AML D Series User manual

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D-Series UHV Stepper Motors
Ultra-High Vacuum Compatible Stepper Motors
USER MANUAL
ISSUE 20220314

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1. Summary and Warnings............................................................................................................................................3
2. Packing ........................................................................................................................................................................3
3. Installation..................................................................................................................................................................3
4. Load Connection ........................................................................................................................................................3
5. Vacuum Conditioning................................................................................................................................................4
6. Drive Requirements...................................................................................................................................................4
7. Over-Temperature Protection .................................................................................................................................4
8. Identifying and Connecting the Leads....................................................................................................................4
9. Feedthrough Requirements .....................................................................................................................................6
10. Connection to MLF18VCF or MLF18F .......................................................................................................................7
11. Preparation of Motor Leadout Wires for use with Other Feedthroughs...........................................................7
12. Reversal of Rotation ..................................................................................................................................................7
13. Troubleshooting.........................................................................................................................................................8
14. General Operation Considerations for UHV-Compatible Motors .......................................................................8
14.1. Temperature Rise and Run Times ........................................................................................................................................................... 8
14.2. Outgassing and Bakeout .......................................................................................................................................................................... 8
14.3. Rotating Mechanisms Holding Torque................................................................................................................................................. 9
14.4. Translation Mechanisms Shaft End-Float ............................................................................................................................................ 9
14.5. Resonances................................................................................................................................................................................................ 9
14.6. Load Inertia and Reduction Gearing ....................................................................................................................................................... 9
14.7. Magnetic Fields Near the Motor ............................................................................................................................................................ 10

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1. Summary and Warnings
AML stepper motors are specifically designed for use in UHV environments making them ideally suited for low-speed precision
in-vacuum manipulation. The model D motors are two phase hybrid stepper motors, available in a range of standard sizes
and torque ratings. Standard motors provide 200 full steps per revolution, are suitable for use below 1x10-10 mBar and
temperatures between -65 °C to +190 °C. Extended low temperature range (-196 °C) and radiation hard versions (1 x 106Gy)
are available options.
The mounting screws are fitted with metered torque. Do not disturb. Do not drop, demagnetise, disassemble, modify or
overheat the motor or allow particles to enter the bearings or pumping ports.
Do not touch the stepper motor with bare hands!
The published performance was obtained using an SMD210 drive operating with standard settings for step division. The
SMD210 is a bipolar, switch-mode, current-regulating drive, optimised for use with vacuum stepper motors. Different drives
will produce different speed/torque curves. Drives capable of producing a total phase current of more than 1A RSS (root sum
of squares) may damage the insulation, even if the current is claimed to be adjustable.
Design mechanisms with balanced rotating loads and/or friction to maintain position with reduced or zero phase current for
minimum outgassing. Use step division only to smooth transitions: increase resolution by reduction gearing.
Ensure ice cannot form in the motor if testing at low temperature in air. Avoid thermal shocks, for example, plunging in liquid
nitrogen.
2. Packing
AML motors are vacuum sealed packaged. Wear gloves to remove the motor from its vacuum packaging. Once removed it
must be stored in a cleanroom or clean, closed container.
3. Installation
Use the extended screws for mounting but do not adjust them. Take normal vacuum precautions,
avoid creating trapped volumes when mounting the motor and obstruct as few as possible of the
pumping holes in the end faces. The location spigot projecting from the face of the motor is
accurately concentric with the shaft and intended for precise location in a recess.
Warranty is void if screws are tampered with.
4. Load Connection
The preferred method of coupling a load to the shaft is by a setscrew or collet fixing. AML do not recommend users modify
shafts.

