Abstract: A shutter, positioned between an electronic imaging device (such as a laser printer) and a film processor, is actuated by a solenoid which is driven by a driver network which isolates the solenoid coil when it is deactuated. If, after deactuation, a jam occurs, such that the shutter does not close completely, the jam is detected by a solenoid engagement sensing circuit. The sensing circuit includes a function generator, an operational amplifier having the solenoid coil connected between its input and output, an envelope detector and a comparator. The comparator produces a jam signal in response to an increase in the voltage detected by the presence of the solenoid plunger within the solenoid coil.

Abstract: A method for detecting the motion and position state of a component of an inductive electric load. The component can be moved between two end positions by magnetic interaction, in which the total waveform of the drive current is divided into a plurality of states. By evaluating these individual states, comprehensive error diagnostics and a setting of operating parameters of the inductive electric load to operating conditions are possible.

Abstract: An apparatus for detecting a failure in a solenoid valve having an electrical winding electromagnetically coupled with a plunger. The failure detecting apparatus comprises a current sensor sensitive to a current through the electrical winding for producing a signal indicative of a sensed winding current. Changes in the sensed winding current indication signal with time are monitored to provide a failure signal which indicates a failure in the solenoid valve when no inflection point exists in the change of the sensed winding current indication signal for a predetermined period of time after power is supplied to the electrical winding.

Abstract: An alarm for a photosensitive detector used for detecting registration marks on a web or the like measures the voltage levels of an intensity signal derived from the registration marks and the background as it moves past the photosensitive switch. Pulses that are sufficient to operate the photosensitive switch but are below a level deemed to indicate reliable operation are counted and set an alarm if a predetermined number occur without an intervening pulse of a higher amount.

Abstract: A safety arrangement for an automatic warehousing facility is disclosed. A traveling crane adapted to perform goods take-in and take-out operations is provided which is movable on a path provided between and extending along a plurality of three-dimensional racks arranged in parallel for storage of goods. There is provided a safety arrangement for safeguarding any operator from possible danger when he inadvertently enters the path of crane movement while the crane is in automatic operation, and for preventing the crane from moving when the operator is present within the path of crane movement. The safety arrangement includes a device for stopping the crane when an open / close door of a safety enclosure provided at one end of the crane movement path is opened.

Abstract: A solid state relay circuit which utilizes a metal oxide power semiconductor field effect transistor (MOSFET) as an output switching device to switch power to load circuitry by an electrically-isolated coupling arrangement including light emitting diodes (LEDs) and a photodiode array. An arrangement is provided for sensing the current through the LED coupling arrangement to determine when the solid state relay circuit has been operated and, in turn, to operate indicating circuitry. The circuitry may also be utilized as self testing circuitry to check the integrity of components of the input portion of the circuitry.

Abstract: A remote monitoring apparatus comprises a computer program controlled monitor for detecting changes in condition responsive relay switches to generate a data signal identifying the change of switch condition, a report assembler which prepares a report in a preselected format identifying the apparatus location and including the data signal generated, and a modem for automatically transmitting the assembled report to a selected number of remote locations connected with the monitoring site by a telecommunication network. The monitoring apparatus repeatedly accesses the telecommunication network until a successful communication has been transmitted to each remote location. The apparatus also senses the loss of a continuous, primary power source and includes a back-up power supply. The program limits the number of unsuccessful attempts which can be made with the back-up power supply and preserves the assembled reports for later transmission when power has been fully restored.

Abstract: A relay device having a relay element mounted on a circuit board, the relay element having a pair of contacts which are closed and electrically conducted by a control signal of a first level and opened by the control signal of a second level to form a contact gap; an LED for emitting light toward the contact gap; a phototransistor for receiving light from the LED, the phototransistor not receiving the light while the pair of contacts are closed, and receiving the light passing through the contact gap while the pair of contacts are opened; and a checker circuit for checking if the relay device operates normally or abnormally in accordance with the relationship between an output from the phototransistor and the level of the control signal.

Abstract: A power apparatus in which a particle in an enclosure of the apparatus containing an electric conductor therein is moved in the enclosure by an electric field so as to cause an abnormality of insulation. The apparatus includes a first sensor for detecting an existence position of the particle on the basis of a sound of collision of the particle against the enclosure, a second sensor for detecting a partial discharge generated in the enclosure, and a device for receiving respective detection signals from the first and second sensors, for judging the existence of insulation abnormality caused by the particle inside the enclosure, and for outputting a result of the judgment.

