Design Article

Use micro-stepping techniques to smooth operation of step motors

Arasch Lagies, Toshiba America Electronic Components, Inc.

2/3/2009 2:50 PM EST

Over the last two decades, stepping motors (or step motors) have increasingly been applied in electronic-control applications throughout all segments of the electronics industry. Its simplicity of design has made the stepping motor an ideal choice in applications where size, weight and low power consumption are primary requisites.

Indeed, the rotor, which is the only moving part of a stepping motor has, unlike typical DC motors, no windings, commutator or brushes. The rotor is held at its position solely by the action of the magnetic flux between stator and rotor. But stepping motors need a special, repeating sequence of pulses to generate a more or less smooth rotation. Every pulse sequence results in the turning of the rotor with a fixed known angle. The position of the rotor can easily be calculated by knowing the starting position of the rotor and the number of pulses (as long as the maximum loading is not exceeded).

To make the rotation of the rotor smoother (i.e., to make the angular steps smaller), a technique of mini- or micro-stepping technique can be applied. This article will detail a micro-stepping design technique that improves the precision of the rotor movement and smoothes out the motion of stepping motors.

Micro-stepping
Micro-stepping is a technique based on two-phase-on operation, which provides for the subdivision of each full step into a number of "sub steps" of equal size. In contrast with the two-phase, full step mode where the two currents of two poles have to be kept equal, the currents are deliberately made unequal in the design technique detailed below. By correctly choosing and controlling the relative amplitudes of the currents, the rotor equilibrium position can be made to lie anywhere between the step positions for each of the two separate phases.

In terms of input control for stepping motor driver, two groups can be named: clock-in type controllers that include the complete sinus stepping function internally, and phase input controllers, which need all controlling signals from an MCU.

Stepping Motor
Stepping Motor
The (simplified) stepping motor shown in Figure 1 has four stator phases and a two-pole permanent magnet rotor. Usually, a bipolar motor is built with two adjacent stator phases connected in one current loop.

Figure 1: Stepping motor with four stator phases and a two pole permanent magnet rotor. Phase A1 and A2 (yellow) are connected and phase B1 and B2 (green) are connected.

Driving a stepping motor as shown in Figure 1 in the full-step two phase-on, bipolar mode, requires a signal pattern like the one in Figure 2. As detailed, all phases are active at any time (I_A (Phase A) and I_B (Phase B)). Two neighboring phases are always carrying the same polarity. This increases the force on the rotor poles, but as a rotor pole is attracted at any time by two stator poles, the rotor is always aligned midway between two phases, as long as the current through phase A and phase B is equal.

Figure 2: Full step, 2-phase excitation mode of the stepping motor in Figure 1.

Figure 3 shows one full cycle for the full step, 2-phase excitation mode of a stepping motor with four stator phases and a two pole, permanent magnet rotor.

Figure 3: One full cycle of a 2 phase excitation of a stepping motor with a two-pole permanent magnet rotor. Here for every step, the yellow lines are connected and the green lines are connected, respectively.

To align the activation/deactivation of the phases A and B in Figure 2, one clock (CLK) is needed. This CLK signal must trigger with the rising edge the current I_A and with the falling edge the current I_B. To achieve this, an edge-triggered flip-flop is implemented in our circuitry as shown in Figure 4.

Figure 4: Bipolar stepping motor in a full-step, 2-phase excitation mode.

The (rising) edge triggered Flip-Flop switches between I_Amax and I_Amin at every rising edge of CLK. The (falling) edge triggered FlipFlop switches between I_Bmax and I_Bmin at every falling edge of the clock.

Stepping Motor (cont.)
The full step, 2-phase excitation mode of the motor is not suited for applications that need a fine resolution of the rotor revolution. If we want to achieve a finer resolution, a more complicated logic for the (rising/falling) edge triggered Flip-Flop is needed. For further analysis of this, see Figure 5.

Figure 5: Simplified block-diagram to drive a stepping motor in micro-steps.