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5. Vacuum Conditioning
Motors are supplied pre-baked to 200 °C in 1 x 10-6 mbar pressure for 24 hours and vented with dry Nitrogen before vacuum
packaging. They may still adsorb water in storage and handling so a 24-hour bake by an SMD3 drive, with an adequate pump
is recommended to achieve UHV-compatibility.
6. Drive Requirements
These motors are specifically designed for use in conjunction with an AML SMD3 current-regulated switch-mode drive. If
another drive is used it must be a bipolar 2-phase drive and capable of providing a selection of well-regulated currents of less
than or equal to 1 Amp per phase. If the drive can provide simultaneous currents to both phases (for step division) then the
root of the sum of the squares of the phase currents must never exceed 1 Amp.
The source voltage of alternative drives should be greater than 45 V. Lower voltages will result in loss of torque at higher
stepping rates. Voltages over 100 V are not suitable for vacuum use. Switching frequencies should not exceed 22 kHz and
peak-to-peak switching ripple on the phase current should not exceed 15%.
The drive current should be reduced or removed when the motor is stopped. For this reason, attempts to improve angular
resolution by step division are not recommended.
Refer to the current motor datasheet for electrical, mechanical and thermal characteristics.
7. Over-Temperature Protection
All AML motors are provided with a K-type Thermocouple (NiCr-Ni) embedded between adjacent windings as standard, or
alternatively, the PT100 temperature sensor option. The drive current must be removed when the indicated temperature of
the windings reaches 190 °C. AML drives provide this function. Simple on/off temperature controllers may be used but ensure
that the adverse electrical noise environment within the motor underdrive does not affect the temperature measurement.
8. Identifying and Connecting the Leads
The lead-out wires are self-coloured polyimide film-wrapped, silver-plated OFHC copper and each is fitted with a 1.5 mm crimp
socket terminal. They are supplied fitted with UHV compatible coloured glass beads for identification. Lead-out wires are
supplied 1.5 metres long but can be shortened if required. For replacement crimp terminals use Souriau SC24ML1S31
(AML part no. MLF18SKT) or equivalent.
If the identification beads have been removed, then use
the following procedure. An ohmmeter with resolution
down to about 1 ohm is required to identify the two phase
windings: most inexpensive multimeters are suitable.
The phase lead-out wires are much thicker than the
temperature sensor wires. Radiation-hard motors may
have multi-strand leads. Identify the two motor phases by
their resistance, which will be in the range of 3 to 15 ohms,
depending on the motor type. There is no electrical
connection between the two phases or to the
temperature sensor or the case of the motor.
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
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
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Most of the resistance is in the windings of the motor and is virtually unaffected by shortening of the leads. Connect each
phase to the appropriate drive terminals. The resistance
of the wires from the feedthrough to the drive must be
less than a few ohms.
The thermocouple wires are much thinner than the phase
leads. The thermocouple is insulated from the rest of the
motor. The Alumel wire may be identified with a magnet
since it is weakly magnetic. At the controller the Alumel
lead should be connected to the terminal marked Alumel,
N, -, or coloured blue, and the Chromel lead should be
connected to the terminal marked Chromel, P, + or
coloured brown.
The temperature measurement is not required to be very
precise, so it is not necessary to use thermocouple-compatible feedthroughs or extension wires. If compatible materials are
used then they must be connected the correct way round.
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


