AMIS-3062x and NCV70627 Micro-Stepping Motor Driver Family, Robust Motion Control Using the AMIS-3062x

AND8404
AMIS-3062x and NCV70627
Micro-Stepping Motor
Driver Family, Robust
Motion Control Using the
AMIS-3062x and NCV70627
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APPLICATION NOTE
Introduction
The AMIS−3062x and NCV70627 family is fully
compatible with the operating voltages in automotive and
industrial systems.
This article provides guidelines for arriving at a set of
robust motion parameters for the AMIS−3062x and
NCV70627 stepper motor drivers. In this context, robust
motion control implies reducing the occurrences of
step−loss as well as false step−loss flags.
The AMIS−3062x and NCV70627 is a family of
integrated micro−stepping stepper motor driver solutions
which are designed with a number of innovative features,
including a position controller as well as the control/
diagnostics interface. A number of devices in this product
family are targeted at dedicated mechatronics applications,
whereby they are coupled to a remote LIN master.
Alternatively, other versions of these devices can be
interfaced to an external microcontroller via an I2C bus.
In these applications, the motor driver IC receives
positioning instructions via the bus (LIN or I2C) and
subsequently drives the motor coils to the desired position.
The on−chip position controller is configurable (OTP) for
different motor types and positioning ranges, as well as speed,
acceleration and deceleration parameters, respectively. In
addition, the “micro−stepping” feature allows for silent
motor operation and increased positioning resolution.
C7
100 nF
C5
CPN
VDD
C9
HW0
9
1 kW
C1
HW2
CPP
10
C3
VCP
11
VBB
12
C4
VBB
20
1
AMIS−30621
8
LIN
VDR 27V
5
4
7
14
SWI
C2
MOTXP
16
MOTXN
13
6
1 kW
18
15
2,7 nF
LIN bus
100 nF
19
3
HW1 2
Connect
to VBAT
or GND
C6
220 nF
100 mF
100 nF
C8
The typical application diagram for AMIS−30621 is
shown in Figure 1. Here, SWI is the position switch input.
If not used, Pin 20 should be left open. HW2 is an external
address pin capable of withstanding high voltage transients.
If not used, this pin should be connected to ground via a 1 kW
resistor.
220 nF
VBAT
Typical Application
2,7 nF
MOTYP
Connect
to VBAT
or GND
M
MOTYN
17
TST
GND
Figure 1. Typical Application Diagram
1.
2.
3.
4.
5.
6.
All resistors are 1/4 W, ± 5%.
C1, C2 : Minimum value is 2.7 nF, maximum value is 10 nF.
Depending on the application, the ESR value and operating voltage of C7 must be carefully chosen.
C3 and C4 must be close to Pins VBB and GND.
C5 and C6 must be as close as possible to Pins CPN, CPP, VCP, and VBB to reduce EMC radiation.
HW0 and HW1 are to be connected to VDD or GND, respectively.
© Semiconductor Components Industries, LLC, 2013
May, 2013 − Rev. 3
1
Publication Order Number:
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AND8404
Resonance
the resonant frequencies rather than calculating them is the
preferred technique. These frequencies can in turn be
measured by means of the step−response of a loaded motor
or a velocity sweep.
Important: The resonant frequencies of an unloaded
motor are typically different from those of a loaded motor
(due to the differences in m and couplings). This affects the
selection of robust motion control parameters in the
AMIS−3062x and NCV70627 devices. For this reason, it is
essential to determine these parameters for a specific motor
and load configuration. In all cases, first ensure that the
selected motor matches the load requirements, and then
determine the proper value of current to generate the desired
torque during all conditions (“Irun[3:0]” & “Ihold[3:0]”
parameters). This is key to achieving the correct set of
resonant frequencies.
In conjunction with resonance effects, some degree of
velocity ringing can occur following abrupt speed changes.
The positioning controller of AMIS−3062x and NCV70627
introduces four corner points (Figure 2), which will be
discussed later in this application note.
