General Description

This user manual explains the installation, function and use of the FT20 and FT50 Isometric force transducers.

FT20 Type 385 Item Number 734986 and 735035

FT50 Type 385 Item Number 734987 and 735036

This force transducer series is designed for measurement applications in animal experimental research or other technical uses in general laboratory, light industrial or office environments. The installed sensor system is based on a resistance full bridge circuit, which can be directly connected to any HA-HSE bridge amplifier, e.g. the PLUGSYS TAM amplifier module or on a variety of AD Instruments bridge amplifiers. If using other DC bridge amplifiers, e.g. Grass or Gould, there are different connection cables available.

Features

• Isometric (low displacement of the measurement cell)
• Can be used in both directions, pull or push
• Two ranges available ±20 g, and ±50 g full scale (FS)
• Suitable for small muscle and tissue samples like papillary muscle, Purkinje fibers and vessel rings
• Resistance full bridge circuit (Wheatstone bridge) can be used with most commonly used DC bridge amplifiers
• Supply voltage range 5 to 10 VDC (max. 15 mA)
• Removable connection cable with 6-pin binder connector for HA-HSE bridge amps or 8-pin binder for ADI amps, special cables for products from other manufacturers, e.g. Grass or Gould, are available on request.
• Full metal housing made of aluminum and stainless steel
• Removable holder with two M5 fastening threads for horizontal or vertical rod mounting
• Compact design, two mechanical end stops as overload protection and additional front protection around the actuator cap.

Detailed Description

Spring Hook & Handling 

The spring hook is a specially designed flat spring used to affix a muscle preparation with a thread to the transducer for recording a dynamic force contraction signal. To remove the hook press both spring legs together and remove from stylus tip. In the event of an overload the spring hook will pull off from the stylus reducing the likelihood of damage.

Test Weight 1 cN

Part of the standard accessories is a small test weight 1 cN (~1 g). This test weight can be used to calibrate a Force Transducer FT20 or FT50 to the attached bridge amplifier. Such calibration sets the amplification of the amplifier to a self-defined measuring range, such as a 2 V output voltage per 1 cN load.

Electrical Connection

The transducers are connected to a suitable bridge amplifier via a removable and exchangeable cable. This allows connection of the transducer to different amplifier models by simply changing the connection cable. 

Note: The connector on the transducer side is not a USB port; therefore do not use any extension cable between the transducer and amplifier. As an HSE standard, we offer a 2 m length connection cables with either 6-pin binder suitable for all HSE amplifiers, or 8-pin binder suitable for AD Instruments bridge amplifiers*. Special cables for amplifiers from other manufacturers e.g. Grass, Gould, etc. are available on request.

*Compatible with AD Instruments bridge amplifiers models ML110, ML112, ML221, ML224, ML228, FE221, FE228 and FE224. 

Mounting & Handling in Operation

The transducer can be mounted in any direction, but make sure that there is no risk that the transducer becomes contaminated directly by liquids or indirectly through condensation. The removable holder is attached to the housing with a dovetail joint and is fixed with an M5 hexagon socket set screw. The required retention force is very low; please use only a little force to fix it to prevent damage to the housing. An appropriate screwdriver is part of the standard accessories. The support rod can be screwed into one of the two M5 mount threads of the holder in a horizontal or vertical direction. When measuring very small forces, especially when using an FT20, it is recommended that you place the setup on a vibration-free platform or equivalent vibration isolation equipment.
To minimize temperature drift, ensure that the transducer is not exposed directly to heat sources (lamps, heaters, direct sunlight, etc.).

If the transducer is currently not in use, place it back into its storage box. Under normal conditions and with good handling practices, the force transducer will be a precise measuring tool for many years.

Removable Holder

Transducer with Vernier Control

For precise adjustment of a given preload, e.g. for certain isolated muscle preparations, a Vernier control is important. The transducer position can be finely set using the Vernier control to obtain the required preload force.

Technical Specification

Basics about Wheatstone Bridge

This section describes some basics about how a Wheatstone bridge transducer works, how to calibrate the transducer and set up the connected amplifier. All information hereinafter describes additional technical details which are important to understand your measuring setup in more detail.

Wheatstone Bridge Circuit

The installed full bridge magneto-resistive sensor chip consists of four equal magnetic field-dependent
resistors. Through the movement of a magnetic anchor plate along the sensitive sensor area, the ohmic bridge
resistance is changed by the displaced magnetic field. The sensitivity of the sensor is very high; thus the displacement of the anchor is very low. 

The diagram to the below illustrates the basic circuit of the bridge sensor.

There are two separate half-bridges: R1.1 and R1.2; and R2.1 and R2.2. All four resistors are magnetic sensitive and all have the same value.

Thus if there is no difference in the magnetic excitation. The measured output voltage between the (+) Out and
(-) Out is zero. A displacement of the magnetic anchor plate of the transducer shift s the magnetic excitation
between the half-bridges; thus we can measure a positive or negative voltage on output (+) Out and (-)
Out. This output voltage is very low, in the microvolt range. The size of the measured signal is directly proportional to the applied bridge excitation voltage. A fixed 5 VDC supply, as quasi standard, is mainly used
by many manufacturers. The 5-volt supply offers a sufficient output signal and the self-heating of the sensor element is still very small, so the temperature drift within the switch-on phase is not critical.

An outstanding feature of a bridge circuit design is the ability to add an electrical zero balance. The correction is made directly into one of the two half-bridge sides. Derived from the bridge excitation voltage through a potentiometer or a digital to analog converter (auto zero circuit), a very small current is fed into the bridge circuit. Thus it is possible to compensate for the gravity error and user preload setting. The zero balance circuit is part of the connected bridge amplifier and not the transducer. The ability to compensate for any kind of transducer off set (non-zero output) before amplification allows distribution of any transducer sub-range, e.g. from 20% to 100% full scale amplifier output. However, for a relatively accurate measurement, the sub-range should be not less than 10% full scale. For a FT50 transducer with ±50 cN the maximal range spread is ±5 cN full scale amplifier output. This means that a FT50 can be adapted to a given amplifier only by gain adjustment in the range ±5 cN to ±50 cN. Values higher than 50 cN are limited by the deflection of the sensor mechanic and values lower than 5 cN are electrically limited by amplifier noise, temperature drift , max. gain, etc.

Bridge Excitation Voltage

Bridge excitation voltage is the supply voltage for the transducer. In our case we are using a DC supply in the range 5 to 10 V. The maximal current is below 20 mA. The special design feature of an additional installed preamplifier from an FT20 needs an extra current but is below 50 mA.

Note: Never apply the excitation supply with reversed polarity. This will damage the electronic sensor.

Connection Cable and Pin Assignments

The standard connection cable is based on a modified mini USB cable 2 m long. On the transducer side there is a 90° angled mini USB plug and on the amplifier side a 6-pin binder plug according the HSE standard for bridge amplifiers. The optional available cables for other DC bridge amplifiers, e.g. Grass or Gould, are equipped with their appropriate plug.

The use of a mini USB connector has several benefits such as small size and high reliability even at a high number of mating cycles. Additionally, the cable offers a very good electrical shielding and is still handy and flexible. In case of accidental connection of a FT transducer to a real USB port, there is no risk of damage. Both systems have a DC supply voltage, a positive and negative signal line, and the same pin assignment.

Mini USB Connector

6-pin Binder connector HSE cable type

6-pin Binder connector HSE cable type

*Excitation voltage programming resistor should be 470kΩ to get 5V excitation voltage

Calibration

The FT20 and FT50 transducers are supplied with a calibration test weight of 1 cN (~ 1 g). We recommend that there be one suitable set of Newton calibration weights available in your laboratory. The number of weights and their gradation depends on the measuring range of your application. In all cases, for maximum accuracy it is important to have a calibration weight in the same range as your measuring range or no less than 80% of the
maximum force you want to measure.

Through the calibration procedure, all components of the setup are to be adjusted according the desired measuring range. The following is an example procedure for a HA-HSE TAM-A bridge amplifier.

Example Calibration Procedure for Transducer and Amplifier

• Install all devices in their permanent location in your laboratory. The force transducer must be placed precisely in a vertical position because we are using the weight force as a calibration reference.
• Power on the complete measuring system and allow it to warm up for 5 to 10 minutes to get stable conditions.
• Set the variable GAIN setting to 100%, the mode switch to measure, and the filter to maximum frequency (300 Hz). Perform a zero adjustment without any calibration load. Place the selected calibration load (80 to 100%) on the transducer’s tip. The amplifier output voltage now should be higher than 8 to 10 volts based on the load of 80 to 100% of the measuring range. If not, change the internal jumper settings for gain and auto zero range (low, medium or high). Please consult the instructions for the TAM-A bridge amplifier. For good measuring accuracy it is important to set the amplifier gain large enough so that the amplifier range is used full scale.
• Once the correct gain range is selected, start the calibration procedure. A good additional tool is a small handheld voltmeter connected to the front panel BNC output. In this example we are using an FT50 and the desired measuring range should be ±6 cN. To get an evident relationship between the measured force and the amplifier output voltage we will set 1 cN to 1.5 V. Our calibration weight is a combination of a 5 and 1 cN item which we have attached. The resulting output voltage is 13.6 V which is more than the regular full-scale range of ±10 volts. Based on this, we reduce the gain with the variable gain trimmer to get an amplifier output of exactly 9.0 V. Because of the large amplification change there is also a small shift of the zero line. We remove the calibration weight and perform an additional zero adjust and then retry the calibration procedure and set the amplifier output to exactly 9.0 V such that 1 cN = 1.5 V amplifier output.
• Next adjust the CAL simulation value.

CAL Simulation Value Adjustment

The following adjustment is a specific feature of the TAM-A (D) amplifier. Through the mode switch CAL - OFF  - MEASURE it is possible to emulate the CAL-values for a 2-point calibration procedure of a connected data acquisition, scope or recording device. Thus there is no need to attach a calibration weight to the transducer tip.

The first calibration point ZERO is generated by setting the mode switch into the OFF position. The amplifier output is then 0 volts. The second calibration point (Mode switch = CAL) we will now adjust the output with the front panel trimmer CAL (for example to 2 cN = 3 V amplifier output). This adjustment can be done very easily if you have a voltmeter connected to the amplifier output. Our amplifier TAM-D has an integrated voltmeter (DVM) on the front panel and if the ADJ. trimmer of the DVM is on the clockwise right end, the reading is 10.00 V. 

If there is no voltmeter available, it is also possible to attach a calibration weight to the transducer and adjust the simulation CAL value equal to the amplifier output voltage. For the adjustment of the CAL value the mode switch must be in the CAL position and for the output reference switch to the MEASURE position. This approach requires several mode switches CAL <> MEASURE for an exact adjustment. 

Once the simulation CAL value is set, please make a note with a pencil in the designated field. If all settings are accurately done, it is now very easy to output the references for a two-point calibration, or you can add a calibration reference mark e.g. +2 cN to the currently running data acquisition.

IMPORTANT: We cannot give general advice on how often a real calibration procedure must be done. This depends on your laboratory Standard Operation Procedures. It is a good practice to make a full calibration check at least every month or if there was an appreciable overload of the transducer to check if there is a damage to the sensor mechanics.

Once per day, before and at the end of each measurement, please check zero and the simulated CAL value to be sure that the measuring setup work well.

Manual Zero Adjust and Auto Zero Function on TAM-A(D)

Like every modern electronic design, the TAM-A (D) amplifier has an auto zero function installed. The auto zero makes it easier to establish a zero offset exactly. But it must interact together with the manual zero trimmer settings. 

First set the adjustment range of the auto-zero circuit into its middle position, making it possible to compensate positive and negative offsets in the same way later. This can be done by disconnecting the transducer and then starting the auto zero function. In case there is no transducer connected, zero cannot be adjusted and after an error signal the controller sets the auto-zero into the middle. Now reconnect the transducer and make first the zero adjustment with the FINE and COARSE trimmer of the TAM and then press the auto-zero key for an accurate zero baseline.

Natural Frequency and Damping

The term "natural frequency" describes the oscillation behavior of the sensor after a mechanical excitation. Like a tuning fork, the sensor parallelogram spring system has a specific individual natural frequency. This value depends on the spring constant and is therefore different according to the type of transducer (FT20 or FT50). The next three screenshots illustrate the behavior for an FT50 transducer, free swinging without any additional load.

Initial mechanical excitation and the resulting free swinging of the moving sensor shaft. The measured natural frequency of the FT50 is 300Hz.

The damping factor specifies the amplitude reduction of the freely oscillating sensor. it is calculated from swing to swing. The damping factor DF = 1.045 and is calculated [Amplitude 1] divided by [Amplitude 2].

For the force measurement it is important that the natural frequency of the transducer be significantly higher than the force curve to be measured so that there is no risk that the mechanical parts come into resonance. The frequency parts of a derived muscle contraction is in all cases below 100Hz, so there is no falsification of the measurement through resonance side effects.