Cooper Instruments ELF 4200 User Manual

A201 & B201

USER’S GUIDE

INTRODUCTION

This manual describes how to use A201/B201 sensors. These sensors are ideal for designers, researchers, or anyone
who needs to measure forces without disturbing the dynamics of their tests. The sensors can be used to measure both
static and dynamic forces (up to 1000 lbf.), and are thin enough to enable non-intrusive measurement.

The sensors use a resistive-based technology. The application of a force to the active sensing area of the sensor results
in a change in the resistance of the sensing element in inverse proportion to the force applied.

OVERVIEW

This section outlines Sensor Construction and Application.

Sensors

The A201/B201 sensor is an ultra-thin and flexible printed circuit, which can be easily integrated into most applications.
With its paper-thin construction, flexibility and force measurement ability, the force sensor can measure force between
almost any two surfaces and is durable enough to stand up to most environments. A201/B201 sensors have better force
sensing properties, linearity, hysteresis, drift, and temperature sensitivity than any other thin-film force sensors. The
"active sensing area" is a 0.375” diameter circle at the end of the sensor.

The A201 sensor is available in the following force ranges:

• Sensor A201-1 (0-1 lb. force range)

• Sensor A201-25 (0-25 lb. force range)

• Sensor A201-100 (0-100 lb. force range)*

* In order to measure forces above 100 lbs. (up to 1000 lbs), apply a lower drive voltage and reduce the resistance of the
feedback resistor (1k min.). See the sample drive circuit below.

The sensors are constructed of two layers of substrate. This substrate is composed of polyester film. On each layer, a
conductive material (silver) is applied, followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the
two layers of substrate together to form the sensor. The silver circle on top of the pressure-sensitive ink defines the “active
sensing area.” Silver extends from the sensing area to the connectors at the other end of the sensor, forming the
conductive leads.

A201/B201 sensors are terminated with a solderable male square pin connector, which allows them to be incorporated
into a circuit. The two outer pins of the connector are active and the center pin is inactive. The length of the sensors can
be trimmed by Cooper Instruments to predefined lengths of 2”, 4” and 6” or can be trimmed by the customer. If the
customer trims the sensor, a new connector must be attached. This can be accomplished by purchasing staked pin
connectors and a crimping tool. A conductive epoxy can also be used to adhere small wires to each conductor.

The sensor acts as a variable resistor in an electrical circuit. When the sensor is unloaded, its resistance is very high
(greater than 5 Meg-ohm); when a force is applied to the sensor, the resistance decreases. Connecting an ohmmeter to
the outer two pins of the sensor connector and applying a force to the sensing area can read the change in resistance.

Sensors should be stored at temperatures in the range of 15°F (-9°C) to 165°F (74°C)

Application

There are many ways to integrate the A201/B201 sensor into an application. One way is to incorporate it into a force-to­voltage circuit. A means of calibration must then be established to convert the output into the appropriate engineering
units. Depending on the setup, an adjustment could then be done to increase or decrease the sensitivity of the sensor.

An example circuit is shown below. In this case, it is driven by a -5 V DC excitation voltage. This circuit uses an inverting
operational amplifier arrangement to produce an analog output based on the sensor resistance and a fixed reference

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resistance (RF). An analog-to-digital converter can be used to change this voltage to a digital output. In this circuit, the
sensitivity of the sensor could be adjusted by changing the reference resistance (RF); a lower reference resistance will
make the sensor less sensitive, and increase its active force range.

In the circuit shown, the dynamic force range of the sensor can be adjusted by changing the reference resistor (RF) or by
changing the Drive Voltage (VO). Refer to the Saturation section for additional information.

Sensor Loading Considerations

The following general sensor loading guidelines can be applied to most applications, and will help you achieve the most
accurate results from your tests. It is important that you read the Sensor Performance Characteristics section for further
information on how to get the most accurate results from your sensor readings.

Sensor Loading

The entire sensing area of the A201/B201 sensor is treated as a single contact point. For this reason, the applied load
should be distributed evenly across the sensing area to ensure accurate and repeatable force readings. Readings may
vary slightly if the load distribution changes over the sensing area.

Note that the sensing area is the silver circle on the top of the sensor only.

It is also important that the sensor be loaded consistently, or in the same way each time.

If the footprint of the applied load is smaller than the sensing area, the load should not be placed near the edges of the
sensing area, to ensure an even load distribution.

It is also important to ensure that the sensing area is the entire load path, and that the load is not supported by the area
outside of the sensing area.

If the footprint of the applied load is larger than the sensing area, it may be necessary to use a "puck." A puck is a piece of
rigid material (smaller than the sensing area) that is placed on the sensing area to ensure that the entire load path goes
through this area. The puck must not touch any of the edges of the sensing area, or these edges may support some of the
load and give an erroneous reading.

The A201/B201 sensor reads forces that are perpendicular to the sensor plane. Applications that impart "shear" forces
could reduce the life of the sensor. If the application will place a "shear" force on the sensor, it should be protected by
covering it with a more resilient material.

If it is necessary to mount the sensor to a surface, it is recommended that you use tape, when possible. Adhesives may
also be used, but make sure that the adhesive will not degrade the substrate (polyester) material of the sensor before
using it in an application. Adhesives should not be applied to the sensing area; however, if it is necessary, ensure that the
adhesive is spread evenly. Otherwise, any high spots may appear as load on the sensor.

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