9.1.1 Bridge-force sensor for bowed strings

Figure 1 shows a violin bridge equipped with four force sensors, to allow the transverse force from the vibrating strings to be captured as electrical signals. Two small rectangles of piezoelectric crystal are used, oriented to respond to compressive force through the thickness. Such sensors have high electrical impedance, so the output of the sensor needs to be fed into the same kind of pre-amplifier that would be used with a piezoelectric accelerometer: a “charge amplifier”.

Figure 1. A violin bridge with four force sensors installed. The wires carrying the signals away can be seen behind the bridge.

The construction of each sensor is shown in diagrammatic form in Fig. 2. The string rests on a small triangular piece of insulating polymer, such as perspex. This triangle has conducting copper wrapped around its two lower faces. These make contact with one face of the two crystal pieces. The other faces of the crystal pieces rest on two more copper electrodes, fixed to a V-shaped supporting piece of perspex. Wires are connected to these, to take the signal to the charge amplifier. In reality these wires need to be either coaxial cable or a twisted pair, to minimise electrical hum pickup. All the contacting surfaces will be held firmly together by the static force from the string once the sensor is assembled on the instrument, but for initial assembly it can be helpful to use electrically-conducting adhesive of some kind, for example the carbon-loaded sticky pads used to secure specimens in a scanning electron microscope.

Figure 2. Diagram of a bridge force sensor. Blue rectangles indicate blocks of piezoelectric crystal, red lines indicate copper conducting strip, and attached wires. The shaped pieces above and below the crystals are made of insulating polymer such as perspex.

The sensor will respond either to the vertical component of force from the string, or the horizontal component: it will depend on the relative orientations of the two crystal pieces. The crystals are connected in series, so that the combined output will be the sum of the two voltages generated. A downward vertical force component will put both pieces in compression, and if that is the desired output then the two crystal pieces would need to be placed opposite ways up, so that both give signals with the same polarity in the series combination. But it is more usual to want the transverse force component. A transverse force will increase the compression in one crystal while reducing the compression in the other. For this output, the two crystals need to be arranged the same way up: the series electrical connection will then result in a voltage corresponding preferentially to transverse force. When making sensors like this, it is easy to lose track of which way up the crystals are! A useful tip is to mark one face with graphite pencil (which is electrically conducting and will not interfere with the contacts).