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9. Feedthrough Requirements
Each motor requires six (seven for PT100) vacuum feedthrough pins. It is not necessary to use a thermocouple vacuum
feedthrough, as the error introduced by incompatible feedthrough material is usually less than 5 degrees.
AML feedthrough MLF18F is recommended, since it mates directly with the 1.5 mm crimp socket terminals fitted to motor
leads and simplifies installation.
The MLF18F has 18, 1.5 mm gold-plated feedthrough pins and is suitable for one to three motors. An internal bakeable
connector, MLF18VCF, is available into which the crimp terminals on the motor leads are inserted. This significantly reduces
the risk of short-circuits and makes the installation more convenient.
AML supply an 18-way non-bakeable, screened external lead, MLF18SMD3 for direct connection to an SMD3. The use of
this lead ensures compliance with the EU EMC Directive. Air-side connectors, MLF18AC are available for users wishing to
make their own cables.
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10. Connection to MLF18VCF or MLF18F
If you are connecting crimp terminals directly to the feedthrough (i.e., not using MLF18VCF) do not forget to thread the
wires though the copper gasket before connecting to the feedthrough. To avoid short-circuits insulate the crimp terminals
with PTFE sleeving.
If you are using MLF18VCF it will be useful to have a socket extractor tool. AML part no. XPX24 or RS Components stock
number 466-876 is suitable and inexpensive.
Make connections according to the following conventions. This will ensure compatibility with MLF18NBL leads. The diagram
shows the rear of the connector or the vacuum side of the feedthrough.
Connection
Colour
Motor 1
Motor 2
Lack of dot indicates non-mating side.
Insert terminals from this side.
Phase A1
Green
1
7
Phase A2
Grey
3
9
Phase B1
Black
2
8
Phase B2
White
4
10
Thermocouple +
Brown
5
11
Thermocouple -
Blue
6
12
RTD A
Blue
13
16
RTD B1
Brown
14
17
RTD B2
Brown
15
18
11. Preparation of Motor Leadout Wires for use with Other
Feedthroughs
It will be necessary to cut off the crimp terminals fitted to the leads and re-strip them. Standard motors are fitted with
Polyimide film-wrapped leads: radiation-hard motors are fitted with Polyimide lacquer-coated leads.
Polyimide is strong, flexible and abrasion-resistant and therefore difficult to strip. The simplest method of stripping
polyimide film is to cut a ring with a sharp knife and withdraw the cylinder of insulation over the end of the wire. Be careful
not to mark the conductor surface with the knife. Strip lacquer-coated radiation-hard leads by scraping with a sharp knife.
Either type of lead may be stripped with a suitable high-speed rotary stripper. Do not use a thermal stripper.
12. Reversal of Rotation
There is a 50% probability that the direction of rotation will be reversed from the desired or conventional sense. To reverse
the direction, exchange the connections to one of the phases.
Check that the drive, wiring and motor combination work properly before closing the vacuum system.

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13. Troubleshooting
If the motor rotates at the wrong speed, frequently changes direction, or has low torque this is probably due to one of the
phase connections being open circuit.
If the drive indicates that the motor temperature exceeds 190 °C when the motor is not hot, then a connection to the
thermocouple is open circuit.
14. General Operation Considerations for UHV-Compatible Motors
It is recommended that motors and mechanisms are operated in air during early commissioning. This has the advantage that
more cooling is available and that the operator can see and hear that the motor is stepping.
14.1. Temperature Rise and Run Times
The maximum recommended operating temperature of AML motors is 190 °C, as measured by the embedded type K
thermocouple.
The motor should be run at the minimum phase current consistent with the requirements of the load. This will reduce the
maximum temperature of the motor and outgassing from the motor. Resistive heating losses in the winding resistance, R, are
given by I2R. The winding resistance is approximately proportional to absolute temperature so even small reductions in phase
current, I, produce worthwhile reductions in temperature rise and outgassing. For phase currents down to about 50% of
maximum the output torque is reduced roughly in proportion to phase current.
The minimum practical phase current is determined by the load friction and inertia, and the required acceleration and
maximum speed. It is best found by experiment. A reasonable margin of safety should be allowed for any expected increase
in load friction, which might occur after bakeout.
Vacuum stepper motors achieve maximum efficiency at full-step rates between 500 Hz and 2 kHz.
The SMD3 allows the reduction of the phase current dynamically during each step at low step rates, with separate control of
initial and final currents and transition times. Use of this technique can dramatically reduce the power dissipated in some
applications. The provision of heatsinking will improve the performance.
Irreversible deterioration of the winding insulation will occur at 230 °C and the motor will subsequently produce large amounts
of gas, even at lower temperatures.
14.2. Outgassing and Bakeout
A newly installed motor will outgas in vacuum, mainly due to water-vapour retention in the polyimide. As this material is micro-
porous the water is released rapidly and the rate will subside after a few hours. The rate may be accelerated by running the
motor to self-heat it.
Baking at up to 200 °C is required for operation at UHV. Motors are typically operated at some distance from the chamber
walls where the bakeout temperature is most often controlled. The motors will not reach a high enough temperature in such
cases, and it may be increased by using the windings as heating elements. The SMD3 includes a bake program, which
automatically controls the motor temperature at 200 °C by applying phase current. Maintain the motor temperature above
that of the rest of the system during cooling, as this will prevent condensation on the motor. Where internal infra-red heaters
are used for bakeout it is advisable to shield the motor from direct radiation and to achieve controlled temperature during
bakeout solely by self-heating.

