Linear Motors

A: A motor with a standard winding has a higher inductance with a higher force constant than does a motor with the high response winding. A standard response motor is used in systems with large mass and/or in systems that do not need high precision. A high response winding is used for high precision and/or highly dynamic motion.

Q: How are the Moog motors thermally protected?

A: Moog motors have 95° C thermal switches inside the stator. They are designed to be connected to and monitored by a control system. If the stator exceeds the motor’s rated temperature the switches will open. When the control system observes the open switch it needs to shut down the motor.

Q: What voltage drives can be used with Moog motors?

A: Servo drives that use 240 VAC or less are acceptable for use with Moog motors. Using voltages less than 208 VAC will result in a reduction in performance from the published peak speed and dynamic response.

Q: What type of commutation signal and sequence do Moog motors with Moog feedback devices have?

A: Moog feedback devices mounted on Moog motors provide digital 120° commutation signals. Motors without Moog feedback devices do not have commutation signals.

Q: What is the electrical cycle length of a Moog motor?

A: Electrical cycle of a Moog motor is .922 inches (0.0234 meters).

Q: How much side load can be applied to the motor shaft?

A: Moog motors are not designed to have an external side load applied. Any lateral loading of the bearings will increase wear of the bearings and reduce the life of the motor. Side loading of the shaft can also cause misalignment of Moog provided feedback devices.

Q: How much torque can be applied to a motor shaft?

A: Applying torque to a Moog motor shaft is a problem only when using an LC type Moog encoder. When the encoder is being used any applied torque will result in wear or binding of the internal anti-rotation surfaces and could eventually result in a loss of feedback signal as a result of encoder misalignment. On motors without an LC type Moog encoder the shafts are free to rotate

Q: Can the motor shaft rotate?

A: Rotating of the Moog motor shaft does not affect the ability of the motor to produce force. A rotating shaft motor can only be accomplished with a position feedback sensor that would allow shaft rotation. The LC type Moog encoder does not allow rotation. Continuous rotation of the shaft could decrease the bearing life of the motor.

Q: Can the motor be used to control force?

A: Moog motors work very well and accurately in a closed loop force servo system for controlling force. Controlling force without a closed loop force servo control tends to yield poor accuracy. It is possible to increase the accuracy of open loop force control by adding compensation tables (position, temperature, speed) to the control loop.

Friction

Q: What is the static and dynamic friction drag force of the current motors?

A: Static friction is greatly variable since it is time dependent. It ranges from just greater than dynamic friction at very small dwell times at a stopped position, to approximately twice the dynamic friction when left stationary for several days.

Q: What is the range of variation in this force on normal production units?

A: For 50206 motors, motor to motor friction varies from a high of 55 lbf to a low of approximately ½ of the high or 27 lbf.

Q: Can you characterize the source and variability of the normal force that creates that friction drag force?

A: The normal force that creates the drag is primarily based on two phenomena.

1-The magnetic attraction between the shaft and the stator causes the majority of the drag force. This force is dependent on the magnetic strength of the magnets, the area of interaction of the magnets and the stator, and the distance from “on center” the shaft is in the stator. Ideally the shaft would be centered in the stator. However, there are variations due to physical limitations. The bearings cannot be operated at zero clearance. In addition, the shaft and bearing diameters must have a tolerance. There is also a variation in the magnetic center of the shaft vs. the physical center of the shaft based on the tolerance build up in the production of the shaft. The straightness of the shaft also has an effect on the magnetic centering of the shaft. And finally, there is a tolerance in the assembly of the end plates to the stator. All of these clearances and tolerances must be accommodated within a very small air gap in order to obtain the high thrust capability of the Moog linear motors.

2- The tightness of the wipers in the groove between the end plates and the sh aft causes an additional drag force. This force has a variation dependent on the tolerances of the shaft, end plate, wiper, and motor assembly.

Detent Force

Q: The Moog brochure lists a specification “Detent Force (peak)”. How is this specification defined and measured?

A: This specification is defined as the peak (as opposed to peak to peak) variation in the drag of the motor. The drag described in the friction section above, could be considered the dc offset and the detent could be considered the ac variability. The detent, as well as the drag is measured by pulling on the shaft with the motor de-energized from one end of stroke to the other end of stroke. A force transducer reports the force and a position sensor reports the linear position of the shaft. A chart is then made of the force vs. position. The drag force and the detent force must not exceed the specifications. This test is performed on all motors as part of the acceptance test.

Q: Can you clarify the magnetic detent force specification? I have an old spec sheet for your 50206A/B motor that lists a higher number than the 10lbf listed in the current literature. What is the current specification?

