Recently, someone asked me why a stepper motor driver might be behaving erratically. The concern is a lack of steps at low speeds. The current waveform might jump to a certain level and remain there until the steps catch up, or it might increase beyond the maximum chopping current. Users typically set mixed decay, but many devices use slow decay as the absolute value of the current increases.
In most cases, distorted current waveforms appear during hold or low speeds, where slow decay doesn't remove as much current as a driver inserted during normal drive time. The problem typically worsens as the motor voltage increases. The image below shows an example of this loss of regulation at low speeds. The motor is driven at 200 steps per second (1/8 microstep). The motor voltage is 12V.
For example, there is a new feature called adaptive decay for creating the desired waveform at a low speed. This new feature automatically adjusts the percentage of slow and fast decay to create a near-optimal current waveform, using the DRV8846 driver chip.
The DRV8846 provides a highly integrated stepper motor driver for applications such as cameras, printers, projectors, and other automation equipment. This device features two H-bridges and a microstepping indexer, and is specifically designed to drive a bipolar stepper motor. Each H-bridge driver's output block contains N-channel and P-channel power MOSFETs configured as a full H-bridge for driving the motor windings. The DRV8846 is capable of driving up to 1.4A of full-scale output current (with proper cooling and TA = 25°C).
A simple stepper/direction interface allows for easy connection to controller circuitry. Pins enable motor configurations from full-step to 1/32-step modes. Attenuation modes can be configured for automatic tuning, slow attenuation, fast attenuation, and hybrid attenuation. The PWM current chopper shutdown time can also be selected. A low-power sleep mode shuts down some internal circuitry, resulting in extremely low quiescent current and power consumption. This sleep mode can be set via a dedicated nSLEEP pin.
It provides internal protection against UVLO, overcurrent, short circuit, and overheating. Fault conditions are indicated via an nFAULT pin.
• Pulse Width Modulation (PWM) Microstepper Motor Driver
o Built-in micro-stepping indexer
o Maximum 1/32 microstep
o Step/Direction Control
• Multiple attenuation modes
AutoTune™ technology
o Mixed attenuation
slow decay
o Rapid decay
• Configurable off-time pulse width modulation (PWM) chopping
10, 20, or 30µs shutdown time
• Features adaptive blanking time for smooth stepping.
• Operating power supply voltage range is 4V to 18V
• The full-scale (maximum drive) current of each H-bridge is 1.4A (at 25°C).
Low-current sleep mode
• Regulate motor current using a 3-bit torque DAC
• Heat-resistant reinforced surface mount packaging
Features in DRV8303
o VM Undervoltage Lockout (UVLO)
o Overcurrent protection (OCP)
o Thermal shutdown (TSD)
o Fault condition indicator pin (nFAULT)
Typical waveforms use slow decay when the absolute value of the current increases and mixed decay (50% fast and 50% slow) when the absolute value of the current decreases. Note the missing step, as the current transitions from zero, as highlighted in the yellow circle. This is because the motor driver injects more current during the drive state than it removes during the slow decay state. The missing step can also be viewed as the current starting to become negative.
Using these new available features (such as adaptive decay), we can quickly adjust the stepper motor and move on to the next task.
