Technical Specifications

MECHANICAL OVERVIEW
To produce the linear movement, a ~.2" OD Thruster protrudes on-axis from a nominal 1" OD housing in standard actuators. Depending on the model, the Thruster may be rotating or non-rotating and push up to 300 lbs. Ironless-core brushed DC, or stepping motors power the actuators.

Brushed DC motors feature --

Precious metal brushes,

3-28 vdc ironless-core armature.

Stepping motors feature --

2-phases,

24-steps/rev,

Maximum 0.25A phase current.

These high-speed motors rotate a drivescrew through a spurgear or planetary-gear reducer. In the 1100, 1200, 2100, and 2200-series, the drivescrew itself if the thruster and protrudes through a threaded Nose Mounting. They can provide pull with the Push/Pull Adaptor [PPA] option. In contrast, the 1000 and 2000-series have push-only plungers independent from the rotating drivescrew. Hardened Push Pads [PP#] and a protective Boot [B] for the drivescrew are available as options. In all versions, we lap the drivescrew and its mating nut for a long life of smooth, repeatable motions.

To mount, simply drill a 3/8" hole in a plate, insert the actuator, and secure with the Nose Nut. The shorter travel devices are specifically-designed to replaced typical micrometer spindles traditionally used in precise alignment of optics and translation slides.
The actuator's compact package permits a user to nestle it into tight places without having to design a complete precision motion. For many practical applications, they are buried deep inside equipment to provide remote final alignment in difficult-to-reach locations.

MOTOR ALTERNATIVES
You can choose between the standard stepping, and brushed ironless core DC motors to propel the Linear Actuators. Each motor type has its strengths and weaknesses. A Selection Guide included in the catalog will help you make a choice.

ANALOG VELOCITY
Although you can drive brushed DC-motor actuators from your own electronics, we supply the SER3000 Analog Velocity Servo as well as manual and µ computer-based controllers. The servo expects a low-current +- 10 vdc velocity steering voltage from external sources such as our 1000INT, 1000MOD, or CONTROL3 controllers In spite of load variations, the servo drives the motor at a programmable constant velocity by sensing the motor's BEMF. Additional circuitry senses unusual motor conditions -- moving too slow and pushing too hard -- to indicate a limit of travel or stall conditions.

A-QUAD-B ENCODER
(Closed-loop)
Both 1000/1100 and 2000/2100 series have a built-in rotary encoder directly coupled to the motor shaft. With 15PPR (Pulses per Revolution), it provides 60 counts in 4X decode. Because the motor drives the drivescrew through a gearhead, the ratio directly multiplies the 60 counts. For example a 262:1 ratio will produce 15,720 encoder counts per revolution of the drivescrew. With a 32 TPI drivescrew, these counts produce an incremental resolution of 2 uin. The 2-phase outputs from an A-QUAD-B encoder are for direction and position information. Its magnetic construction assures high immunity to induced EMI current at either TTL or CMOS levels.

LEADSCREW PITCH
Most actuators are available with either 32.3885 or 40 TPI drivescrews. The first pitch produces an exact resolution of 0.1 um (a common optical research standard) through the 262:1 gearhead if using 2X encoder decode. Only 40 TPI drivescrews will be available when the 32.3885 TPI drivescrewshave been depleted.

GEARHEAD RATIO
Since the comparatively small motors have extremely high rotary velocity but low torque, we gear them down to perform useful work. the 15/5 and 16/5 gearheads are available in 18 different ratios producing an incremental resolution from 50pm to 3um at 4X encoder decode. The 17/5 gearhead series has 14 different ratios to produce incremental resolutions from 2nm to 4um. Some gearhead are available in a zero-backlash configuration that removes the 4o clearance between drive gear teeth by pre-loading.

How to choose a gearhead ratio.

Closed Loop
While examining the Motor/Gearhead Selection Tables from the catalog, pick a ratio producing less than your desired resolution. Check the maximum velocity for that gearhead. You may have to compromise your initial specifications to select the best actuator for your application. Because the motor has a fixed maximum power and encoder frequency, you must juggle Velocity against Load Capacity against Resolution.

