User Manual

ManualsBrandsPololu ManualsMotor ControllersAMIS-30543 Stepper Motor Driver Carrier

August, 2010 − Rev. 2

1 Publication Order Number:

AND8399/D

How to Measure Bemf on

the SLA-pin

Prepared by: Tom De Ryck

ON Semiconductor

Abstract

To enable the possibility to build very accurate stall− and

steploss algorithms as also torque adaptive applications,

AMIS−305xx has a Speed and Load Angle (SLA) pin which

outputs a voltage that reflects the Bemf (Back Electro

Magnetic Force) voltage of the motor.

To prevent incorrect use of the SLA−pin, this application

note describes how the motor driver should be operated to

measure the correct Bemf voltage on the SLA−pin.

Introduction

The Bemf is sampled every so called “coil current zero

crossing”. Per coil 2 zero−crossing positions exist per

electrical period, resulting in a total of 4 zero crossings per

electrical period. Or in short, the Bemf voltage can be

measured 4 times per electrical period. Although the Bemf

voltage can

be measured 4 times per electrical period, it does

not mean that it will

be measured 4 times. The Bemf voltage

will only be sampled by the motor driver if a microstep

position is located on the “coil current zero crossing”. Only

then a correct representation (Note 1) of the Bemf voltage

can be measured on the SLA−pin. If no microstep position

is located on the “coil current zero crossing”, an incorrect

value will be measured on the SLA−pin.

Coil Current Zero Crossings

Next figures display 4 of the in total 7 stepping modes

possible with AMIS−305xx.

Start

Figure 1. Full Step Mode

Start

coil

1. The voltage measured on the SLA−pin only represents the Bemf voltage. Depending on the SLA Gain setting (see Ref 1) the SLA voltage

will be equal to 1/2 or 1/4 of the real Bemf voltage.

http://onsemi.com

APPLICATION NOTE

Summary of content (26 pages)

