A stepper motor system is an open-loop position control system. The system controller or the motor driver IC has no information on how much load torque is being applied or what should be the optimal current for operation without step loss. Since the driver is imperceptive of the load torque demand, motors are generally driven at constant full-scale current which can sustain maximum load torque. However, the use of large operating current is needless at lighter loads as it results in unnecessary I2R losses. Apart from lowering overall system efficiency, high coil current leads to thermal issues due to motor heating reducing the durability and longevity of the motor.
Texas Instruments recently introduced the DRV8462, DRV8452 and DRV8461 stepper motor drivers, which include several new features including the auto-torque algorithm. Auto-torque boosts system efficiency by adjusting the stepper coil current automatically according to the load torque. Auto-torque does not require any external sensor. Instead, by monitoring the power delivered to the motor, it generates an internal signal which varies linearly with the load torque, with fast sensing capability. This application report aims to highlight the advantages of the auto-torque algorithm and how it can be tuned for maximum benefits.
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Stepper motors are popular due to their simplicity of translating excitation changes on the input to precise positional changes on the output without using any external sensor to monitor position. The currents in stepper coils are regulated to achieve precise position and velocity control.
Torque equation for a motor is given by Equation 1. It depends on coil current and motor construction:
where τmax is the maximum supported torque, KT is motor’s torque constant and I is the coil current.
Equation 1 can be interpreted as torque capability offered by coil current I. To sustain a given load torque, the motor driver must always operate at a coil current which can offer more torque than demanded.
Conventional motor drivers configure operating full-scale current based on the peak load torque demand. This ensures that the motor does not lose steps any time peak load is demanded. The current therefore is constant irrespective of the load torque. As a result, when load torque is lower than the peak load, the driver and the motor dissipate some of the input power as resistive power loss as represented in Figure 1-1.
In most systems, the demand for peak load torque occurs only rarely. For example, in an ATM machine, the stepper motors might be needed to deliver peak load for less than 15% of their overall run time. A typical stepper driver though ends up delivering full-scale current to the motors all the time - leading to lower system efficiency due to the unwanted power loss, larger system size and shorter lifetime of components.