Abstract: A circuit configuration for monitoring the final stages of a plurality of valves, particularly the power transistors (LT1-LT4) and their circuitry, for defects as well as for leakage currents or by-path currents comprises a comparator circuit (1), an OR-link (2) and of a further comparator element (XOR4). By way of the monitoring circuit, the actuation connections (S1-S4) of electromagnetically operable valves are connected to a sole output (A3). By applying short test pulses to the actuation inputs (E1-E4) of the final stages (LT1-LT4) and by comparing the signal variation at the output (A3) with the test pulse variation defects, leakage currents and the like are detected.

Abstract: An in-circuit contact monitor for providing reliable and safe detection of the position of fail safe, or vital, circuit contacts in railroad signaling and inter-locking control circuits comprises an impedance network arranged in a bridge configuration, a low-power, high-frequency signal source, an isolation transformer, and a pair of DC blocking capacitors. The low-power, high-frequency AC source provides energy for an isolated circuit which is used in detecting the toggling of an individual circuit contact. The circuit is configured such that the vital circuitry, which is connected to the circuit contact, is completely isolated from the DC components generated by the low-power, high-voltage AC source. The isolation transformer acts to isolate the monitor system from the in-circuit contact monitor circuitry and thus the vital circuitry.

Abstract: An indicator circuit for indicating the states of various electric components such as limit switches, comprising a light emitting device such as a light emitting diode, and a constant current device, such as a constant current diode, connected in series with the light emitting element. The constant current element significantly expands the operable range of the input voltage to the indicator circuit, and keeps the brightness of the light emitting device constant over a wide input voltage range. For high voltage applications, a plurality of constant current devices may be connected in series, and, preferably, balancing resistors and/or zener diodes may be connected in parallel with them.

Abstract: A single module for relay, limit switch or related devices for self-powering the set of contacts and sensing the state of the contacts, compatible with Opto 22 and related I/O systems interfacing microprocessors and industrial processes and the like and with further features, if desired, of a single isolation device for both power and signal isolation such as a transformer, preferably employing conversion to D.C. in the tranformer output and optional resonant mode flyback pulses to step up the primary drive voltage.

Abstract: A relay control for a cooking appliance having a heating element includes a sensor for detecting the occurrence of an arc upon a change of state of a relay coupled between the heating element and a power supply. Upon detecting an arc, the sensor provides a timing signal which is coupled to a microprocessor that controls the energization and deenergization of the relay. The microprocessor is responsive to the timing signal from the sensor to determine the time delay between the last energization/deenergization of the relay and the changing of the relay's state. From the time delay, the microprocessor determines a delay constant. Subsequently, the microprocessor energizes the relay at a time after the detection of a zero crossover point of the power supply signal, wherein the time is equal to the determined delay constant to cause the relay to change state at or slightly before a zero crossover point of the power supply signal.

Abstract: An electric motor controller comprises a circuit for selecting between driving and braking an electric motor in accordance with the health or otherwise of a pair of main electric motor control switches (14 and 16). In a preferred embodiment the health of the switch is determined by the voltage (A) between the switches. The controller also comprises apparatus for detecting the sense of movement of the motor. In the event that the motor is moving in a sense opposite to that selected, the apparatus increase field current to reduce the armature current thereby increasing the braking effect of the motor. Also in the controller are apparatus for changing from armature to field current control and vice versa, in response to a given magnitude of control signal. In order to provide an unambiguous change-over, a lockout circuit is provided to inhibit change-over in a transition region. The lockout circuit maintains the mode of control to above/beneath the change-over level when changing from one level to another.

Abstract: The appliance according to the invention has a front face with retention means adapted for cooperating with fixing means belonging to a removable label with rank or function identification characters. This label is molded over an electronic circuit comprising at least one visible light-emitting component, this circuit being connectable to terminals provided on the appliance by means of electric conductors.

Abstract: The disclosure describes a tester for determining the operating condition of a Reed Relay Printed Circuit Board (PCB) device. A Reed Relay PCB to be tested is connected directly to the tester for testing in a data flow per path environment as it would be functionally connected with a data acquisition system, such as a "Ramp Scanner" model 128RR by Kaye Instruments, Inc. The tester develops accurate voltages for the test and connects them for powering components as well as for simulating data flow. As each data flow path is sequenced in its turn, the time of opening of a previous relay contact as a next contact is closed is monitored, and if it exceeds a predetermined amount of time, a visual indicator light is energized and latched in its energized state. Also, the tester includes other test circuits for testing the operating functionality of each data flow path.

