The secret to a smaller, quieter, smoother stepper motor system

火辣西米秀

The secret to a smaller, quieter, smoother stepper motor system [Copy link]

 

Stepper motors are open-loop control motors that convert electrical pulse signals into angular or linear displacements. They have penetrated into multiple economic fields including computers, communication equipment, office automation, industrial automation, and banking equipment, and are still deepening. Relevant data show that from 2018 to 2023, the market size of China's stepper motor manufacturing industry will maintain an average annual growth rate of about 5.1%. Optimistic estimates show that by 2023, the industry market size is expected to exceed 18 billion yuan.

However, with the development of IoT and AI technology, portable intelligent products have increasing requirements for stepper motors in terms of package size, power consumption, control accuracy, noise, drive response speed, etc. 256 subdivisions, 80nA sleep power consumption, 12V/2A drive capability, TQFN-3×3-16L/MSOP-10 or even smaller packages have become indicative parameters for measuring product performance. At the same time, a full range of safety protection mechanisms must be established for short circuits (between outputs and between outputs to ground and power supply), overheating, overcurrent, etc. All of this poses considerable challenges to stepper motor drive design engineers.

Up to 256 subdivisions
It is well known that in the case of non-overload, the speed and stop position of the stepper motor only depend on the frequency and number of pulses of the pulse signal, and are not affected by load changes. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in the set direction, called the "step angle", and its rotation runs step by step at a fixed angle. We can control the angular displacement by controlling the number of pulses to achieve the purpose of accurate positioning; we can also control the speed and acceleration of the motor by controlling the pulse frequency to achieve the purpose of speed regulation.

Research on subdivision drive technology is very active at home and abroad. At present, high-performance subdivision drive circuits can be subdivided into thousands or even arbitrary subdivisions. After complex calculations, the step angle after subdivision can be made uniform, which greatly improves the pulse resolution of the stepper motor, reduces or eliminates vibration, noise and torque fluctuation, and makes the stepper motor more "servo-like" characteristics.

SGM42622 is a representative stepper motor driver of Shengbang Microelectronics. It adopts true 256 subdivision and small TQFN-3×3-16L green package, integrates logic control and low RDS(ON) power stage circuit, and can realize PWM current control with adjustable off time and subdivision accuracy up to 1/256.

Figure 1 SGM42622 uses 256 subdivisions

In order to make the device suitable for different application scenarios, it is a common practice in the industry to reserve pins for customers to configure functions in
advance. Taking a 28-pin traditional stepper motor driver chip as an example, the chip body package area is about 9.7×4.4mm2, plus more than 20 discrete devices with various functions on the periphery, the PCB board area is about 30×40mm2. Assuming that the device's own sleep power consumption is 10μA, after taking into account the power consumption of peripheral devices, the total power consumption will be more than 5mA. Although chip manufacturers have tried their best, such as using packages with lower pitch or QFN packages, the required discrete resistors and capacitors have not been reduced, and the power consumption reduction effect is still limited.

For example, MCU is used to run GPIO to simulate I2C functions. Within 1ms after the general I/O control chip is turned on, it first writes various parameters to the register during the device standby period, and reads out to check whether the writing is correct. After that, the MCU switches back to the general I/O to control the chip to exit the standby mode and work normally, thereby reducing the MCU's overhead and power consumption.

SGM42622 is powerful. Customers can program the switching frequency, subdivision accuracy, attenuation mode, torque size and other parameters of the device during the chip standby period. This is also the secret of SGM42622 being so compact.

Figure 2 SGM42622 has both “small size” and “big wisdom”

The beauty of quietness and smoothness
Generally speaking, the current zero-crossing distortion of stepper motors will lead to uneven and uneven steps, and may even cause loss of steps or audible noise. The main reasons for this phenomenon are the length of the blanking time (tBLANK) and the total off-time (tOFF) of PWM. In other words, the blanking time needs to be shorter in the case of low current, so it is more appropriate to use an adaptive blanking time; if the off time is too long, the current will decay to zero in the case of low current, resulting in the average current of each step being significantly less than the target value.

In SGM42622, customers can flexibly set the off time (slow decay + fast decay) by connecting an external resistor between the TOFF pin and ground. However, in order to increase the stability of the regulation circuit, a small RC series must be inserted in parallel with the regulation resistor.

Figure 3 Users can flexibly set the total off time (tOFF) of SGM42622

Data shows that this product can drive an output of 10V@1.8A (Peak) and can drive a motor of size 57. It has very strong driving capability and is very suitable for 1.8V to 12V DC bus, ECG printers, 3D scanners, home domes, mobile phones, tablet computers, televisions, camera telescopic mechanisms and other market segments.

Figure 4 Actual current waveform of the motor drive @3.3V

Conclusion
SGM42622 is just one of Shengbang Microelectronics' motor driver product family. In the future, stepper motors can be smaller, have better performance, and be more cost-effective. They can also be gradually developed into highly integrated and intelligent ones, and achieve high-precision control.