ISSUE: August 2012 Non-Dissipative Control Saves Power And Cost In Stepper Motor Applications by Enrico Poli and Jean-Jacques Meneu, STMicroelectronics, Agrate, Italy A stepper motor needs to control the current in the coils in order to control the torque and speed of the motor. To obtain the desired current, the traditional method senses the current flowing through the motor and provides feedback to the controller chip, which then decides whether to increase or decrease the current in the coil. In general, this method has required the use of expensive, large and highly dissipative shunt resistors. However, this article will present two new innovative control techniques that avoid using these shunts resistors, and hence, decrease the cost, complexity and power loss of the application. Traditional Current-Mode Control As shown in Fig. 1, the current drained through a bridge is monitored at the bottom of the bridge thanks to a shunt resistor. The voltage across the shunt resistor is compared to a reference. If the voltage is higher than the reference, the logic of the driver performs a decay to decrease the current in the motor. On the other hand, if the sensed voltage is lower than the voltage reference, the driver will increase the current in the motor. Fig. 1. In the conventional approach to current control, a high-wattage shunt resistor is placed at the bottom of the bridge to sense the motor current. This method has several major drawbacks. First, it s highly dissipative. Among stepper motor applications, it s very common to see values of motor current in the range of 500 ma to 3 A rms. As the desired voltage across the shunt resistor is around 1 V to 2 V, that current range leads to resistor values falling somewhere in the range of 0.25 to 0.5 Ω. These very low value resistors values must be able to dissipate fairly high power levels and so require that the components be rather large. For example, a 1-W resistor in this range may measure around 3.2 mm x 6.3 mm and ratings anywhere from 1 W to 3 W are common. Worse yet, in high-power applications, where a current of 10 A may be used, several resistors will have to be put in parallel. Such solutions take up a lot of space on the board. In addition to large resistor size, another drawback of using shunt resistors to sense motor current is that lowvalue resistors with high power ratings are expensive. And the higher the current, the more power must be dissipated, and therefore the more expensive the solution will be (Fig. 2.) In a high-power application, the required shunt resistors may cost a total of several dollars. As will be explained later on in this article, the new, non-dissipative current-control solutions are more elegant from a technical point of view. But on top of that, these solutions do not increase the cost of the application nor require extra room on the PCB as the power level increases. 2012 How2Power. All rights reserved. Page 1 of 5
Fig. 2. In the traditional approach to current control, the cost of the shunt resistors rises as motor current levels increase. However, such is not the case with the new non-disspative methods of motor current sensing. Two solutions, or control modes, are now available for stepper motor applications: Current-mode control where current sensing is integrated on the silicon. This sensing circuitry has no dissipation, so the current capability of the driver is not limited by it. Voltage-mode control. In this configuration, the current is not controlled through a closed-loop approach. There is no feedback loop, and hence, no dissipative sense resistor. The voltage mode makes it possible to reach higher microstepping resolution when compared to current-mode control. The voltage mode allows a much greater smoothness at low speed, but it does not work well in full step driving. For an application with several amperes of motor current, a discrete power stage with separate gate drivers is the approach that must be chosen. In that case, voltage-mode control is the only option. Now, let s investigate how these two modes are implemented. Current-Mode Control As we have seen previously, the feedback loop is highly dissipative due to the high current flowing through the bridge and the sense resistor. If we can find a way to decrease significantly the current in the sense resistor, it won t be dissipative anymore and the problem is solved. Obviously, the current flowing through the bridge cannot be decreased as it directly controls the torque desired to drive the motor. This contradiction can be solved with a special design of the MOS inside the H-bridge as shown in Fig. 3. 2012 How2Power. All rights reserved. Page 2 of 5
Fig. 3. A specially designed H-bridge integrates a current mirror that generates a small current that is proportional to the current flowing in each MOSFET of the H bridge and also the motor coil. In STMicroelectronics products such as the easyspin L6474 and the dspin L6472 two fully integrated microstepping motor drivers a dual H-bridge contains additional circuitry that generates a small current, proportional to the actual current flowing in each MOSFET. This tiny current is therefore proportional to the current flowing through the coil of the motor. Hence, instead of having feedback loops that must handle several amperes, the feedback loop of these devices handles only a few microamps. This low current is not high enough to increase the temperature of the die and can be considered non-dissipative. This method is in fact an optimization of the traditional current-control loop in stepper motor designs. A more radical change is the voltage-mode control. With this method, there is no feedback to monitor the current. Instead, it s an open loop and hence no dissipative elements are used. Voltage-Mode Control The shaft of a stepper motor has a magnetic field that, while spinning, creates a back electromotive force (BEMF). This BEMF is simply a voltage opposing the voltage that is supplied to the coil of the motor. In current-mode control, the current to the coil is imposed by closed-loop control, but not without complications due to the BEMF. Since the BEMF effectively reduces the voltage across the coil, the time needed to reach the desired current in the coil is lengthened (V=Ldi/dt). Depending on how great the BEMF is relative to the voltage across the bridge, the BEMF can ultimately limit the speed of the motor. So while the system does not need to know the value of BEMF under current-mode control, not knowing and not compensating for the BEMF can come at the cost of system performance. With voltage-mode control, a smart system is implemented to compensate the BEMF. Knowing the characteristics of the motor (resistance, inductance and BEMF versus speed), a voltage is calculated and applied to reach the desired current (Fig. 5.) 2012 How2Power. All rights reserved. Page 3 of 5
Figure 4. Motor phase electrical model (left) and vector representation (right). With this method, a voltage is applied to the motor instead of trying to impose a constant current. The applied voltage compensates and eliminates the effects of the BEMF. As the BEMF increases with the speed of the motor, the voltage applied to the motor is increased in the same proportion, allowing a flat current amplitude whatever the speed of the motor. So, as in the case shown above in Fig. 4, knowing the desired current, it is easy to determine the value of applied voltage that will achieve that current. In this way, the current is indirectly controlled by the voltage. Fig. 5 below also shows the effectiveness of this method. Fig. 5. In the voltage-control mode approach, the controller compensates for the back EMF of the motor coil to maintain constant torque as motor speed increases. To conclude, highly dissipative and expensive shunt resistors are no longer needed to control the current in stepper motors. New motor drivers from STMicroelectronics such as dspin and EasySPIN offer a choice of either current-mode or voltage-mode control methods that sense current within the moter driver chip. In both cases, the method does not dissipate power, decreasing the cost of the solution and improving the thermal efficiency of the application. 2012 How2Power. All rights reserved. Page 4 of 5
About The Authors Enrico Poli is a senior application engineer in the Industrial & Power Conversion Division of STMicroelectronics. Since 2006, Poli s work at STMicroelectronics has focused on the development of monolithic solutions for motor driving. Poli holds a degree in Electronic Engineering from the Politecnico di Milano, Italy. Jean-Jacques Meneu is a technical marketing engineer in the Industrial & Power Conversion Division of STMicroelectronics. Since joining the company in 1997, he has focused on industrial applications. Meneu received a degree in Electronic Engineering from the National Superior Electronics School in Paris, France. 2012 How2Power. All rights reserved. Page 5 of 5