Open-Loop Pick a ratio producing more than your desired thrust. check the maximum velocity. You may have to compromise your original specifications to select the best fit for Velocity and Load Capacity.

How to choose a motor size.
Brushed DC-Motor
If Load is not a significant problem, there is only one reason for seleting the 1616 motor -- increased Velocity. Because the 1624 motor produces 4X torque compared to the 1616, it has 30% lower maximum velocity. When Load Capacity is important, pick the 1624 motor but expect it to move more slowly. For even higher loads, consider the 1717 and 1724 motors -- the 16/7 gearhead can push up to 300 lbs.

Stepping Motor -- Only one model available.

Load-carrying capacity.
The specified Load Limit is to protect the individual teeth on the spur gears. Because the drivescrew pitch is fixed (32.3885 or 40 TPI), the output drive gears see the load independent of ratio. Therefore, we limit the motor current in the Servo and thus the actuator's ability to push more than 100 or 300 lbs. Without the Servo, you may elect to push higher loads but you will sacrifice some of our mechanical We specify two load capacity numbers -- 1. @ maximum servo and 2. @ stall (limit-of-travel). The maximum servo value represents the maximum load to remain inside a constant velocity servo window such as with our SER3000 Analog Velocity Servo. Outside this window, the motor's velocity is inversely dependent of the load.

Stepping motor or DC motor?
The following table will help you choose between brushed, and a stepping motor. Although many facts are presented, often people choose one motor style over another because of personal experience or familiarity.

MOTOR PERFORMANCE COMPARISON
Category	Stepping (Bipolar) 2000-family		DC (Ironless-core)
Category	Stepping (Bipolar) 2000-family		Brushed 1000-family
ELECTRONIC	Open-Loop¹	Closed-Loop²	Open-Loop	Closed-Loop
Digital Coordinate Control	Yes	Yes	No	Yes
Run-away possible?³	No	No	Yes	Yes
Attitude of being In Control?⁴	Yes	Yes	No	No
Resume Running after Stall?⁵	No	No	Yes	Yes
Sensitive to acceleration?⁶	Yes	Yes	No	No
Single-Polarity Power Supply?⁷	Yes	Yes	No	No
Complex-Drive Electronics?⁸	Yes	Yes	No	No
Category	Stepping (Bipolar) 2000-family		DC (Ironless-core)
Category	Stepping (Bipolar) 2000-family		Brushed 1000-family
MECHANICAL
Acoustic Noise?⁹	Yes	Yes	No	No
Excessive Vibration?¹⁰	Yes	Yes	No	No
Motor runs hot?¹¹	Yes	Yes	No	No
Brushes to wear-out?¹²	No	No	Yes	Yes
High inertia?¹³	Yes	Yes	No	No
Inaccuracy under load?¹⁴	Yes	Yes	No	No

TABLE FOOTNOTES
        1. Open-Loop?
        2. Closed-Loop?
        3. Run-away?
        4. Attitude of Being IN CONTROL?
        5. Resume Running after Stall?
        6. Sensitive to Acceleration?
        7. Single-Polarity Power Supply?
        8. Complex Drive Electronics?
        9. Acoustic Noise?
        10. Excessive Vibration?
        11. Motor runs hot?
        12. Brushes to wear-out?
        13. High inertia?
        14. Inaccuracy under load?

BACK TO TABLE

1. Open-Loop?
Open-Loop control of a stepping motor requires counting the step commands to control the position. If you loose step-synchoronization, the controller will be unaware of this condition unless some expected external event is also missing.

BACK TO TABLE

2. Closed-Loop?
Closed-loop control of a stepping motor means that an A-QUAD-B position encoding device is somewhere in the motion subsystem. Although you issue individual step commands, you determine the final position by the encoder, independent of the number of steps. Even with loss of step synchronization, the encoder information is accurate.

BACK TO TABLE

3. Run-away?
In contrast to easily-controlled brushed DC motors, and stepping motors require complex switching of current flow through the motor windings. Therefore, failure in a servo, amplifier, or power-supply can falsely move the brushed DC motor -- causing the motor to run-away! While the DC motor's tendency is to move, the stepping motor's tendency is to not move. Although a simple battery will move a brushed DC motor, only complex electronics will move a stepping motor.

