Nearfield Systems Inc.
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3. What is Bending Moment or Overturning Moment?
6. What is Maximum Load Inertia?
10. What is Maximum Full Load Operating Speed?
11. What is Maximum Acceleration?
15. What is Maximum Static Positioning Error?
16. What is Open-Loop Position Repeatability?
21. Is a system with feedback better than an open-loop system?
22. Do stepper systems lose position?
26. What is difference between rms error, peak error, and p-p error?
Axial load limit (measurement unit: [N]) is the maximum axial load capable of being safely supported by the stage with the spin axis vertical. Any force component in a direction collinear with the spin axis. Examples of axial forces include the vertical load on an azimuth positioner, vertical wind load in outdoor applications, vertical load during test article installation and the vertical load component during a seismic event.
Radial load limit (measurement unit: [N]) is the maximum radial (normal to the spin axis) load capable of being accepted by the positioner without damage. Examples of radial forces include the vertical load on an elevation or Pol positioner, wind or seismic forces and forces produced during the installation of the test article.
3. What is Bending Moment or Overturning Moment?
The Bending moment or Overturning moment (measurement unit: [N*m]) is the maximum moment that the positioner is capable of safely supporting about the axis of rotation. The applied moment is calculated by multiplying the magnitude of the applied force by the perpendicular distance from the platen surface. Example moment forces may include weight and position of additional axes above this axis, weight and position of fixtures above this axis, weight and position of the test article, inertial loading, wind loading, seismic loading, installation shock loading.
The delivered torque (measurement unit: [N*m]) is the available dynamic torque to rotate the stage at any speed under any load conditions. This torque is available to overcome forces due to an unbalanced load, load inertia and wind forces (if used outside).
Withstand torque (measurement unit: [N*m]) is the amount of external torque that can be safely applied to the spin axis. This value represents the safe design limit.
6. What is Maximum Load Inertia?
The maximum load inertia (measurement unit: [kg*m^2]) is the maximum mass moment of inertia around the spin axis that can be smoothly started and stopped without damaging the positioner.
The spin axis wobble (rms or peak, loaded or unloaded) (measurement unit: [°]) is a measure of how the spin axis pointing direction changes as the platen is rotated. This is not the same as table tilt or coning. The specification can be in terms of peak or rms spin axis error and can be with the stage loaded at rated capacity or unloaded.
Table tilt (measurement unit: [°]) is defined as the angle between a plane perpendicular to the spin axis and the best-fit plane of the platen surface.
Coning is the accumulation of table tilt plus any level offset relative to the spin axis (which may include measurement errors).
10. What is Maximum Full Load Operating Speed?
Maximum full load operating speed (measurement unit: [°/s] or [m/s]) is the maximum speed at which a positioner can reliably move when operating at the worst case loading condition.
11. What is Maximum Acceleration?
Maximum acceleration (measurement unit: [°/s^2] or [m/s^2]) is the maximum acceleration that the positioner can reliably use when accelerating from zero to the maximum speed while operating at the worst case loading condition.
Drive power (measurement unit: [W]) is the continuous duty power rating of the positioner motion with the rated current applied to the motor terminals.
Step size (measurement unit: [°] or [m]) is the change in the stage position for a single step pulse from the controller.
NSI defines backlash (measurement unit: [°] or [m], peak to peak) as the difference in measured angles (rotary positioner) or position (linear positioner) when the positioner is commanded to the same position from opposing directions while loaded to some percentage of the rated load. This definition is for open loop operation if no feedback sensor is used.
15. What is Maximum Static Positioning Error?
Maximum static positioning error (measurement unit: [°] or [m], peak) is the maximum position error under a full load condition with bi-directional operation. The positioner is operated closed loop if it includes a feedback sensor.
16. What is Open Loop Position Repeatability?
Open loop position repeatability (measurement unit: [°] or [m], peak) is how accurately a position can be repeatedly found, if approached from the same direction under a constant (although not zero) load, without making use of a feedback loop.
