Capacitance Probes vs. Strain Gauges in Piezo Flexure Stages

Capacitance probes with pico-positioning sensors can be embedded in piezo flexure stages. With minor mounting modifications, these non-contact probes provide significantly greater accuracy than strain gauges.

Piezo flexure stages are positioning devices with a high degree of resolution. These motion stages contain a piezoelectric actuator that converts an electrical signal into displacement based on the reverse piezoelectric effect, which is the ability of a piezo electric material to elongate when a voltage is applied. When a voltage (e.g., 0-150 VDC) is applied to the actuator, the stage moves in proportion to the applied voltage. Yet, the exact position remains unknown, because piezo electric actuators are inherently non-linear and exhibit hysteresis.

Fig 1 Schematic view of a Piezo Flexure Stage

Fig 1 Schematic view of a Piezo Flexure Stage

Fig 2 Piezo stage with MTI Cap probe embedded

Piezo Stage with Embedded Sensor Piezo in-situ to improve wafer quality

Fig 3 Piezo stage in-situ wafer quality

Embedding a displacement sensor in the stage enables precision measurement of this motion. By using displacement as feedback, a stage controller can move the piezo stage to an exact position. Typically, the displacement sensor is either a capacitive probe or a strain gauge. Capacitive sensors measure absolute distance directly and without contact. Strain gauge sensors are bonded to the structure of the stage, derive distance information from strain, and are categorized as either piezo resistive or metal foil.
Capacitive sensors provide very high linearity and the best long-term stability. Strain gauge sensors cost less than capacitive sensors and are sufficient for many applications. However, strain gauge sensors cannot match the linearity and stability that is associated with the direct measurement of capacitive sensors. To highlight these differences, the basic operating principles of both types of displacement sensors are explained below.

How Capacitive Probes Work

Capacitive probes use electric fields to measure the distance from the sensor face to the target surface. A guard ring steers the direction of sensing practically straight toward the target, ignoring the potential influences of adjacent surfaces that could disrupt the reading. Consequently, only the target area is measured. The guard ring minimizes external influences and helps to produce a linear sensing field.

Typically, the targets for capacitive probes are conductive and grounded. The electric field senses only the surface layer of the conductive target and is neither affected by the type of metal nor its magnetic properties. In addition, capacitive sensing is not adversely affected by temperature, vacuum, air pressure, or magnetic fields.

Figure 3: Capacitance probe with stainless steel body (Credit: MTI Instruments)

Figure 4: Capacitance probe with stainless steel body

Target under the field measured

Figure 5: Only the target under the field is measured. The guard ring minimizes external influences and helps to produce this linear sensing field

Despite their advantages, capacitance probes are limited by sensing distances. They are fine for motion stages with limited travel such as 1µm to perhaps 5mm. For greater motion, the diameter of the capacitance probe is too large to easily embed in the stage. However, because piezo flexure stages rarely travel more than 1mm, capacitance sensors are ideal. Importantly, capacitance sensors can also have resolutions in picometers.

How Strain Gauges Work

Strain gauges are sensors that convert mechanical force into a change in electrical resistance, which can then be measured. Strain is defined as the displacement that occurs when an external force is applied to a stationary object such as a stage. Typically, a bridge or half-bridge measuring circuit is used to convert this change in resistance into a voltage. The resulting voltage is proportional to the strain and, in the case of a piezo flexure stage, position.
A strain gauge sensor is bonded between the flexure and the stage’s fixed frame. (See Figure 1 above.) As the flexure moves, the sensor is stretched and produces strain. Although the resistance changes in proportion to the movement, strain gauges do not measure displacement directly. Because the device measures strain, performance depends upon the strain gauge’s sensitivity, which may not have high linearity or high correlation to the actual motion. Strain gauges are used when low cost is the major consideration, but they are not as accurate or as high-resolution as capacitance probes.

Strain Gauge (credit Wikipedia)

Figure 6 Typical strain gage image (Credit: Wikimedia)

Piezoresistive Strain Gauge (credit Wikipedia)

Figure 7: Piezoresistive strain gauge (Credit: Wikipedia)

MTI Instruments of Albany, NY (USA) provides pico-positioning sensors that can be easily embedded into piezo positioning stages with only minor modifications. The result is significantly greater accuracy than can be achieved with strain gauges. MTI’s Accumeasure HD Series, a digital capacitance measurement system, can achieve resolutions as low as 20pm with a 10µm range probe. Learn more about the Accumeasure HD high resolution capacitance sensor.