BACK TO TABLE

4. Attitude of Being IN CONTROL?
Each step is the result of a deliberate electronic event. In contrast, the brushed DC motor only needs a voltage source to run. In a closed-loop environment, instead of thinking GO TO HERE, you ask a DC motor, WHERE ARE YOU? In DC controllers, the encoder counts maintain either a hardware or software position counter. Stepping controllers issue timed pulses to a driver that produces the complex switching to move the motor.

BACK TO TABLE

5. Resume Running after Stall?
A stepping motor can lose step-synchronism easily if the load increases beyond the operating window for the current velocity or acceleration. If the time is long enough, the motor will stall until the motion sequence starts again. Even if it is operating with an encoder in closed-loop fashion, the motor will stall. The controller must handle this error condition and restart the motion toward the final coordinate.

In comparison, the DC motor will continue to try to move in the face of adversity. Although it may slow down, it will resume running normally after removing the offending load. However, in a closed-loop environment with digital velocity servoing, the controller may sense a following error and terminate the motion.

BACK TO TABLE

6. Sensitive to Acceleration?
Stepping motors will lose step-synchronization if accelerated or decelerated too fast. Taking the next step too early or too late causes this error and results in loss of step synchronism -- it may even run backwards! If the load varies, it is difficult to exactly find the correct acceleration to guaranty faultless stepping. One common aapproach is to accelerate much more slowly to assure not losing synchronism. Although you don't lose position when the motor stalls in closed-loop environment, it is inconvenient to restart the motion.

BACK TO TABLE

7. Single-Polarity Power Supply?
We use balanced-bridge analog velocity servos with our DC motors to achieve a smooth, quick-response movement. In contrast, some pulse-width modulated (PWM) servos may create audio noise, mechanical vibration, and cogging at low velocities. The balanced-bridge servo by necessity has a bipolar power source (+- 15vdc).

On the other hand, both the brushed DC-motor and stepping motor operate from unipolar supplies.

BACK TO TABLE

8. Complex Drive Electronics?
Although brushed DC motors can run from a battery, stepping motors require more elaborate electronic controllers. Eventually, both motors often have complex controllers due to all the collateral events they are controlling -- limit handling and encoder decoding.

BACK TO TABLE

9. Acoustic Noise?
10. Excessive Vibration?
The stepper's action is violent -- especially for full steps -- the magnitude of the switched elecrical current is independent of motor velocity! This jolt produces both audio noise and mechanical vibration that is especially noticeable at lower velocities. However, the DC brushed motors feature seamless transitions from winding to winding. Except for vibration from bearings and air currents, these motors are noise and vibration free!

Fortunately, microstepping reduces the current differential being switched and the acoustic noise. Due to the motor's construction, we recommend limiting microstepping to 4X -- unfortunately, this is still not sufficient to completely eliminate noise and vibration.

BACK TO TABLE

11. Motor runs hot?
A stepping motor is often fully-powered all the time to retain its maximum holding torque. If the system friction is high enough, holding current can be substantially lowered. For example, we recommend reducing the current to 100mA from a running 250mA when stopped. In either case, this continuous current makes the stepping motor hotter than equivalent DC motors. Because the motor gets hot, the linear actuators should be in contact with a heat conducting element such as a thick metal plate. Without adequate heat conduction, it is possible to overheat and destroy the motor windings.

BACK TO TABLE

12. Brushes to wear-out?
Obviously, stepping DC motors do not have brushes to wear out -- this is one of their primary strengths. In contrast, even the precious metal brushes of normal DC motors eventually wear out. The only question is: how long do they last? While the motor manufacturer conservatively specifies the brushes for 1000 hours at maximum conditions, we anticipate 2000-5000 hours. In stepping motors, the bearing should last for 50,000 hours.

BACK TO TABLE

13. High inertia?
Since they have no moving magnets, only brushed, ironless-core DC motors can exhibit ultra-low rotor inertia. In contrast, the stepping motors have moving permanent magnets. This increased mass invariably results in slower starts and stops when compared to ironless-core DC motors with identical magnetic materials. However, the effects of higher inertia are effectively negated by high-strength samarium-cobalt rotor magnets in the stepping motor.