Feedback is the mechanism where the state of a system is measured and where those measurements are used to generate signals that can bring the system into a wanted state. For a positioner this means that e.g. position, speed and acceleration are measured and that from these measurements signals like number of steps and steprate are calculated which are sent to the positioner.
Open loop is a system without feedback. If the mechanical behaviour of a positioner is well known and has limited influence of external factors, a feedback system can cause unnecessary system complexity, system cost and calculation overhead. In some cases a feedback loop can even cause unwanted behaviour (if a feedback loop is not well-designed).
An encoder is a device that measures the actual position of a positioner.
A tacho is a device that measures the speed of a positioner.
21. Is a system with feedback better than an open-loop system?
No, not necessarily. Both systems have their advantage and disadvantages.
Open loop systems are usually faster and require less tuning when loads are varied. They are more predictable in their dynamic behaviour. Closed loop systems may be capable of compensating external influences.
NSI has chosen to use primarily open loop stepper motor systems. The reason for this is that stepper motor systems do not require tuning of feedback parameters and are very predictable in their state (position, speed, acceleration). In order to verify that during a measurement a series of steps were missed, one can use the drift check which ensures that the AUT and/or probe can return to the exact location as that it started from.
In cases where there are external factors that may influence the position of an actuator (like temperature that makes that a linear slide expands so that a step on a stepper motor no longer equals a known displacament), we can apply a feedback system on top of our stepper motor system. In that case, we have the best of both worlds: a feedforward system that has the advantages of fast and load independent dynamics and a feedback system that ensures proper compensation of external factors.
For positioners that require to move real large loads, we do use servo systems because of the limited maximum torque for stepper motors.
22. Do stepper systems lose position?
Under normal circumstances they don't. However, if they are overloaded they can stall. Fortunately, this is usually very obvious: as a stepper motor stalls, it does not just miss a step, but it misses a whole series of steps and the actual end position is far of the commanded position. It is therefore always smart to do some check at the end of a measurement whether the positioner is still in the commanded position. One can use the drift check for that, which compares the AUT response on a certain AUT/probe postion before the measurement starts with the response on that same AUT/probe postion after the measurement has completed.
Dead-band is the phenomenon in a positioning system with feedback when the error signal is not large enough to let the feedback system compensate for it. This can be intentional (like thermostat control to prevent the furnace continuously switching on and off) or unintentional (like friction in a positioning system with analog proportional feedback).
If a positioning system with feedback needs to compensate a difference between an actual position and a commanded position, the steering is based on the difference between actual and commanded position (the error signal). If no error is allowed, the compensation process will theoretically take infinitely long (unless the parameters are set with infinite accuracy). In order to avoid a long time of ‘hunting' for a final position, a feedback system designer incorporates some small tolerance (allowed error between commanded and actual position).
If a servo system is supposed to assume a certain state (position, speed, etc) as a function of time, it can happen that the dynamics of the system does not allow this (the actual state does not follow the commanded state). Even though the final (static) state may be as commanded, this does not mean that the movement has been accurately performed.
Since an antenna measurement system relies on a very accurate relation between position and time (a measurement sample must be taken with the antenna under test or probe in a position accurately known), following error need to be avoided or taken into account.
26. What is difference between rms error, peak error, and p-p error?
If one assumes a series of measurements of the actual position of the positioner versus the commanded position, for each measurement the error is the difference between the measured and the commanded position. The peak error is the maximum value of this error, taking into account that this error can also be negative. The p-p error (peak to peak error) is the difference between the maximum positive error and the maximum negative error. The RMS (root mean square) error takes the whole series of errors, squares them (‘calculating the energy in the error') takes the mean of those values (‘determining the average energy') and take the root.
NSI defines its planarity and positioning error in terms of RMS. This gives more information on the overall performance since RMS takes all measured errors into account, as opposed to the peak error or p-p error that depends basically on a single coincidental value.