Abstract: A monitoring system for use with a conventional actuator system which includes an actuator and control system. The monitoring system comprises a wiring harness including at least three and preferably five ring type current sensors, such as current transformers or Hall effect devices, which are permanently mounted on various leads associated with the control system. The sensors sense currents carried on the leads which are indicative of the state of control system switches connected with the leads. The sensors generate signals responsive to the currents and indicative of the switch states. Each of the sensors is spaced from and wired to a single electrical connector. The connector is adapted to removably mate with a complementary connector to simultaneously electrically couple the complementary connector with each wired sensor.

Abstract: A novel electro-mechanical switch device is disclosed which is suitable for use in a factory production line to detect and control the flow of workpieces or articles of manufacture along the line. The switch device includes a pair of switches (101, 102) operative in response to the workpieces passing thereby along the production line, a switch circuit (13) electrically connected to the switches (101, 102), and a moisture detector (15). The pair of switches are so designed that the operation of one switch is delayed with respect to the other. The switch circuit (13) functions in response to the operation of the switches and the moisture detector (15) to provide a warning signal when one of the switches fails to operate within a predetermined delay time with respect to the other switch, as well as when the moisture detector senses a presence of moisture.

Abstract: A method and apparatus detect a faulty air switch in a heating system which has an operation cycle comprising a plurality of operating periods. The air switch senses air flow in the heating system and causes air flow limit contacts to open when the air flow is outside a predetermined limit. The air flow is outside the predetermined limit during a first operating period of the operation cycle. A series circuit comprises the air flow limit contacts, coupled in series with other limit contacts between a limit sense terminal and an energized input terminal. The other limit contacts should be closed during the first operating period of the operation cycle. The limit sense terminal is sampled during the first operating period to determine whether the limit sense terminal is energized. Based on whether the limit sense terminal is energized, the air switch is determined to be faulty or operating properly.

Abstract: An improved sender circuit which provides increased resolution measurements from a sensor by using two or more separately applied driving voltages. Only two connections to the sensor are required, the sensor having one or more switches closed in relation to the parameter being measured.

Abstract: A novel electro-mechanical switch device is disclosed which is suitable for use in a factory production line to detect and control the flow of workpieces or articles of manufacture along the line. The switch device includes a pair of switches (101, 102) operative in response to the workpieces passing thereby along the production line, a switch circuit (13) electrically connected to the switches (101, 102), and a moisture detector (15). The pair of switches are so designed that the operation of one switch is delayed with respect to the other. The switch circuit (13) functions in response to the operation of the switches and the moisture detector (15) to provide a warning signal when one of the switches fails to operate within a predetermined delay time with respect to the other switch, as well as when the moisture detector senses a presence of moisture.

Abstract: In a process for automatically detecting the response of an electromagnetic valve, in particular an electrovalve, an increasing testing voltage is applied to the valve and the instantaneous testing voltage at which the electromagnetic valve responds is fixed and indicated in that the monetary decrease of the current flowing through the valve is detected and utilized for fixing and indicating the instantaneous value of the testing voltage. A device for automatically detecting the response voltage of the electromagnetic valve includes an indicating device (8) which is controllable in such a way that it indicates and fixes the value of the testing voltage applied to the electromagnetic valve as soon as the valve responds to the testing voltage.

Abstract: By providing two comparators, each of which have a reference voltage level source connected to one input, and separate voltage level producing/enhancing circuits connected to the contacts of a relay, so that each alternate switch configuration produces a different voltage level at the two comparators, a unique relay switch detection circuit is attained. Preferably, three separate voltage level producing/enhancing circuits are employed, with one circuit connected between the relay output and the two comparators, while the other two circuits are connected to the primary or the secondary relay contacts. Futhermore, the voltage level resulting from each particular switch configuration is pre-set relative to the reference voltage level source imputs to assure that the two comparators produce different combined outputs for each alternate switched condition.

