed. Increasing Current Speed-torque curves for terminal voltages of 10, 20, and 30 V. Source: DC Motors, Speed Controls, Servo Systems, Electro-Craft Corp., 1980 PWM • efficiency • waveform • filtering structure • frequency setting PWM Generation Simple Indication Blue W aveform – Motor Voltage Red W aveform – Filtered motor voltage Center-Aligned PWMs H-Bridge Half-bridge – When two transistors are connected in a totem-pole arrangement as shown below, they are said to be in a halfbridge configuration. By turning each transistor on and off in a complimentary fashion, the half-bridge can drive the load voltage alternately high and low to produce a PWM waveform at the motor terminal. V+ V+ V+ DC Bus A Motor + _ motor Mod Carrier A B GND Inverter B (Dead-Time not shown) H-Bridge Dead-Time C urrent with C orrection Disabled With a half-bridge power structure, you must make sure that the bottom transistor and the top transistor NEVER get turned on at the same time. Doing so will result in large currents flowing through the transistor pair, as a short is created across the DC bus. This condition is called shoot-through. ON ON Top transistor Dead-Time Bottom transistor ON ON ON Example of Dead-time Distortion Current Re-circulation Inductor character and behavior (video) Current need to be continuously maintained What will happen if we turn off SW1 ? Current Re-circulation V+ V+ A Motor + B _ Mod Carrier A B (Dead-Time not shown) H-Bridge Solution: provide a current path for recirculation A-sync and Sync mode recirculations Current Recirculation: Fast Decay Mode • Current flowing through the motor winding will be working against the full supply voltage, plus two diode drops. • Current decays quickly. Current Recirculation: Slow Decay Mode • Current re-circulates through power MOSFETs presenting a resistive path to the current • Current decays slower (directly proportional to the LR) Mixed Decay Mode: The best of Both? • • • The idea behind mixed decay mode is to provide faster decay than slow, but slower than fast. FET switching is coordinated so that Fast Decay mode is engaged for a fixed period of time, followed by slow decay mode Mixed decay mode is necessary for reliable micro-stepping Brushed DC Motors: Dynamic Braking VM VM (-) BEMF (+) (-) BEMF (+) (+) VBB (-) Normal Operation VM > BEMF Braking BEMF Stops Motor Critical Points: By shorting the motor leads, you allow the Back EMF voltage to drive current in the opposite direction of the supply current, quickly bringing the motor to a stop. The energy of the motor dissipated by the “resistive load” Current ModeisProfiles Stepper Winding Current Brushed DC Motors: Coasting VM VM (-) BEMF (+) (-) BEMF (+) (+) VBB (-) Normal Operation VM > BEMF Coasting Critical Points: By opening the circuit, only the friction of the motor rotation slows the motor down, bringing the motor to a slow stop. The energy of the motor is dissipated by the friction of the motor DC Motor BRAKE IM RM RM LM IG L di/dt = 0 LM DISABLEMENT OCCURS VS RDSON * 2 BEMF BEMF VS Current in the winding decays in a few micro seconds. Motor becomes a Generator and pushes current in the opposing direction. Motor drive protection VBB AH High Side PreDriver High Side PreDriver + AL M- Low Side PreDriver Thermal Shutdown BH Low Side PreDriver Current Control BL Over Current Protection Protection circuits for a variety of conditions enable a simplified design while providing protection at circuit, motor and board levels against potentially damaging events Over Current Protection (OCP) VM/RDS(ON) Without OCP: Current rises to destructive levels TIME CURRENT CURRENT POOF! With OCP: Current is limited, then driver turns off IOCP TIME TOCP • Designed to protect the device from damage in the event of a fault in the motor or wiring, such as a short to ground, power, or across the winding. • Each power FET is protected individually, so is protected against shorts to any other signal • Protection must be quick enough to prevent damage but not trigger on false trips. Body diodes must be robust enough to handle shorts. • Can be a latched or auto-recovery. Some devices alert the system MCU via a Fault pin on an OCP event. Current Regulation Winding Current VM IMAX