PAGE 1
AND8399/D How to Measure Bemf on the SLA-pin Prepared by: Tom De Ryck ON Semiconductor http://onsemi.com APPLICATION NOTE Abstract measured 4 times per electrical period. Although the Bemf voltage can be measured 4 times per electrical period, it does not mean that it will be measured 4 times. The Bemf voltage will only be sampled by the motor driver if a microstep position is located on the “coil current zero crossing”.
PAGE 2
AND8399/D Icoil IY “coil current zero crossing” IY Start IX IX Start t “coil current zero crossing” Figure 2. Half Step Compensated Mode IY Icoil IY Start “coil current zero crossing” IX IX Start t “coil current zero crossing” Figure 3. 1/4th Stepping Mode IY “coil current zero crossing” Icoil Start IY IX IX Start t “coil current zero crossing” Figure 4.
PAGE 3
AND8399/D The several stepping modes of AMIS−305xx offers a lot of flexibility but at the same time could also cause problems when not used in a correct way. Figure 5 displays what could go wrong when switching between stepping modes is done in an incorrect way. IY IY IY IX IX IX Motor is in full step position, stepping mode is 1/8 stepping NXT−pulse is applied. 1/8 of a full step is set. Stepping mode is changed to half stepping and several NXT− pulses are applied Figure 5.
PAGE 4
AND8399/D In the second plot (Figure 8) an offset is created. No microstep position is located on the “coil current zero crossing”. The voltage measured on the SLA−pin will not represent the real Bemf voltage of the stepper motor. Below oscilloscope plots display the coil currents and the SLA voltage measured at 1/8 microstepping. In the first plot (Figure 7) no offset is created resulting in a microstep position on the “coil current zero crossing”.
PAGE 5
AND8399/D X−coil VSLA X−coil Y−coil “Coil current zero crossing” Because there is an offset created, no microstep position is located at the zero current crossing (= full step position). The measured SLA voltage (VSLA) will not represent the real Bemf voltage. VSLA Figure 8. Oscilloscope Plot of 1/8 Microstepping with No Microstep Position Located at the Zero Current Crossing http://onsemi.
PAGE 6
AND8399/D SLA Check IY Although the microcontroller controls the stepper motor driver, it’s always possible that an offset is created unwontedly. By reading out the Microstep Position (Status Register 3) the microcontroller can check if an offset is created or not. Table 1 displays all possible Microstep Positions for all stepping modes except full step.
PAGE 7
AND8399/D Table 1.
PAGE 8
AND8399/D Transparency Mode get all the current out of the coil), the coil voltage will clamp to the power supply voltage (VBB) + 0.6 V. Once the coil current is zero, the coil voltage will decay (transient behavior). The coil voltage will decay to the Bemf voltage of the stepper motor.
PAGE 9
AND8399/D In this mode the coil voltage is also sampled at the PWM rate when the coil current is zero but the voltage on the SLA−pin is only updated from the moment the “coil current zero crossing” state is left (see also Figure 12). The transient behavior will not be seen by the microcontroller (which is also not needed for most applications). Keep in mind that the SLA voltage is only updated when leaving the current less state.
PAGE 10
AND8399/D Icoil t Icoil Current Decay Next Microstep Next step Coil Current Zero Crossing Next step Previous Microstep PWM Rate t Vcoil VBB + 0.6V VBEMF Transient behavior VSLA t Last sample before leaving zero crossing is retained. Transparent Mode t VSLA Bemf of previous zero crossing. SLA voltage is only updated after current−less state is left+ a certain delay (50us at room temp). Not Transparent Mode t Figure 12.
PAGE 11
AND8399/D Maximum Operating Speed Step 5: As explained in Transparency Mode, one needs to be careful when to measure the BEMF. After the start of the coil current zero crossing it takes some time before the real BEMF can be measured (see Figure 11). Leaving the coil current zero crossing stage too early will result in an incorrect BEMF sampling (= the measured voltage on the SLA−pin will not represent the real BEMF). This will limit the maximum operating speed.
PAGE 12
AND8399/D 155us Latest noticeable step in SLA voltage (SLA voltage approx. 0V). Figure 14. Define Maximum Operating Speed The time measured between the rising edge of the NXT signal (purple curve) and the latest noticeable step in the SLA voltage (yellow curve) will define the maximum operating speed. As can be seen in Figure 14 this time is about 155 ms.
PAGE 13
AND8399/D Figure 15. SLA Voltage at 800 Full Steps / sec Notice the stable SLA−voltage in Figure 16 when the SLA Transparency is set to Not Transparent (Control Register 2). In Not Transparent the clamping and transient behavior is removed from the SLA signal giving a better view on the BEMF (see also Figure 12). http://onsemi.
PAGE 14
AND8399/D Figure 16. SLA Voltage at Maximum Speed in Not Transparent Mode too fast resulting in an incorrect SLA voltage (see Figure 17). The jumps in the SLA voltage is because the coil current zero crossing is left too early and the transient part is sampled (see also Figure 11). Notice in Figure 16 that the measured SLA voltage is about 2.5 V. If the measured voltage would be clamped to 5 V it’s possible that the generated BEMF is too high. Set the SLA Gain to 1/4 (Control Register 2).
PAGE 15