Abstract: A contact abnormality detection system for use in a gas-insulated switchgear or other power transmission or distribution apparatus. A gas-insulated switchgear includes a hermetic housing, an insulating gas sealed in the housing, and conductors in the housing having contacts for providing a path for a large current and being electrically insulated from the housing. An abnormality in the contacts causes the contacts to generate vibrations when a large current flows through them. A vibration detector detects vibrations generated by the switchgear including the vibrations generated by the contacts. The frequency spectrum of the vibrations detected by the vibration detector is analyzed to detect if the vibrations contain frequency components in the range of 200 to 500 Hz which meet predetermined criteria.

Abstract: A system for debouncing a switch circuit includes a sample circuit for providing an initial determination of the switch status, and a debounce circuit connected to operate on the output of the sample or sensing circuit to verify the actual switch status by providing a confirmation signal. The system of the invention accumulates a count related either to the rate at which the sample circuit operates, or the actual changes in state indicated by latches coupled to the sample circuit. If the count accumulates to a predetermined number before being cleared by a confirmation signal verifying the switch status, then the irregular operation is signalled. This identifies a continuous, rapid alternation between different switch states without a confirmation of an actual state change, which would otherwise be undetected by system operation.

Abstract: A portable testing device for seed tube and combine sensors is provided for checking the operability of the sensor in situ at the time a problem arises in the agricultural implement. The testing device includes a housing in which is mounted a printed circuit board and a battery for providing an electrical current to the components on the circuit board. The circuit board includes an on/off switch and an LED. An electrical coupler allows the device to be quickly and easily attached to the coupler of the planter harness so as to check the operability of a seed tube sensor. In operation, the LED blinks on and off when the sensor is properly operating, and remains lit when the sensor is malfunctioning.

Abstract: A system for detecting load has a first detector for detecting load on a generator based on the voltage in a circuit of an armature of the generator, and a second detector for detecting the load on the generator based on the voltage in a circuit inductively coupled to the armature circuit. An output of the first detector is compared with a first reference value by a first comparator and a first overload signal is produced when exceeding the first reference value. A second comparator is provided for producing a second overload signal when exceeding a second reference value. In response to the first and second overload signals alarm lamps are turned on for providing an alarm of the overload.

Abstract: A visual indication system for easily providing information about the status of the output terminals of a solid state relay. A two-electrode neon glow lamp is connected across the output terminals of the relay in series with a current limiting resistor. The shunting impedance of the relay output terminals prevents voltage from inducing a glow in the neon lamp in certain modes of terminal configuration. When the terminals are ON, neither electrode glows. When the terminals are OFF, both electrodes glow. If the AC relay fails in a half-wave mode, only one of the electrodes will glow.

Abstract: A protected digital output circuit coupled to a programmable controller, for controlling a load supplied through alternating-current (ac) power lines, including a fuse and a single controlled output switch connected in series with the load, and a pair of series-connected crowbar switches connected together across the ac power lines. The crowbar switches can be commanded closed together in the event that the output switch is commanded open but fails to open, thereby shorting the ac power lines and disconnecting the load by blowing the fuse. In routine diagnostic testing, the crowbar switches can be commanded closed separately to check their operation without blowing the fuse. A current detection circuit is used to check for current in the closed crowbar switch. A current sensor is also used to detect current through the output switch, when commanded closed for switching the load on, or for diagnostic turn-on testing when the load is disconnected.

Abstract: A method and system for monitoring the condition of a plurality of electrical devices, particularly electrical switches actuated by sensors, to determine the condition of each and the location of e.g., a malfunctioning device. The electrical devices are connected in parallel across a pair of line conductors. A voltage-dropping element is inserted in series with the line at the input side of each device, and a current-limiting element, passing a fixed current independently of voltage above a predetermined threshold, is inserted in series with the respective device. Initially, a voltage is applied to the line of greater magnitude than the sum of the predetermined threshold and the voltage of the first device, and is sequentially increased in a plurality of steps, while the current passing through the line initially and after each of the voltage steps is measured.

Abstract: A system and method for communicating control signals between a digital mobile transceiver, a digital control head, and a mobile data terminal over a common serial peripheral bus uses a message protocol and architecture which is efficient and yet provides great versatility and functionality without adding undue complexity. As mobile data terminals are becoming increasing important in digital trunked repeater systems, the efficiency provided by the signalling protocol disclosed herein (which is optimized for mobile data terminal messages) is a very important advantage.

Abstract: There are provided a first switch which is switched to the on state or the off state in association with a displacement of an actuator which causes a displacement by a contact with an object and a second switch which is switched to the on state or off state later than the first switch in association with the displacement of the actuator. A switch output is produced and an abnormality is detected in accordance with the on and off states of these switches.

