Bosch Rexorth Speed sensor DSM1S18K1000A/20P R917011888 Rexroth

The Hall effect-based DSM1-10 speed sensor has been specially developed for use under harsh conditions in mobile working machines. The sensor collects the speed signal of ferromagnetic gear wheels or punching sheets. As an active sensor, when it does this it delivers a signal with a constant amplitude that is independent of the speed. The sensor distinguishes itself not only due to the fact that it can detect the direction of rotation, but also because of its additional diagnosis functions such as:

• Standstill detection

• Critical air gap

• Critical installation position

Example applications
Due to its compact and robust design, the sensor is suitable for integrated use

• In the wheel bearing for wheel speed acquisition

• In the transmissions or gear stages

• Fan drives in buses, trucks and construction machinery (7 to 20 kW)

• In vibration drives for road rollers and pavers

Example:
Axial piston motor with DSM external gear motor
Electrical circuit

A two-wire current interface is used for signal transmission. The sensor supplies a current signal. The low current (Ilow = own current of the active element) is interpreted as a low signal. The high current (IHigh =Ilow + ∆I; ∆I = additional current from a path parallel to the active element) is interpreted as high current. The current received in the control unit from the sensor at a measuring resistor RM is converted to a voltage signal. The evaluation circuit detects whether a high signal or low signal is present on the basis of the voltage level.
Electrical circuit diagram when the sensor is supplied by the on-board power supply battery

Output signals
The DSM1-10 output signal is made up of square-wave signals of constant amplitude which are produced by the DSM1-10 evaluation electronics. The length of the individual pulses provides information about the direction of rotation and any errors in the installation position.
The evaluation electronics generates a high pulse of a defined length after each edge of the sensor-internal speed signal, whereby the length of the high pulse is defined by the information to be transported. This, for example, the information direction of rotation left is described by a 90 µs long pulse and the information direction of rotation right by a 180 µs long pulse.
In order for the rotational speed information to still be output when there are long, high pulses at higher speeds, a low-time (pre-bit low) is always inserted ahead of the high pulse. So although the additional information within the signal is lost at higher rotational speeds (pulses are truncated by the low-time feature), reliable output of the rotational speed information is possible up to a maximum frequency (upstream low time + shortest high pulse).
If the air gap reserve signal is output, the other signals are overlayed (AR is dominant), i.e. neither a direction of rotation signal (DR) nor the installation position signal (IP) are output.

The resistor R generates a voltage that is present at the frequency input of the RC control units.
For an example with R = 200 Ω, the following voltages are read:
The minimum pulse width is 52 µs. This corresponds to a frequency of 10 kHz.
To interpret the signal, it must be ensured at 30 kHz input frequency that the signal (after any low-pass filter present) still has a sufficient voltage difference (ΔV) for evaluation.

The current I Supplies the sensor information in the form of pulses (see chapter “Output signals” or details), whose low and high levels are as follows:
Application with other control unit
Basic use
RC10-10/31
6 inputs
RC36-20/30
6 inputs
RC28-14/30, RC20-10/30, RC12-10/30
5 inputs
RCE12-4/22
2 inputs
RC2-2/21
2 inputs
Application at control units
Application with Rexroth BODAS controllers
The DSM1-10 can be read with the following BODAS control units: RC Series 21, 22, 30 and 31.
Notice:
The current data sheet for the BODAS RC control unit used is to be considered.
Vibrations
Vibrations in the encoder wheel at standstill can produce sensor false signals.

Signal tolerances
The following durations (minimum, nominal, maximum) are determined from the tolerances of the internal components in the sensor for the individual cases:
Description
When the vehicle is at standstill, the sensor outputs pulses with a length of 1.44 ms every 0.7 s. These pulses are also output after an undervoltage as long as no speed signal is detected.
An initialization is also carried out at standstill. This initialization lasts between 255 and 345 µs. No change of signal can be detected during this time.
Signal on exiting standstill and/or start-up
When determining the output values (frequency, direction of rotation, etc.), a certain number of pulses may be required to ensure the appropriate information is supplied.
When starting up from standstill or after the undervoltage state, the sensor is first set to a non-calibrated state (signal not offset-compensated). Also during this phase, the sensor supplies a correct frequency signal with the start of the second signal pulse, and additionally, under typical conditions, a correct direction of rotation signal with the third signal pulse. The correct output of the direction of rotation requires a maximum of seven teeth/edges, dependent on the installation position. In this mode, the minimum and maximum values of the magnetic input signal are used as trigger points.
During output of the signal in non-calibrated mode, the sensor performs calibration (offset compensation) of the signal. The sensor then switches automatically to calibrated mode. From this point on, the zero-crossings of the magnetic input signal are used as trigger points. On switchover to the calibrated mode, a phase shift of the output signal (maximum –90° and/or +90°) can occur in rare cases.
The number of signal pulses output in non-calibrated mode is a maximum of five.
Behavior at a standstill
Sensor signal after no speed signal was detected
within one second:
Behavior as rotational speed increases
As rotational speed increases, the next surface on the wheel is detected before the planned length of signal is output. In these cases, the signal is shortened and the zero time (45 μs) that occurs after each edge overwrites the signal. This ensures that the pulse frequency, and consequently the rotational speed, is always transmitted correctly. The loss of the rotational speed information is not critical, since due to the high rotational speed the direction of rotation cannot change at that point in time. If the rotational speed reduces, (e.g. deceleration until the direction of rotation changes), then the signal is output fully again and the change in direction of rotation is detected.
Limit range LimitFlux alteration
Less than LimitFlux alteration for magnetic flow changes Signal dropouts may occur.
Near range NearFlux alteration
Less than NearFlux alteration for magnetic flow changes AR bit is output.
Installation position InstallFlux alteration
Less than InstallFlux alteration for magnetic flow changes IP bit is output.
Air gap reserve (AR) and installation position (IP)
The sensor reacts to changes in the magnetic flow. If the air gap between the gear wheel and the sensor is too high, the signal output may be adversely affected:
Signal shape
The resistor to be installed R must be selected such that:

• The voltage difference for internal signal evaluation in the control unit is sufficient.

• The maximum voltage at the resistor R does not become tool high (adapted to the sensor supply), so that at least 4.5 V are present at the sensor pins.

If these conditions are satisfied and the signal is present internally in the control unit, the sensor information can be determined.
Rotational speed
Due to the properties of the DSM, which sees both sides of the wheel tooth, the actual speed difference of the wheel is determined as follows
fWheels = fread / 2
Speed, critical air gap, standstill
To determine this information, the length of the pulses must be measured. This can be done in the control unit by measuring the start and end time of the pulse, for example.
The speed can always be read from the frequency without this evaluation, however. The behavior at standstill should always be taken into account, however (1.44 ms every 0.7 s). An overlength of the pulse (1.44 ms) can be detected.

• Hall-measurement principle
• Measuring range 1 … 5000 Hz
• Output signal current square-wave signals
• Supply voltage 4.5 … 20 V
• Protection class IP69K