AND8399/D Figure 17. Too High Speed Results in Incorrect BEMF Sampling − Choose a lower stepping mode (1/4 instead of 1/8 for instance). This will result in longer coil current zero crossings. A trick to increase the maximum operating speed without the need of changing any of the above parameters is by stretching the coil current zero crossing. This principle is also used in AMIS−3062x (see www.onsemi.com) and is called MinSamples. See Application Note AND8371/D for more info on this.
PAGE 16
AND8399/D Full Step Stepping Mode 2. Set coil current. Set the stepping mode to compensated half step. 3. Enable the motor driver (stepper motor should be located in a full step position) 4. Every time a full step has to be set, apply two NXT−pulses fast after each other (keeping NXT−pin limitations in mind). In the next figure one can see that the stepper motor is running in full step mode although the stepping mode of AMIS−305xx is set to compensated half step.
PAGE 17
AND8399/D Y−coil X−coil VSLA VNXT The current rise and fall time are too high to reach the half step position. The stepper motor will run in full step. Y−coil X−coil VSLA VNXT Apply two NXT−pulses fast after each other. Figure 18. Operating Stepper Motor in Full Step by Using Half Step Stepping Mode http://onsemi.
PAGE 18
AND8399/D High Temperature When operating the stepper motor driver at high temperature it is possible that the SLA voltage drops between two samples taken. This could create unwanted results. For this reason it’s important to measure the SLA voltage as fast as possible after it has been updated. Next oscilloscope plot displays the problem. “Coil current zero crossing” Between two SLA measurements (= full step positions), the SLA voltage drops because of the high IC temperature.
PAGE 19
AND8399/D Sample The SLA Voltage Figure 20 gives a typical application schematic. 100 nF VDD D1 100 nF 100 nF VBAT + 100 nF C5 R2 C4 R3 DIR NXT DO DI CLK mC CS CLR ERR R1 SLA C8 C2 C3 VDD 32 VBB 14 VBB 27 VCP 11 9 CPN 5 10 4 31 2 AMIS−30521/ NCV70521 3 C1 100 mF C6 25,26 21,22 13 12 15,16 6 19,20 220 nF C7 220 nF CPP MOTXP MOTXN MOTYP M MOTYN 7 HV20081114.1 1, 17, 23, 8 18 24 30 28 29 TSTO GND Figure 20.
PAGE 20
AND8399/D Icoil t Icoil Next Microstep Next step Coil Current Zero Crossing Next step Previous Microstep t Vcoil VBB + 0.6V VBEMF t VSLA Earliest moment to sample Easiest moment to sample is at next step . Transparent Mode t VSLA Earliest moment to sample in Not Transparent Not Transparent Mode t Figure 21. Difference Between Transparent and Not Transparent Mode http://onsemi.
PAGE 21
AND8399/D Examples Below some examples are given on how to use the SLA−pin. The flowchart given in these examples represent the microcontroller firmware (see Figure 20). All examples and flowcharts given in this document are solely intended to simplify the explanation and to help you better understand how to use the SLA−pin. be initialized. When this is done, the coil current and stepping mode will be set. The motor driver will be enabled and NXT pulses will be send to rotate the motor.
PAGE 22
AND8399/D Power Up Initialize microcontroller Set Coil Current Set Stepping Mode Enable Motor Send NXT Pulse Start Timer Read Microstep Position Status Register 3 Values only valid for 1/8 micro−stepping See Table 1 for other stepping modes If MSP = (4 OR 36 OR 68 or 100) Y Delay N Sample SLA−pin Timer Overflow ? N Y Figure 23. Sample SLA voltage (1/8 microstepping) If Transparent mode is chosen, the SLA−pin could already be sampled during the coil current zero crossing.
PAGE 23
AND8399/D Power Up Initialize microcontroller Set Coil Current Set Stepping Mode Enable Motor Driver Transparent Mode Send NXT Pulse Start Timer Read Microstep Position If MSP = (0 OR 32 OR 64 OR 96) Y Delay N Sample SLA−pin N Timer Overflow ? Y Figure 24. Sample SLA voltage in Transparent Mode Stall Detection Implementing the flowchart as given in Figure 25 will most probably fail. This because the SLA voltage is already sampled at the first coil current zero crossing.
PAGE 24
AND8399/D Power Up Initialize microcontroller Set Coil Current Set Stepping Mode Enable Motor Transparent Mode Send NXT Pulse Start Timer Read Microstep Position If MSP = (0 OR 32 OR 64 OR 96) Y Delay N Sample SLA−pin SLA below Threshold ? Y N Stop Timer Overflow ? N Y Figure 25. Simple Stall Detection SLA Check The SLA voltage can not be sampled nor can stall detection be done. Figure 26 gives a simple solution to this problem. If an offset is detected, motion is stopped.
PAGE 25
AND8399/D Power Up Initialize microcontroller Set Coil Current Set Stepping Mode Enable Motor Transparent Mode Send NXT Pulse Start Timer Read Microstep Position If MSP = (0 OR 32 OR 64 OR 96) MSP = Microstep Position MOD = Remainder of a division Y = 2 for 1/16 microstepping, 4 for 1/8 microstepping 8 for 1/4 microstepping and 16 for half stepping Y Delay N Sample SLA−pin If MSP MOD Y = 0 SLA below Threshold ? N Timer Overflow ? N Y Stop Figure 26. SLA Check http://onsemi.
PAGE 26
AND8399/D Microcontroller without ADC By sampling the SLA−pin as given in Figure 26, one samples the SLA−pin at the correct moment to make sure that the real BEMF is measured. An offset will be detected as well as a stall condition. By sampling the SLA−pin at the right moment, temperature effect will be eliminated (see High Temperature). To implement above flowcharts one needs to use a microcontroller with ADC. In some cases this ADC is not available.