A temperature controller is a device that allows the actual value of a temperature to be recorded with the aid of a temperature sensor , to compare it with a predetermined target value and to set the desired (predetermined) target value via an actuator . A thermostat (from ancient Greek θερμός thermós , German 'warm, hot' and στατός statós , German 'standing, set' ) can designate both a temperature controller and just the temperature sensor without an actuator.
The Dutch physicist, chemist, designer and instrument maker Cornelis Drebbel (1572–1633) is considered to be the inventor of the first modern thermostat, which forms the basis of the incubator. He developed it for alchemical ovens and incubators for hatching chicken eggs. In this case, a fire burned in the incubator under the actual incubator, the hollow walls of which were filled with water. The combustion gases rose on the outside of the walls of the incubator and were able to escape through a smoke vent. To regulate the water temperature, there was a glass temperature sensor in the water-filled bottom of the incubator. A cylindrically shaped part of this elongated glass body was filled with alcohol , a second, U-shaped part with mercury . With the water temperature, the mercury column also moved due to the thermal expansion of the alcohol. A floating rod lying on it was moved with the mercury level. The smoke outlet and thus also the oxygen supply were closed or opened via a lever connected to it, which was used to regulate the fire in the furnace and thus also the temperature.
In the 18th century, it was the French inventor and chicken farmer Jean Simon Bonnemain (~ 1743–1828) who was the first to invent an industrially produced temperature control - again for an incubator that was used in chicken breeding. It is based on the principle that different metals expand differently under the influence of temperature. An iron rod in a lead or zinc pipe acted as a temperature sensor in the water. The lead pipe is closed at the lower end and the iron rod is firmly screwed to this closure. A copper or brass holder is soldered to the upper end of the tube and is connected to the fresh air flap via a lever arm. Since the coefficient of linear expansion of lead is greater than that of iron, the pipe expands more than the iron rod when the temperature rises, the fresh air flap closes and the temperature falls due to the falling oxygen supply.
The control principles remained mechanical for a long time, however, and mercury was often used because its thermal expansion is largely directly proportional to temperature. With the invention of contact thermometers , they became the most common switching measuring probe well into the 20th century. At first they had fixed contacts and could only switch an electric heater on or off via a relay. In 1926, the Juchheim company from Ilmenau applied for a patent for the first glass contact thermometer, with which variable temperature settings were possible using adjustable metal threads.
From the 1970s onwards, fully electronic thermostats, later with the beginning of the digital age, microprocessor-controlled controllers in combination with highly sensitive temperature sensors ensured precise control behavior. Resistance thermometers with platinum temperature sensors are usually used.
The control circuit is used to bring a specified temperature ( controlled variable x) to a desired value ( setpoint w) and to keep it as constant as possible. In order to fulfill this task (x = w), the controlled variable must be measured with a temperature sensor . If there is a difference between the setpoint and actual value of the controlled variable ( control difference e), the controller must counteract this. The variable that has to be changed for this purpose is called the manipulated variable (y). The change in the manipulated variable must lead to a change in the controlled variable. As a result, the deviation is reduced or eliminated entirely. The adjusting device required for this consists of the actuator and the actuator ; in the case of the temperature controller, it either controls a heating element, possibly also a cooling element via the energy supply (example: iron), or consists of a control for an external heat supply (example: radiator).
The temperature controller intervenes either by changing the setpoint (temperature selection) or by a fault. A disturbance can e.g. B. be the change in ambient temperature . It is called the disturbance variable (z). Every change in the disturbance variable causes a change in the controlled variable. If the disturbance variable did not change and if there were no changes in the setpoint, the temperature controller that has reached the setpoint would remain in this state. The element that compares the actual value and the setpoint value and specifies the value for the control device (control variable u) is called a controller.
In practice, the controllers are often combined with the sensors and control units in one device or component.
- Thermostatic valves on radiators. They are also available with remote sensors and with radio control, in case the radiators are not so easily accessible. They regulate the room temperature by controlling the amount of water entering the radiator (and thus the amount of heat that can be emitted by the radiator).
- Thermostatic valves in thermostatic mixer taps . They regulate the temperature of the drinking water exiting the tap by controlling the mixing ratio of incoming cold and warm water.
- Devices that work with water or other media, are equipped with a thermostat and a pump and are used to thermostate laboratory setups: immersion thermostat, bath thermostat . In addition to the temperature controller, laboratory thermostats also have a pump and a storage vessel for a heat exchanger liquid.
- Temperature controllers in irons , refrigerators and ovens consist of a temperature switch .
- Thermostat in anesthetic vaporizers to hold (Vaporen) variation of surrounding temperature, the pre-set anesthetic gas concentrations constant.
- Thermostat-controlled connection of the cooler circuit in the liquid cooling circuit of internal combustion engines .
More complex control systems for the temperature
- Electronic heating controllers on radiators that have a temperature setpoint based on the time of day can often be programmed based on the time of day or the day of the week and may also take into account the outside temperature to control the flow temperature (control circuit with several manipulated variables).
- Self-regulating electrical heating elements only consist of a material with an extremely temperature-dependent electrical resistance ( PTC ceramic) and combine all components of a control loop - the target value is a fixed material parameter. Typical for small devices with widely spread operating voltages (typically 110–240 or 12–24 volts), such as hot glue gun , hair straightener, hair dryer , kettle .
- Electronically controlled soldering iron with temperature sensor near the soldering tip, current regulator in the handle or an extra housing.
- Electrical temperature controllers for industrial applications are often housed in a DIN installation housing and can be configured for a wide variety of control tasks and characteristics. They can often be configured for operation with platinum measuring resistors (Pt100 or Pt1000) or various thermocouples as temperature sensors and contain a relay for controlling a heater or a cooler.
- Werner Kriesel : ursastat - measuring sensor with relay output (relay transmitter) and regulator without auxiliary energy (direct regulator). In: H. Haas, E. Bernicke, H. Fuchs, G. Obenhaus, G. (overall editor): ursamat-Handbuch. published by the Institute for Automatic Control Berlin. Verlag Technik, Berlin 1969.
- Heinz Töpfer , Werner Kriesel: Small automation through devices without auxiliary energy. (= Automation technology. Volume 173). Verlag Technik, Berlin 1976. (2nd edition. 1978, with Ekkehard Reimann and Mertik Quedlinburg )
- Heinz Töpfer, Werner Kriesel: Functional units without auxiliary energy. In: Functional units of automation technology - electrical, pneumatic, hydraulic. Verlag Technik, Berlin and VDI-Verlag, Düsseldorf 1977. (4th edition 1983, ISBN 3-341-00290-1 )
- Werner Kriesel, Hans Rohr, Andreas Koch: History and future of measurement and automation technology. VDI-Verlag, Düsseldorf 1995, ISBN 3-18-150047-X .
- Werner Kriesel: Automatic Museum in Leipzig. In: Association of German Engineers, VDI / VDE-GMA (Hrsg.): Yearbook 1997 VDI / VDE-Gesellschaft Mess- und Automatisierungstechnik. VDI-Verlag, Düsseldorf 1997, ISBN 3-18-401611-0 .