Abstract: The operation of an electromagnetic valve is monitored by connecting the coil of the valve to a d.c. source for a period of time sufficient to permit the current in the coil to reach its maximum steady state value then disconnecting the coil to permit the current to fall and re-connecting the coil to the source again and measuring the current which is established in the coil.

Abstract: A system for testing the conditions of at least one pair of contacts to determine if the contacts are opened or closed comprising a transmit circuit and a receive circuit. The transmit circuit generates and outputs a test signal having a known frequency and the test signal is optically coupled to the contacts. The receiving circuit receives the test signal from the contacts and outputs one output indication indicating the contacts are closed in response to receiving the test signal and outputs one other indication indicating the contacts are opened in resposne to not receiving the test signal. The receiving circuit also optically couples the test signal from the contacts to the indicating means.

Abstract: Disclosed is a switch operation monitoring apparatus for quickly and accurately detecting an abnormal operation of an actuating mechanism actuating a switch such as a power circuit breaker. A normal characteristic stroke of a moving part of the actuating mechanism is previously calculated on the basis of an interrupting current waveform, an environmental gas pressure and an actuating fluid pressure while taking into account the structure of the switch so as to use the calculated characteristic stroke as a reference stroke, and this reference stroke is compared with an actually measured characteristic stroke to decide whether the operation of the actuating mechanism actuating the switch is normal or abnormal.

Abstract: A circuit arrangement is disclosed for monitoring the connecting lines of a switch or sensor open in the normal condition, with a first ohmic resistance connected in series with the switch or sensor. That together with the switch or sensor and a second ohmic resistance connected in parallel to both forms a parallel circuit, whose first connecting line is connected to ground and whose second connecting line is connected, on the one hand, to a voltage supply by way of a third ohmic resistance and, on the other, with a voltage discriminator unit. The second ohmic resistance is adapted to be disconnected.

Abstract: Apparatus for testing an electrical switch responsive to fluid under pressure and a solenoid valve that controls the flow of fluid under pressure. Fluid under pressure is injected into a socket holding the switch to be tested. The magnitude of the pressure is predetermined in accordance with the rating of the switch. An electrical circuit is connected to the switch to determine whether the switch opens and closes at the rated range of the switch. A solenoid valve communicates with the socket. Fluid under pressure is conducted along a path. The solenoid valve is disposed in the path of fluid flow to control the flow of fluid under pressure. The electrical circuit is connected to the solenoid valve to energize and deenergize the same, thereby opening and closing the valve. A meter communicating with the flow of fluid under pressure indicates whether the solenoid valve is operating properly.

Abstract: An automatic testing method and apparatus for proximity switches toggles a proximity switch while storing the initial state of the proximity switch until the toggling is successfully completed. Both the proximity switch position and the validity of the information are provided as outputs.

Abstract: A semaphore for use in a detection system for a high voltage device and for signalling a predetermined state of the device. The semaphore comprises a housing supporting therein a retroreflector element. The housing has an aperture and the retroreflector has a reflective surface supported in a alignment with the aperture. The housing is mounted in close proximity to the device. A linkage is associated with the housing and the device and is actuated upon the occurrence of a predetermined condition whereby to expose the reflective surface in the housing for the reflection of signals through the apertures. A remote scanning device continuously generates infrared signals in the direction of the aperture and detects reflective signals from the reflective surface.

Abstract: The invention relates to a manually controlled static switch. This switch comprises a body having a slot with a base wall, bordered on either side by first and second recesses, so that the slot has at least two walls transparent to electromagnetic radiation positioned laterally with respect to the base wall and which are substantially parallel to one another, an electromagnetic radiation beam emitter disposed in the first recess, so as to be able to supply beams to said second recess, a radiation--sensitive receiver located in the second recess and which detects the beams, the receiver emitting an output signal having two different levels depending on whether the receiver does or does not receive the beam.

Abstract: A remote indicator for displaying the position of a power control switch whether or not power is applied to the switch. An auxiliary DC power supply provides a reference potential to a plurality of voltage dividers. One voltage divider provides a first reference potential to a comparator. A second voltage divider and the reference power supply are connected through the power control switch so that a second reference potential is applied to the comparator when the power switch is closed. Differences of the two reference potentials corresponding to the open and closed switch positions are sensed by the comparator and used to activate an indicator accordingly.