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The outgassing rate for well-baked motors installed on a typical mechanism and run below 120 °C winding temperature is in
the order of 10-8 millibar litres sec-1. This represents high duty-cycle operation at rated phase current for size 35, 42 & 57
motors. The gas species are H2(90%) and CO (10%) and originate mainly from the windings and laminations. As a rule of
thumb, an additional 100 litres per second of pumping capacity per motor will be required for UHV. This gas load is insignificant
at HV and higher pressures.
14.3. Rotating Mechanisms Holding Torque
Design rotation mechanisms with balanced loads to reduce or eliminate the necessity for holding torque. If the torque imposed
on the motor by any imbalance of the load is less than the detent torque, then the motor will hold position without power.
The gearing required to achieve the desired angular resolution or to match the load inertia will increase the effect of detent
torque and also add friction.
14.4. Translation Mechanisms Shaft End-Float
The motor shaft has a compression spring, which pushes the shaft toward the mounting-face of the motor. The amount of
end-float is 100 to 200 µm for D35.1 and 200 to 400 µm for D42 motors. The spring is fully exercised with an axial force of 3 kg
toward the rear of the motor. For linear mechanisms where the motor is directly coupled to a leadscrew use gravity and/or
apply an opposite axial pre-load to avoid adding end-float to backlash.
There may be a significant static friction component added to the compression spring force, which may give the impression
that the end-float is less or that the spring is stiffer than specified. This should not be relied on to reduce backlash, as
repeatedly exercising the end-float will reduce the static friction and may also produce particles.
14.5. Resonances
Stepper motors are classic second-order systems and have one or more natural resonant frequencies. Operation at step rates
around these frequencies will excite the resonances, resulting in very low output torques and erratic stepping. The resonant
frequency is modified by the friction and inertia of the load, the temperature of the motor and by the characteristics of the
drive and therefore cannot be stated with any precision. Fortunately, coupling a load normally reduces the resonant
frequencies, which for unloaded AML motors occur below 300 Hz. The drive circuits of the SMD3 are optimised to produce
heavy damping of mechanical oscillations in the motors.
The simplest method of controlling resonances is to avoid operation of the motor close to the resonant frequencies. It is almost
always possible to start a motor at rates in excess of 400 Hz if the load inertia is matched as described in the next section.
Resonances are not usually a problem when the motor speed is accelerating or retarding through the resonance frequency
region.
If it is necessary to operate at slower speeds than this, the step division feature (micro-stepping) helps by effectively increasing
the stepping rate by the step division factor and reducing the amplitude of the step transients which excite the resonances. In
particularly difficult cases modifying the step frequencies at which transitions to micro-stepping occurs can be useful.
14.6. Load Inertia and Reduction Gearing
The load inertia coupled to the motor shaft should ideally be comparable to the rotor inertia of the motor where accurate
position control is required. The load inertia can be very much larger for speed control applications where some slip of
absolute position is unimportant. Where reduction gearing is used for load-matching the spur gear meshing with the motor
pinion will normally dominate the load inertia and it is important to keep its diameter small. Loosely coupled loads may give
rise to additional resonances at higher frequencies: these can usually be damped by substituting either anti-backlash or helical
gears in the gear train or arranging additional friction in the train.

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14.7. Magnetic Fields Near the Motor
It is recommended that motors are not operated in fields of greater than 50 millitesla (500 gauss), as this will affect the
performance while the field is present. Fields significantly greater than this may cause partial demagnetisation of the rotor,
permanently affecting the performance.
The leakage field of a motor is less than 100 microtesla (1 gauss) at 1 cm from the surface of the motor and in an axial direction.
It is due to the permanent magnet in the rotor and is present when the motor is stationary and unpowered. Under drive an
alternating component is added at the fundamental and harmonics of step frequency, up to a few kHz. This field is easy to
screen at the sides and non-shaft end of the motor, but more difficult at the shaft end because of the projection of the shaft.
Early consideration of the interaction of stray fields on nearby equipment is recommended.