In this example, the CLK frequency of the circuit in Figure 5 is the same for all excitation modes. The control logic A and the control logic B in Figure 5 provide the right output to change the step widths for the stepping motor rotor. The control logic A and B change the input voltage to the H-Bridge A and the H-Bridge B at the rising edge of every new cycle. The output voltage from the control logic A and B to the H-Bridge A and B has certain values according to the chosen excitation mode. The voltages lead to specific values of the currents I_A and I_B, which lead to certain intermediate values of the rotor angle between two poles of same polarity.

Figure 6: 2-phase excitation mode with a standard CLK frequency.

The 2-phase excitation mode with the clock frequency CLK, as shown in Figure 6, requires four CLK cycles for one full revolution of the rotor. This can be seen also in Figure 3, where one revolution is subdivided in four steps.

The next step in the refinement of the rotor revolution is to split every step into two steps. This would lead to an intermediate step, where the rotor is aligned with one phase of the stator. The full revolution in this excitation mode is subdivided in eight steps as shown in Figure 7 (indicated by the "1" in the name "1-2-phase").

Figure 7: One full cycle of the 1-2 phase excitation mode of a stepping motor with a two-pole permanent magnet rotor. For every step, the yellow and lines are connected and the green lines are connected, respectively. The grey lines indicate that there is no current flowing. Red indicates the S-pole of the magnet, blue the N-pole and brown is non-polarized.

Stepping Motor (cont.)
Figure 8 shows the current steps, which are activated at the eight cycles of the standard CLK. Obviously, the speed for one full revolution is halved going from the 2-phase excitation mode to the 1-2-phase excitation mode.

Figure 8: 1-2-phase excitation mode with a standard CLK frequency.

Also noticeable in Figure 8 is that at the intermediate 2-phase-on step the currents I_A and I_B are higher, about 75%. By drawing more current, without overloading the power source, the force on the rotor can be increased. In the extreme case, I_A and I_B can be set to 100% in the 2-phase-on step to have a maximum force on the rotor in this excitation mode.

Moving forward in the process of subdividing every step to make the angular resolution of the rotor smaller, we introduce a further step between the 2-phase-on mode and the 1-phase-on mode, the W1-2-phase excitation mode. Figure 9 shows the current steps for the phase A and the phase B with the standard CLK. With every rising edge of CLK, a new current value for I_A and I_B is set. Going from the 1-2-phase excitation mode to the W1-2-phase excitation mode, the number of cycles of CLK for the rotor to make one full revolution is again doubled.

Figure 9: W1-2-phase excitation mode with a standard CLK frequency.

For the 1-2-phase it was eight cycles, for the W1-2-phase it is 16 cycles. While the rotor needs twice the time to make a full revolution, the movement of the rotor becomes increasingly smooth, less noisy, and the angular resolution improves.

The subdividing of the steps between two phases is called mini- or micro-stepping, which can be achieved by setting the currents I_A and I_B to specified intermediate values between 0% and 100%, so the rotor is aligned at a specific angle between the phases. This is very useful for applications that need a stepping motor with a fine angular resolution and with a low noise and smooth rotation. These include applications like printers, computer numeric control (CNC) machines, automobile dash-board instruments and many other applications that require high-precision movement.

The micro-stepping design techniques described in this article can be implemented with standard off-the-shelf components readily available from a number of industry sources. For a more integrated solution, Toshiba has several stepping motor driver ICs in its portfolio, which are capable of driving bipolar stepping motors in the micro-stepping mode with up to 1/16th steps.

The latest product in this line is the TB6560AHQ/AFG (TB6560AHQ,TB6560AFG datasheet (PDF)), which comes in two different current and package versions and has many more advantageous features, besides the micro-stepping. The 1/16th step excitation mode means that the full step between two phases of the stator is subdivided into 16 intermediate steps. So, as the motor in our example has 4 phases, the rotor needs for a full revolution 4 x 16 = 64 cycles of the standard clock. To increase the motor speed, the clock frequency needs only to be increased.

About the author:
Arasch Lagies is a senior application engineer with the System LSI Group at Toshiba America Electronic Components. He has a PhD in electrical engineering from the Militiary University of Munich and a master's degree in physics from the Technical University of Munich. He has published previously on the topic of device modeling and holds patents in circuit design.