Resonance phenomena in stepper motors can cause
step−loss, leading to random and uncontrollable motion.
The resonant frequency in stepper motor systems (with
rigidly mounted ideal motor) is calculated in Ref.[1] as
follows:
f res +
Ǹ8 @ p @h m @ S
(eq. 1)
where:
fres = resonant frequency of the motor with rigidly coupled
load
h = holding torque
m = moment of inertia of the rotor and any coupled load
S = the step angle in radians
This equation indicates that for a given motor and load
condition, one can affect the resonant frequency by
changing the current/torque. For example, if the current in
the motor coils increases, the resonant frequency will also
increase. In real world systems, several resonant frequencies
are generated, which are mainly due to the elastic couplings
and motor non−idealities. In many of these cases, measuring
Velocity
Acceleration
Vmax
Constant speed at Vmax
Deceleration
3
2
Direction of movement
1
Vmin
Jump to Vmin
4
Pstart
Jump to stop
Pstop
Position
Figure 2. Speed as a Function of the Position as Implemented in the AMIS−3062x and NCV70627 Position Controller
Hint: Step−loss due to resonance or velocity ringing is
minimized by selecting “Vmin[3:0]”, such that the stepping
frequency is at all times above fres (Corners 1 and 4 in
Figure 2).
This equation can be use to arrive at a good approximate
value for the maximum deceleration as well.
Hint: Measure the resonant frequencies in your application
and select “Acc[3:0]” such that the resulting acceleration
and deceleration are at all times below Amax.
Example: If fres = 30 Hz, then Amax = 16000 FS/s2
Velocity ringing due to maximum acceleration and
“Corner 2” (See Figure 2) can lead to false activation of the
stall detection function (Ref. [3]). In order to prevent this
occurrence, a blanking period is to be introduced via the
parameter “FS2StallEn[2:0]”. This parameter represents (in
binary format) the count of the number of full steps to be
skipped, from the onset of “Corner 2” to the instance when
the stall detection function is enabled, respectively. In this
case, it is necessary to first characterize the ringing in the
velocity waveform beyond “Corner 2”, in order to arrive at
the correct “FS2StallEn[2:0]” parameter setting. The length
of the blanking period can be set according to Table 1.
Acceleration and Deceleration
If changes in speed (accelerations and decelerations) are
not in line with certain physical constraints, an increased
chance of step−loss exists. In Ref.[1], the maximum
acceleration is expressed as a function of fres ,as follows:
A max +
8 @ p @ f res
Ǹ2
2
(eq. 2)
where:
Amax= maximum acceleration (in FS/s2 )
fres = resonant frequency of the motor with rigidly coupled
load
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AND8404
Table 1. BLANKING TIME SETTINGS TO PREVENT THE ACTIVATION OF THE STALL DETECTION CIRCUIT
DURING “VELOCITY RINGING”
Index
FS2StallEn[2:0]
Blanking Time (Full Steps)
0
000
0
1
001
1
2
010
2
3
011
3
4
100
4
5
101
5
6
110
6
7
111
7
Hint: The parameter “FS2StallEn[2:0]” allows for reducing
the risk of activation of the stall detection function due to
velocity ringing beyond “Corner 2”. This parameter will
blank the stall detection function during a number of full
steps, at the expense of potentially losing a number of full
steps in the positioner in case the motor encounters a true
stall condition during this blanking time.
the “AbsThr[3:0]” parameter setting is “AbsThr[3:0]nom”
minus 0.5 V.
Hitting a Mechanical Obstruction at Vmax
In this case, the maximum number of lost full steps is only
related to FS2StallEn[2:0]. Please refer to the description of
“DelThr[3:0]” and “DelStallLo” and “DelStallHi” in
Ref. [3]. DelThr, DelStallLo and DelStallHi are not
applicable for the NCV70627.
Hint: To verify “DelThr[3:0]” operation, it is important for
velocity−ringing (parameter “FS2StallEn[2:0]”) to be under
control. A robust rule of thumb estimate for the
“DelThr[3:0]” parameter setting is “AbsThr[3:0]”/2 (in
volts).