Current Limit Kicks In Time RSENSE Amp Winding Current In Volts IMAX Reference Voltage RSA To H Bridge A Enable VREFAB To H Bridge A Why is current regulation needed? • Stepper Motors: Winding resistance is very low and there is little to no back EMF. Typically you must have it for basic stepper operation. • Brushed / Brushless DC: Needed to limit stall / start up currents Configuring Itrip VREF ITRIP GAIN RSENSE M Winding Current In Volts (V) RSENSE Amp I MAX Reference Voltage VREF Low Side Current Sensing To H Bridge Enable Current Regulation Example Using current control with a DC brushed motor can allow you to limit the high starting current, and use a motor driver IC that is rated for less current than the stall current of the motor Motor startup without current control Startup (stall) current over 14 amps! Motor startup with current control Current limited to less than stall current Motor reaches full speed Thermal Protection Normal Operation TSD 170 C 150 C 120 C 80 C 50 C 25 C Temperature • Multiple thermal sensors are placed across the die, continuously monitoring temperature. When temperature reaches over temp, the H Bridge is tri-stated. • Fault can be latched or auto-recovery. Some devices alert the system MCU via a fault pin on an over temp event. • Some devices have 2-stage thermal protection, providing a warning (fault flag) prior to shutting the device down. Undervoltage/Cross Conduction Protection • Undervoltage protection – Supply voltage level is constantly monitored and the device is tri-stated when the voltage level is too low to ensure proper control over the H-Bridge • Shoot-through Protection – High side and low side are never allowed to turn on at the same time. A small amount of delay (dead time) is inserted after turning off the high side and turning on the low side. The longer the dead-time, the safer the operation but the worse the linearity and efficiency. VBB AH BH AL BL Shoot–through!! BIPOLAR vs UNIPOLAR Stepper Drive Topology Used primarily for simple stepped mode operation Compatible with more sophisticated control techniques (Commutation diodes not shown) (Commutation diodes not shown) Source: Airpax Stepper Motor Handbook, 1989 Source: Airpax Stepper Motor Handbook, 1989 Stepper leads Full and Half-Step Operation Current Vectors for Various Stepper Excitation Modes Source: Monolithic, Programmable, Full-Bridge Motor Driver Integrates PWM Current Control and ‘Mixed-Mode’ Microstepping, Paul Emerald, Roger Peppiette, and Anatol Seliversto,, Allegro MicroSystems, Inc. Technical Paper STP 97-5A Microstepped Current Waveforms B A A B Interface Styles ENABLE / PHASE Interface VM BH ENABLE PHASE Logic & Pre Drive AH OUTA AL OUTB BL ENABLE PHASE A B L L HIZ HIZ L H HIZ HIZ H L GND VM H H VM GND • Logic controls all aspects of four FET switching. • ENABLE signal selects whether the entire H Bridge is turned ON or OFF. • PHASE signal selects whether the H Bridge is conducting from side A to side B or vice versa. • A single PWM signal can control speed and/or direction. • Only one Inductive load can be driven. IN1/IN2 Interface VM IN1 AH LOGIC & Pre Drive BH A AL IN2 LOGIC & Pre Drive B IN1 IN2 A B L L GND GND L H GND VM H L VM GND H H VM VM BL • Each Half H Bridge can be controlled independently. • Can be utilized to drive two inductive loads without current control. Per example, two solenoids can be driven with a single H Bridge. • Requires 2 PWM sources for speed control of a DC motor. IN1/IN2 Possible Implementations VM IN1 AH BH IN2 M AL BL VM IN1 AH AL BH BL IN2 • Can drive a single inductive load with current flow in both directions. • Can drive two inductive loads (such as solenoids) with unipolar current drive. • When driving dual load, current regulation must be disabled – SENSE pin can be grounded. Serial Interface VM DATA BH CLK SPI SELECT Logic & Pre Drive AH OUTA AL • Different Serial Protocols are used to sample data which will be transferred as a command into the H Bridge. OUTB BL • Very hard or not recommended for PWM. VM SD BH SCLK I2C Logic & Pre Drive AH OUTA AL OUTB BL • Gives the largest amount of H Bridge devices to be controlled with the least amount of resources. – E.g. A single I2C bus can control 9 DRV8830’s