Abstract: A control unit for use in a vehicle includes a failure detecting apparatus for detecting failures of solenoids which are used for various kinds of controls of the vehicle. The failure detecting apparatus includes a solenoid driving circuit which sequentially supplies currents to the solenoids, and includes an input circuit which receives an output signal from each solenoid which operates normally. When a solenoid doesn't operate normally, no output signal is returned. Therefore, a control circuit of the apparatus detects the absence of the output signal from such solenoid and thereby determines that this solenoid is defective. When the failure of a solenoid is thus detected, the control circuit outputs an alarm signal to the alarm indicator through the alarm driving circuit, the alarm signal identifying the defective solenoid. A light emitting diode corresponding to the defective solenoid may then be lit to visually indicate the failure.

Abstract: A self-testing contact closure test circuit and method in which invalid logic states are artificially created in order to simulate a switch malfunction, and thus to determine whether the testing circuitry correctly identifies the invalid state. The system is designed to be used with switches such as those incorporating form "C" arrangements having two pairs of contacts which under normal circumstances are in opposite states.

Abstract: An electronic bipolar interface driver circuit for interconnecting a microprocessor to an electromagnetic relay load. The driver circuit includes a peripheral adapter circuit having a pair of outputs connected to a respective one of the two inputs of a pair of NAND gates. A source of square-wave signals connected to the other inputs of the two input NAND gates so that a.c. signals are supplied to one of two switching amplifiers. The a.c. signals are transformer-coupled to a pair of rectifiers. The rectified d.c. voltage is supplied to either one of two switching transistors which establishes a given polarity of voltage on a pair of output terminals and which illuminates one of two optical couplers to feedback a checking signal to the peripheral adapter circuit to verify the polarity of the voltage on the output terminals.

Abstract: A circuit status indicating device is used to monitor separate circuits to indicate their on/off status. Each circuit conducts an AC operating signal when switched to "on" status and an AC non-operating signal when switched to "off" status. The AC non-operating signal can be zero or some fraction of the AC operating signal, depending on the type of contact switches used in the circuit being monitored. A separate LED is provided for each circuit monitored. A separate AC input signal corresponding to one of the AC operating and non-operating signals in each monitored circuit is received and compared with a modified AC reference signal to produce an AC comparison output signal. A DC reference signal is compared with the AC comparison output signal and produces a DC output signal when the AC comparison output signal exceeds the DC reference signal.

Abstract: A digital protective relay monitoring system for testing the analog input section of the relay by application of a monitoring signal, with a frequency different from the power frequency, superimposed on the line voltage and current sensor signals. After the signals have been sampled in the form of digital data, the monitoring signal component is extracted through the digital filtering process and compared with the reference value, and a fault in the analog input section of the relay is detected on the basis of the result of comparison.

Abstract: A contact chatter detector includes a bipolar power supply and a switching system which permits the fixed contacts of a relay under test to be connected either to the positive or the negative terminal of the power supply, the movable contacts of the relay under test being grounded. Depending upon the switch position and whether or not chatter is present, the fixed contacts may be at ground potential, a positive potential, or a negative potential. Binary gates continuously monitor the fixed contacts, and are able to accommodate the three input states by receiving power from power supply translation circuits which effectively adjust the gates to be responsive to the negative potential and ground at one switch position and to ground and the positive potential at another switch position. Interruption alarms connected to the gates determine whether chatter exceeds a predetermined minimum, regardless of the rate at which the chatter pulses occur.

Abstract: A switch position sensing device gives visual and recorded information as to the relative open and closed position of a monitored switch whether the monitored switch is found within an AC or DC circuit or a dead circuit and without disturbing the wiring of the switches primary circuit.

Abstract: An electric motor controller comprises a circuit for selecting between driving and braking an electric motor in accordance with the health or otherwise of a pair of main electric motor control switches (14 and 16). In a preferred embodiment the health of the switch is determined by the voltage (A) between the switches. The controller also comprises apparatus for detecting the sense of movement of the motor. In the event that the motor is moving in a sense opposite to that selected, the apparatus increase field current to reduce the armature current thereby increasing the braking effect of the motor. Also in the controller are apparatus for changing from armature to field current control and vice versa, in response to a given magnitude of control signal. In order to provide an unambiguous change-over, a lockout circuit is provided to inhibit change-over in a transition region. The lockout circuit maintains the mode of control to above/beneath the change-over level when changing from one level to another.