The question thus becomes how much current is enough? During the application’s operation there will be moments when the stepper motor is simply holding its position with little load. At other moments the motor may be having to deal with large changes in loading. This requires that more current flows to ensure stalling does not occur. One approach to this is to simply dimension the control electronics so that a constant current is applied, leaving enough headroom for all eventualities. This obviously results in a lot of energy being wasted as heat, both in the stepper motor itself and in the control electronics.
An obvious approach to improving power consumption would be to include a control loop in the power electronics. This could monitor the real-time power demand of the stepper motor, increasing and decreasing the power applied as the required motor torque changes. The latest generation of stepper motor control devices from Toshiba implement a feature known as Active Gain Control (AGC) which takes exactly this approach.
The solution starts with highly integrated current monitoring circuitry, known as the Advanced Current Detect System (ACDS), that does not require any external shunt resistors. This reduces the overall footprint of the circuitry as well as reducing the bill-of-materials (BOM). The measurements made here are used to react to the changing current demands of the stepper motor by the AGC system. This can deliver current reductions of up to 30% when compared to conventional control circuitry.
A state-of-the-art high-voltage 130nm BiCD process is used for the fabrication of these motor control products. This enables the implementation of low-voltage control circuits coupled with high-voltage DMOS devices in the output stage. Resulting devices can be provided in tiny 7 mm x 7mm WQFN48 packages that also contribute to the small footprint. Thermal issues are also reduced, with an AGC stepper motor control generating some 30°C less heat when compared to the circuitry not using the AGC feature.
Using an AGC solution does not mean that designers need to make compromises elsewhere. Anomaly detection for under-voltage, overcurrent and overheating detection are also integrated. Micro-stepping is also fully supported, from full-step through to 1/128 steps.
To find out more about how AGC delivers highly efficient stepper motor control take a look at our reference design available here: