General description

The HSE force transducers F30 and F10 are designed for measuring small tensile and compressive forces and have a wide range of application. The range of the F30 is 0.3 N = 30 cN (approx. 30 g). The range of the F10 with 0.1 N = 10 cN (approx. 10g) is smaller by a factor of 3 and therefore particularly suitable for measuring extremely
low forces. The displacement for the full working range of the F30 is 0.03 mm = 30 µm. The F10 requires twice the displacement (0.06 mm = 60 µm). Both transducers can therefore be considered as highly isometric units. Both the F30 and the F10 are ideally suitable for experiments on small muscle preparations, in particular the papillary muscle, since they are capable of reliably measuring very small forces. The transducers are not only suitable for measuring forces in muscle contractions but can also be employed for weighing tissues or in
evaluating hand tremors.

The characteristic of the F30 is sufficiently linear up to about 80 cN. The noise is of the order of 0.002 cN (2 mg). The corresponding values for the F10 are 20 cN and 0.0005 cN (0.5 mg). This means that very small forces can still be measured. The natural frequency of the measuring system is near 440 Hz on the F30 (180 Hz on the F10) and
is therefore sufficiently high for evaluating rapid contractions without error.
 

Operation

The operation of the F30 and F10 is based on the magneto-resistive principle in which a magnetically sensitive semiconductor resistance made from indium antimonide (InSb) moves in the field of a permanent magnet (field plate principle). The field plate takes the form of a meander-shaped resistance, similar to an etched strain gauge. The field plate is located in the field of a permanent magnet. Changes in the field line density alter the resistance of the field plate.

Figure 2 illustrates the operation of the transducer. Four field plates (FP1 to FP4) are located inside the magnetic field of a U-shaped permanent magnet as shown. The core (A) is mounted on a spring-supported carrier; any displacement causes the magnetic flux to be shifted in the direction of the double arrow. This results in a change in the magnetic flux through the field plates and therefore in a change in resistance.

The field plates are connected electrically into a Wheatstone bridge circuit as shown in Fig. 3. The measuring system is adjusted so that the bridge is zeroed in the rest position. When the rod is loaded by a force (F) in the
axial direction the core (A) is is placed by a distance (D). There is a corresponding change in the magnetic field acting at the field plates and therefore in their resistance, so that the bridge is unbalanced. This out-of-balance of the bridge can be measured as a corresponding bridge output voltage U_o.

In Fig. 4 the output voltage U_o is plotted against the force F for the F30. Note that the strictly linear range is covered by 30 cN (30 g). The accuracy specified in the technical data applies to this range. On the F10 the values are correspondingly smaller.

When applying larger forces the characteristic becomes non-linear. Forces up to about 80 cN (F30) or 20 cN (F10) can be evaluated; however, no information on accuracy can be provided since the measuring systems of the individual transducers differ slightly in this respect.

Mounting Position

The F30 and the F10 are secured by the central threaded boss, either with the knurled nut or by directly screwing it into position.

A mounting adapter with rod is supplied for mounting transducer on a laboratory stand.

Note the following during positioning:

• Place the transducer so that no liquid can pass into it, in particular no Tyrode solution or vapor.
• When measuring extremely small forces, place the transducer in a vibration-free position or ensure that any
  vibrations are kept away from it
• In order to minimize temperature drift, ensure that the transducer is not exposed directly to heat sources (lamps, heaters, direct sunlight etc,)
• Do not overload the force pin on the transducer.

Details of the exploded view

• F30 or F10 force transducer 
• sensing hook cap, screwed on (T16151)
• push-on hook, mounted on (2) (Z28001)
• mounting adapter, side view (T18050)
• mounting adapter, seen from above (T18050)
• rod for mounting on laboratory stand (T15009)
• knurled nut (T16155)

Overload and overload protection

The force is applied to the transducers F30 and F10 as a tensile load through the pushed-on and clamped hook which slips on overload. When a tensile force is applied directly to the transducer (this is not recommended) or when a pressure is applied to the measuring pin it is important to ensure that the measuring system is not overloaded. The transducers incorporate reliable protection up to 10x full scale. Large forces, in particular in a radial direction, may damage the measuring system and result in a permanent zero error. 

Compared with transducers using unsupported wires the F30 and F10 are exceptionally rugged. The reason is that the actual measuring system operates without mechanical contact and that there are no small fragile components. Massive stops protect against damage during normal use.

