The present invention relates generally to the field of conveyor ovens and more particularly to conveyor ovens which include a control system for controlling the speed at which a conveyor moves food products through the oven.
Conveyor ovens typically include an oven housing, a heat source within the housing, and a conveyor configured to move food products through the oven, whereby the food products are heated or toasted by the heat source. The use of an endless conveyor to move food products through the oven typically increases the processing capacity of conveyor ovens over other types of ovens. The degree to which food products are heated depends on the temperature within the conveyor oven and the time period during which food products are heated (which is a function of conveyor speed). Thus, to control the degree of heating, conveyor ovens typically provide operator controls configured to adjust the oven temperature or conveyor speed.
Different types of conveyor ovens are known. For example, conveyor toasters are commonly used to toast food products including bread, buns, bagels, muffins and waffles. A full line of electric conveyor toasters is made, for example, by Hatco Corporation of Wisconsin. These conveyor toasters, which include both vertical and horizontal models, include a toaster housing, electric heating elements mounted within the toaster housing, and an endless conveyor which moves food products adjacent to the heating elements such that the food products are toasted. The food products can be placed directly on the conveyor of a horizontal toaster oven, or can be held within a food-carrying basket of either a vertical or horizontal toaster oven. Food products are manually loaded through an opening in the housing, are toasted as they are moved past the heating elements by the endless conveyor, and are discharged into a product receiving tray located at the bottom of the housing.
Other types of conveyor ovens for processing food products include, for example, conveyorized pizza ovens, microwave ovens and tunnel ovens. However, conveyor ovens may also be used to heat products other than food. For example, conveyor ovens may be used to cure rubber sheets or to dry ink on printed materials. Control over the degree of heating in each type of conveyor oven is typically accomplished by varying the oven temperature or the conveyor speed. Although the description below is focused on toaster conveyor ovens, the term xe2x80x9cconveyor ovenxe2x80x9d is intended to include other conveyor oven types.
In some conveyor ovens using analog conveyor speed control systems, conveyor speed is controlled by an operator using a variable-resistance device (e.g., a rheostat) in parallel with a diode. The rheostat-diode forms part of a speed control circuit which controls the speed of an AC electric motor driving the conveyor via gears. The degree of toasting is set by appropriate adjustment of the rheostat. An increase in resistance causes the speed control circuit to decrease the conveyor speed, thereby increasing the degree of toasting. Conversely, a decrease in resistance of the rheostat causes an increase in conveyor speed, thereby decreasing the degree of toasting. The operator adjusts the rheostat setting such that the food product being processed is properly heated or toasted. An example of a toaster conveyor oven having such a speed control circuit is the TK-105E oven made by Hatco Corp.
The temperature at which food products processed by certain conveyor ovens are heated depends on the status of the oven""s electric heating elements. For example, in the TK-105E oven made by Hatco Corp., one set of elements is always powered while a second set is switched on and off by a temperature-controlled switch (e.g., thermostat). The second set of elements is switched off when the oven temperature exceeds the threshold temperature setting of the thermostat, and is switched on when the temperature drops below the threshold. Thus, the oven temperature is regulated about the threshold setting of the thermostat. The thermostat does not affect the speed of the conveyor.
Despite having control circuits for controlling the conveyor speed and heating elements, such conveyor ovens are unable to control the degree to which food products are heated under certain conditions. For example, assume a speed control circuit is adjusted to properly toast a given load of food products. Then, assume a higher load of food products is introduced. The load increase will cause a drop in temperature, and the thermostat may cause additional heating elements to switch on. After a time, the additional elements will provide additional heat to compensate for the higher load. However, the additional elements may need a significant time period to heat up. For example, metal-sheathed electric heating elements commonly used in such ovens require 3-4 minutes to reach their operating temperatures. During this period, food products being processed will receive an insufficient amount of heat energy and will be undercooked.
Attempted solutions to this problem have been to increase the power of the additional heating elements or to switch on more heating elements using the thermostat. These solutions, however, have several drawbacks. First, the higher power ratings or additional heating elements increases the cost of the ovens. Second, the additional power can result in over-heating or burning the products. Third, the power requirements of the conveyor ovens are increased, thereby increasing operating costs.
Accordingly, it would be advantageous to provide an improved speed control system for a conveyor oven for accommodating loading-induced oven temperature changes. It would also be advantageous to accommodate loading-induced temperature changes without requiring additional heating elements that use excess power, may overheat food or other products, and adversely affect temperature-sensitive oven control components. It would also be advantageous to provide a conveyor speed control system which senses oven temperature and responds to a decrease in steady state oven temperature by slowing the conveyor without manual input to increase the heating time of the food products being processed. It would be further advantageous to provide a simplified conveyor speed control system having decreased complexity, lower costs and increased reliability. Accordingly, it would be advantageous to provide an improved speed control system for a conveyor oven having any one or more of these features.
The invention relates to a control system for controlling the speed at which an electric motor drives a conveyor to move products through a conveyor oven. The system includes a temperature sensor configured to sense the temperature of the conveyor oven at a product heating region near the conveyor and a speed adjustment input for selecting a speed setting corresponding to a desired amount of heat to be transferred to the products that move through the conveyor oven. The system also includes a controller having temperature data storage capacity and programmable logic capability. The controller receives an input signal from the temperature sensor and an input signal from the speed adjustment input and in some cases from a motor speed sensor to perform operations on the stored temperature data and the input signals to provide an output signal to control the speed of the motor according to changes in the temperature of the conveyor oven.
The invention also relates to a conveyor oven having an oven housing, a plurality of heating elements mounted within the oven housing and a conveyor configured to move products through the oven housing adjacent the heating elements such that the products are exposed to the heating elements. An AC electric motor is mechanically coupled to the conveyor to move the conveyor at different speeds in response to the temperature within the oven housing and a speed adjustment input device provides an initial speed setting to the motor.
A temperature-sensing device generates a temperature signal based upon the temperature within the oven housing. A controller is coupled to the speed adjustment input device, the motor, and the temperature-sensing device, and operates to decrease the speed of the conveyor when the sensed temperature drops below a steady state oven temperature and increase the speed of the conveyor when the sensed temperature approaches the steady state oven temperature.
The invention further relates to a method of controlling the speed at which an electric motor drives a conveyor to move products through a conveyor oven and includes the steps of setting an operator-adjustable input to a controller that provides an output signal to drive the motor at a desired speed for a given load of products, sensing a temperature within the conveyor oven, determining when a change in load of products has occurred based on a change in the sensed temperature, and compensating for the change in load by changing the output signal of the controller to the motor, effectively changing the setting of the operator-adjustable input, whereby the speed of the conveyor is changed to compensate for the change in load.