A: The current specification is 10lbf. One of the improvements made with the C/D motors was a reduction in the detent force. We found customers required more smoothness in the motion than the previous design allowed.

Physical Tolerances

Q: I’ve looked at your ICD-50602C & D Interface Control Drawings. Can you tell me the diametric tolerances of the motor shaft? (2.000” +.xxx -.xxx”.)

A: 1.998” +.0010-.0000”

Q: What are the tolerances of the end of the motor shaft, less the end stop? (Flatness and perpendicularity to the shaft outside diameter.)

A: Perpendicularity -.002 to OD, Flatness is not controlled separately.

Q: What is the nominal clearance and tolerance of the shaft-to-bearing interface?

A: Bearing ID dimension is 2.002 _.0010 -.0005”
Shaft OD dimension is 1.998 +.0010 -.0015”
Clearance is therefore .004 +.0000-.0015”

Shaft Bearings

Q: What is the estimated life of the current shaft bearings?

A: In excess of +billion inches of travel.

Q: There was significant testing done in the past. Can you share the results?

A: Two tests of motors with no external side load demonstrated endurance lives in excess of 2 billion inches and 4 billion inches of travel. The two tests were run at two different speeds and axial loads. The slower test with higher load was terminated at the 2 billion inches of travel milestone. The faster test with a lower load was terminated at the 4 billion inches of travel milestone. During these tests, the oil reservoir was topped off after every 100 million inches of travel. The duration of these two tests were approximately 4 years and 3 years of operating 24/7.

Q: How have the bearing solutions changed since the A/B motor series?

A: The A/B motors had the same bearing materials as are currently being used on C/D motors. The failure mode in A/B bearing tests was depletion of oil. Therefore, the oil reservoir behind the bearings in the C/D motors has been increased in volume and the reservoir for the C/D motors is refillable. As is seen from the test results of the C/D motors, by continuing to replenish the oil, the failure mode was eliminated.

Q: What are the side load limitations of the current bearing system?

A: The bearings can operate with a substantial side load. However, the life of the bearings will be decreased. We normally recommend that side loading be kept below 50 lbf when the shaft is within 10 inches of the motor face. For longer strokes, the maximum side load should be decreased.

Skewed Coils

Q: Can you clarify the implementation of “Coil Skewing” in the C/D motor series? What are the trade-offs if any? (i.e. lower force constant, etc.)

A: “Skewed coils” is actually a misnomer or simplification. The reality is that the tooth tips of the stator are angled in relation to a plane normal to the axis of the motor. The skewing reduces the detent force as the shaft magnets do not encounter the tooth edge all at once, but encounter it a little at a time. A consequence of the skewing is a decreased nominal force. Fortunately, the decrease in nominal force is less than the decrease in detent force. Therefore, the result is that the net force at the valleys is the force that can be depended upon to perform a task regardless of position. It is also the force reported in the specification of the motor.

Performance Tolerances

Q: What kind of specification tolerances can we expect motor-to-motor? My assumption is the published specifications are nominal’s? I am most concerned about things like the force constant, back EMF, detent force, and friction. I am not expecting tolerances on resistance or inductance to be an issue.

A: Even though the industry standard is to quote nominal specifications, Moog has chosen to use “no worse than” specifications. The tubular linear motor technology was new so Moog wanted to ensure that customers could meet their specifications consistently.

Q: What can we expect for variations in force constant vs. position?

A: Force constant varies by position primarily due to detent force and variation in drag. These two forces are mostly independent of the force being generated by the motor. As such, it is difficult to give a direct answer to this question.

LP Encoder

Q: What is the accuracy and resolution of the LP encoder?

A: The accuracy of the LP encoder is dependent on the construction of the shaft, which is used as the scale. The accuracy is .005 inch per inch of travel. The resolution is 5 microns. The repeatability is 10 microns.

Q: Can you explain its operation principle? Our understanding is that it utilizes the motors magnets which probably do not have a perfectly uniform magnetic field strength magnet-to-magnet.

A: The basic operation principle is that it measures the magnetic field shape and does not depend much on the intensity to determine the position within the magnetic cycle. This increases the resolution and accuracy capability of the technology.

Q: Is an absolute version planned? If so, what is the estimated resolution and accuracy?

A: Moog is in the process of evaluating what additions to the product line make sense from a business standpoint. The addition of an absolute version of the encoder is under consideration. Once a decision has been made and the engineering work initiated, the estimated resolution and accuracy can be provided.

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