Electrical connection

Operation of the F30 or F10 requires a bridge amplifier with a stabilized bridge supply within the range 0 - 10 Volt. The magnitude of the supply voltage determines the sensitivity of the transducer. It is advisable to set the supply voltage to 5 Volt. This value results in a low temperature drift with a sensitivity adequate for most applications (see Technical Data). The transducer is normally supplied with a 6-pin Binder plug (male). Other connectors can be fitted to special order. It is essential that the connections for supply and signal output are made correctly, otherwise the transducer will not operate properly (in particular an increased temperature drift has to be expected). 

Connector: Binder, 6-pin, male, Type: 09-0321-00-06 The pin connections and the color coding of the wiring can be found in the Test and Calibration Sheet enclosed with each transducer.
 

Calibration

The transducers F30 and F10 are supplied as standard with a calibration weight, a hook weight of 1 cN = 1g approximately. It is advisable to obtain a set of Newton calibration weights as hook weights in order to permit accurate calibration of intermediate values (Item732635). 

A standard measuring set-up consists of an F30 or F10, bridge amplifier and data acquisition system. After connecting them together the complete system has to be calibrated in order to evaluate the amplitudes of the recorded curve. This is done by attaching a suitable calibration weight.

In order to achieve maximum accuracy the calibration weight should be of the same order as the expected force amplitudes. When using an F30 or F10 for measuring contraction forces of isolated tissues the 1 cN calibration weight as supplied is suitable. When measuring larger or smaller forces it is preferable to use suitable calibration weights from a weight set. 

The method described above it of course applicable only if the transducer is mounted vertically. With horizontal mounting the transducer must be calibrated with a suitably sensitive spring balance or a low-friction pulley  has to be used. Another possibility is to rotate the complete apparatus so that the transducer  is positioned vertically. 

In certain laboratories another method is used for regular daily calibration. A calibration step is produced by placing the transducer first into the horizontal position and then turning it vertically. The weight of the moving sensor component itself then acts as a calibration weight. The weight of the moving system does however vary from one transducer to another. For this reason the deflection of each transducer must first be determined by comparison with an accurate calibration weight. With the hook placed into position, the calibration step produced is of the order of 1.5cN.

Reduction Lever

For the measurement of small forces there is an additional accessory for the F30, the Microscale reduction lever  UL5. This attachment is screwed on to the male thread M16x1, metric fine thread, 16 mm ext. dia., 1 mm pitch) on the head of the F30. In theory it might be possible to use the reduction lever also in conjunction with the F10. There is however an increased danger of overloading combined with large hysteresis effects, so that its use is not
recommended.
 

NOTE: when the reduction lever UL5 is fitted to the F30 the transducer has to be handled with particular care to avoid overloading and damaging it.

The lever is intended for horizontal operation. After screwing it into position it must make an angle of 90° with the F30. Larger deviations lead to faulty operation (increased hysteresis!) and errors in measurement. Any necessary adjustments can be made after releasing the clamping screw of the knife bearing (SL, Fig. 11);
however it is advisable first to check for other possible errors.

Figure 11 shows the lever and its operation with the F30 in schematic form.

The lever is mounted on a knife bearing (SL) to ensure low friction. Forces which are applied at the hooks H(x2),  H(x5) or H(x10) act as pressure forces on the sensing pin of the F30. According to the lever formula.

L x LA = F x FA (L = Load, LA = Load Arm, F = Force, FA = Force Arm)

The force acting on the F30 is increased in accordance with the distance of the application point from the center of rotation. For example, a tension force of 1 cN acting at the hook H(x5) results in a compression force F_c of 5 cN on the F30. Expressed differently, in order to have a force of 5 cN acting on the F30 the tension force at the hook H(x5) need only be 1 cN.

The sensitivity of the complete system is therefore increased through the use of the UL5. Obviously the useful measurement range of the complete assembly (F30 plus UL5) is correspondingly reduced. The displacement is also increased according to the higher sensitivity.

The lever can be balanced by means of the nut (counterweight G) which can be moved along the threaded rod.

Natural frequency and damping

Following a sudden displacement the transducers exhibit a damped oscillation as shown in Fig. 12. The following data can be derived from the curve: 

Natural frequency F₀ = 450 Hz (F30), 180 Hz (F10)

Damping factor K = 1.05

The curve shown applies only if the sensing pin of the transducer can oscillate freely and if no tissue is connected to it.

When using the reduction lever UL5 on the F30 the natural frequency is greatly reduced due to the increased moving mass. With a preload of 1 cN the frequency is of the order of F₀ = 50 - 70Hz.
 

Technical Information