BACK TO TABLE

14. Inaccuracy under load?
In unloaded condition, stepping motor's accuracy is +-3% of full step or +-27 arc minutes. Under load, accuracy is further decreased an arc minute for every load change equivalent to 1% of the motor's rated torque. Since the stepper holds position by balanced magnetic fields -- additional force always causes position distortion. In contrast, the closed-loop DC motor has no errors introduced by loading variations. However, the accuracy of both motors is equivalent in a closed-loop environment.

BACK TO TABLE

DRIVESCREW ACCURACY
A Linear Actuator's absolute accuracy is first a function of the drivescrew's pitch accuracy. Even though the gearhead/encoder combination may produce a large number of counts per drivescrew revolution, absolute position directly relates to pitch accuracy.

The drivescrew is fabricated by single-point cutting on a CNC lathe in a temperature regulated environment. If is lapped to produce a fine surface and mated to its Nose Mounting or Nut. However, this lapping does little to compensate for pitch errors.

Error analysis by laser interferometer shows that our drivescrews have maximum peak-to-peak pitch errors ranging from +-0.5 um to +-5 um over 4" travel.

Because of the prohibitive cost of a more accurate drivescrew, one method for improving absolute accuracy is to use a computer look-up table. With a look-up table, you must have a repeatable starting point -- a HOME point. Fortunately, the 2000-family and 1000HES has built-in Hall-effect sensors for both the Forward and Reverse limit. You can use either sensor as the HOME sensor, repeatable to a least 10 uin (0.25 um).

Another accuracy improvement method is a collateral A-QUAD-B external linear encoder such as a glass scale.

RESOLUTION
Resolution is the smallest incremental distance represented by one step or encoder count. Resolution does not relate to absolute accuracy.

BACKLASH
Backlash is present in most all mechanical systems, although it might be too small to be perceived. We define backlash as the absence of physical movement when the direction is first reversed. Clearances between internal parts cause the backlash but also permit smooth operation. There is no resultant motion until all the tolerances in the mechanical system have been taken out and the force is again applied to the output.

Common methods to remove or reduce backlash when using Linear Actuators --

Spring-return of translation stages

Gravity

1. Zero-backlash gearheads (-ZB Option)
2. Motion-control programming that always approaches the final coordinate in the same direction.
3. Motion-control programming that compensates for known backlash every time there's a switch in direction.

ZERO-BACKLASH GEARHEAD
80% of backlash in the in-line linear actuators is caused by the clearance of the teeth in the 15/5 and 16/5 gearhead's output drive gear. The -ZB option removes this major contributor.

The Zero Backlash Gearhead contains two parallel stacks of spur gears in one package with a common attachment to both the output and motor drive gear. Prior to joining with the motor, one gear stack is advanced until all the tooth clearance at the output gear is consumed. Now, it is joined to the motor's drive gear and the gearhead-generated backlash is canceled. This pre-loading increases friction and diminishes lifetime slightly.

BACKLASH
Gearhead Style
Normal	10 um +- 1 um
Zero-Backlash	2 um +- 1 um

EXPECTED BACKLASH
GEARHEAD RATIO	1100-series (60-count Encoder)			2200-series (24 full-steps/rev)
	Distance per Count	EQUIVALENT COUNTS		Distance per Step	EQUIVALENT STEPS
	Distance per Count	Normal	Zero-Backlash	Distance per Step	Normal	Zero-Backlash
6.3:1	2.0 um	5	1	4.1 um	2	0
11.8:1	1.1 um	9	2	2.2 um	5	1
262:1	0.05	200	40	76 nm	132	26
3101:1¹	4.1 nm	2439	488	8.4 nm	1193	239¹ Zero-backlash Gearhead Option is only available through the 1670:1 gear ratio.

SIDELOAD

Sideload on the extended Thruster will increase internal friction and reduce available thrust dramatically, When rotating a tangent arm with a linear actuator, sideload becomes a significant factor influencing the thrusting force. As a rule of thumb, limit the sideload to a maximun of 5 lbs.