Stalled Motor
If a motor is mechanically blocked or is accelerated into
a physical end−stop, then step−loss will occur (see
description of “AbsThr[3:0]” and flag <AbsStall> in
Ref [3]). The maximum number of lost full steps (#LFSmax)
is the sum of following:
{number of full steps in FS2StallEn[2:0]} + {number of full
steps in acceleration ramp}
Or:
#LFS max + FS2StallEn[2:0] )
Torque Reduction at Elevated Speeds
The inductance in the motor windings limits the rate of
rise in motor current. At slow speeds this is hardly
noticeable. However, at elevated speeds, with the rise and
fall times of the current being of the same order of magnitude
as the step time, this results in a reduction in the effective
current in the motor phases, with a consequent reduction of
the torque. Another contributor to the decline of torque is the
“back−emf” or “counter EMF” Ref.[1]. The AMIS−3062x
and NCV70627 stepper drivers have a PWM current control
system. When a motor with high back−emf is operated at
high speeds and low supply voltages, the PWM duty cycle
can be as high as 100%. This indicates that the supply
voltage is too low to generate the required torque. This
situation may also result in erroneous triggering of the stall
detection function (Table 2). As a precaution, the stall
detection function is automatically disabled when the PWM
duty cycle approaches ~100%, while some degree of control
is possible via the parameters “DC100StEn” and
“MinSamples[2:0]”:
(V max 2 * V min 2)
2 @ Acc
(eq. 3)
where:
#LFSmax = maximum number of lost full steps (FS)
FS2StallEn[2:0] = 0 to 7 FS
Vmax = selected maximum speed (FS/s)
Vmin = selected minimum speed (FS/s)
Acc = selected acceleration (FS/s2)
In this case, the actual position in the device’s position
register can be updated (through the master command) to
correct for the lost steps.
Hint: The parameter “AbsThr[3:0]” helps to detect an
indefinitely blocked motor.
Hint: “AbsThr[3:0]nom” is obtained upon conducting
certain motion tests on blocked motors and observing the
flag <AbsStall> Ref. [3]. A robust rule of thumb estimate for
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Table 2. AVOIDING FALSE STALL DETECTION IN CASE OF 100% DUTY CYCLE
Case
DC100StEn
PWM Duty Cycle
Stall Detection Enabled?
Risk for False Stall Detection?
1
0
< 100%
Yes
No
2
0
100%
No
No
3
1
< 100%
Yes
No
4
1
100%
Yes
High
Hint: Set “DC100StEn” = 0 for normal use.
Hint: It is possible (if “DC100StEn” = 1) to verify the safe
operating range (max. speed, min. supply voltage). The
“false stall−detection (case4)” will indicate 100% PWM
duty cycle and torque reduction.
Finally, the parameter “MinSamples[2:0]” provides for a
timing function for sampling of the back−emf voltage,
according to Table 3.
Table 3. RELATION BETWEEN MINSAMPLES[2:0] VALUE AND THE SAMPLING TIME OF THE BACK−EMF
MinSamples[2:0]
Timing (ms)
000
87.7
001
131.6
010
175.4
011
219.3
100
263.2
101
307.0
110
350.9
111
394.7
Hint: Set “MinSamples[2:0]” to the next smaller value that
best corresponds to the duration of one micro−step when
running at Vmax.
Example: When running at 5000 micro−steps/s, the duration
per micro−step is 200 ms and MinSamples[2:0] should be set
to “010”.
References
1. “Stepping Motor Physics”: Part 2 of “Stepping
Motors” by Douglas W. Jones,
http://www.cs.uiowa.edu/~jones/step/
2. Datasheet AMIS−30621 and AMIS−30622
Products, www.onsemi.com
3. Datasheet AMIS−30623 and AMIS−30624
Products, www.onsemi.com
4. Datasheet NCV70627 Products, www.onsemi.com
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