Switches that are designed to activate in response to temperature, pressure or other pre-defined physical variables are found in processes ranging from pharmaceuticals to petrochemicals. At its simplest, a pressure or temperature switch controls electrical power directly, without requiring external electronic circuits. This is a low cost option and it’s robust with respect to electromagnetic interference (EMI) but its simplicity brings some disadvantages too, which become apparent when we examine measurement principles.
Most mechanical pressure switches for process control are activated through deflection of a flexible membrane that separates regions of different pressure. As shown in Figure 1, a membrane (D) deforms depending on the difference in pressure between its two faces. The reference face can be open to the atmosphere to measure gauge pressure. In order to measure differential pressure, the two sides are open to the two pressure sources. Where one side is open to a sealed volume with fixed reference pressure, absolute pressure can be measured.
Figure 1: A process is initiated when an increase in the pressure chamber (P) causes the deforming membrane (D) to expand outward and push into a pin that is connected to a switch.
Temperature measurement can be achieved through pressure measurement. This is done by guiding heat to a sealed enclosure and using it to increase the temperature of the enclosed gas. Established back in 1809, Gay-Lussac’s Law tells us that, for an ideas gas, the pressure exerted on a container’s sides is proportional to absolute temperature.
In the cases of both pressure and temperature, the deforming membrane (D) is in close to the pin of an electrical switch. When the pressure or temperature increases, it causes the membrane to expand outward so that it pushes into the pin, which is connected to the switch. Once the actuating point is reached, the switch commutes from one position to the other, normally closed (NC) to normally open (NO) causing the specified process to initiate.
Snap-action microswitches are the most common types. Here, the changeover movement is initiated through an internal spring-loaded pole, as shown in Figure 2. The actuating force applied to the plunger is transmitted to act on the pole. For activation to occur, the actuating force has to be higher than the internal opposing force, supplied by the spring.
Figure 2: Snap-action switches are designed to change over from their initial NC circuit to their NO circuit when the actuating fore is higher than the internal force.
In a perfect world, the switch would instantaneously change over from its NC state to its NO state, once it reaches this actuating position Pa. But mechanical switches are not ideal switches and their limitations affect final pressure switch accuracy. Two characteristics need to be understood: dead zone passing and hysteresis, sometimes called differential movement.
In most process applications, the temperature/pressure build-up is a slow process, causing the switch, illustrated in Figure 3, to move slowly from its initial position (Pr) to its actuating position (Pa) and on to its final position (Pfc). The same is true as the temperature/pressure gradually decreases causing the switch to move slowly back towards its initial position.
Figure 3: Since temperature/pressure build-up occurs slowly in most process applications, there is a transition zone between the time when circuit 1 is open and circuit 2 is closed.
From the time when circuit 1 is opened (Pa) to the point where the acutating force builds up enough to close circuit 2, (Pa2’), the switch is in a transition zone, also known as a dead zone. To put an end to this dead zone and in turn, activate a process, the actuating force has to increase to the point where it will securely close circuit 2 (ON2). The distance the pin has to travel, starting from the actuating position Pa up to a secured Pa2’ position typically exceeds 10µm for small V4 size switches. Depending on the mechanical construction of the snap action pressure switch, this may represent an important pressure range in which no secure switching can be guaranteed and both electrical circuits remain open.
As the pressure/temperature in the chamber decreases, the actuating force decreases as well, and at a certain point Pdr is no longer higher than the internal force of the spring. When this occurs, the switch changes over again. It opens circuit 2 (Pdr), passes the dead zone and closes circuit 1 (Pdr2’). Ideally, as the temperature/pressure decreases, the switch should open/close the circuits at the same point as they were opened/closed as the temperature/ pressure increased. Real world switches, however, do not move the same way through the switching cycle under decreasing pressure as they do under increasing pressure. Progressing back from its final position (Pfc) through decreasing pressure, the switch does not commute back at the actuating position (Pa) but instead, at a later release position (Pdr2’). This distance between the actuating point and the release point is called differential movement, or hysteresis.
This differential movement is critical in detecting small pressure changes. To maintain optimal switching accuracy for process applications, a switch with low hysteresis should be selected. However, many inexpensive, small switches, usually ‘V4’ types, which are popular due to their compact footprint and low price, often have a large hysteresis. Typical values of 0.15mm or more are common.
Recently, mechanical microswitch performance has improved significantly, sometimes specifically with pressure switch applications in mind. Products such as Microprecision’s MP500 shown in Figure 4 bring dead zone passing down by a order of magnitude over conventional switches, from 10µm to 1µm. At the same time consistent differential movements have been reduced to under 0.05mm, a 3-fold improvement on the performance of apparently comparable products. The limitations are still there, but greatly reduced.
Figure 4: The MP500 microswitch improves dead zone passing performance by 10X and hysteresis by 3X, compared with standard switches.
When used in pressure/temperature switches, these improvements in microswitch characteristics will give more precise control of every kind of process. This can result in improvements in the consistency of end products, higher manufacturing yields and greater productivity.
Dave Mellor, Director, Cyntech Components Tel: 01908 821811