| Encoder |
<1 nm |
High |
Unlimited |
| Laser Interferometer |
<1 nm |
High |
0 mm-2000 mm |
| Capacitance Probe(1) |
<1 nm |
High |
<1 mm |
| LVDT(1) |
>1 nm |
Medium |
0 mm-10 mm |
| Strain Gage(1) |
>1 nm |
Medium |
<1 mm |
Notes:
1. As the travel increases, so does the resolution and minimum step size.
Feedback Devices
Aerotech offers a variety of feedback devices with its
positioning systems. These include linear encoder, laser
interferometer, capacitance probe, LVDT, and strain gage.
The particular device chosen depends on the product and the
application.
 Linear encoder: Linear encoders employ a scale with a
grating period and a read head (see Figure 1). The read
head measures the gratings and generates an analog signal
whose period is the same as the grating on the scale. Typical
periods range from 200 nm to 20 μm. The output of the read
head is either analog (1 volt peak-to-peak) or digital. Analog
output is usually chosen as it gives the ability to have very high resolution systems without sacrificing speed. In addition,
various techniques are employed by Aerotech controllers to
optimize the resolution and allow for exceptional in-position
stability. Linear encoders are suitable for any travel.
 Laser Interferometer: A laser interferometer feedback system
requires a laser, optics, and processing unit (see Figure 2). A
key advantage of the laser interferometer over other feedback
devices is that it allows for measurement at the work point, as
compared to other feedback devices that are usually mounted
near the bearings. Work-point measurements greatly reduce
the effects of Abbé offset errors. Laser interferometers are
often used with environmental stations that allow for
compensation due to external effects. However, they are
expensive and require a clear beam path to the moving
target. Laser interferometers are suitable for travels from 0 mm
to 2000 mm.
 Capacitance Probe: The capacitance probe is a type of
dielectric sensor that uses capacitance to measure distance
(see Figure 3). As the plates move closer to or further apart,
the amount of charge accumulated on the plates changes. By
measuring this charge, the relative change in distance
between the two plates can be measured. While the probe
itself is not too complex, the electronics required to
disseminate the analog signal can be highly complex. Also,
due to the low signal level, feedback signals from
capacitance probes are more susceptible to electrical noise.
Capacitance probe travel is typically limited to less than 1 mm
in high-performance motion systems.
 LVDT: An LVDT is a linear variable differential transformer (see
Figure 4). As the magnetic core moves relative to the fixed
windings, an output voltage is generated that is a function of
the displacement. Advantages of these devices are small size
and lower cost. They are typically used where travel is limited
(<1 mm) and resolution requirements are on the order of 10’s
of nanometers.
Strain Gage: The strain gage measures displacement by
measuring the resistance change as the device (strain gage)
deforms. The output signal is sampled by an A/D converter
and converted to a displacement. To minimize thermal effects,
a Wheatstone bridge is often connected to the unit under
measurement. These devices are popular in short travel stages
(<1 mm) due to the small size and low cost.
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