The OpenGrab EPM v3 is an electropermanent magnet, combining the advantages of electro and permanent magnets. The device creates a very strong magnetic contact with a ferrous target. It supports UAVCAN, RCPWM and push button operation. OpenGrab EPM v3 has been developed by NicaDrone in cooperation with Zubax Robotics.
Theory of operation
Electric shock hazard
The device poses an electric shock hazard. Do not touch exposed parts of the circuit while the magnet is operating.
The NXP LPC11C24 MCU drives a MOSFET connected to a transformer in a flyback configuration to charge the main PET capacitors up to 475 V. A thyristor bridge is used to discharge the capacitor in either direction through the winding inside the AlNiCo material. This results in a short, 20 μs 300 A pulse creating a 100 kAm field in the AlNiCo material. This causes the magnetic domains in the AlNiCo magnets to align in a particular orientation to form a magnetic circuit with a ferrous target. More detailed explanation of the operating principle is available on Wikipedia.
An ON command results in the charging and discharging the capacitors 3 times to achieve full magnetization. An OFF command results in charging and discharging the capacitors several times with changing direction and decreasing amplitude, effectively degaussing the AlNiCo material.
- Cargo lifting in UAV and robotic applications.
- Robot workholding.
- Education, demonstration of magnetic properties.
- Steady state power under 50 mW.
- Short cycle time.
- Variety of interfaces:
- Push button
- Open source firmware and hardware.
- 5 V supply voltage, can be powered via the RCPWM connector or via the UAVCAN port.
The bottom surface of the magnet should be kept clean, because dirt or metal shavings can be crushed into the surface when the magnet is turning on, causing an insulation breakdown.
The diagrams below document the mechanical arrangement and dimensions (click to enlarge):
Version 3R5C and newer
Version 3R4B and older
|Tcycle(ON)||Time to complete one cycle||0.75||s|
|Tcycle(OFF)||Time to complete one cycle||1.2||s|
|Fmax||Max holding force||200||300||N|
|Isteady||Steady state current draw||10||mA|
|Ipeak||Peak current draw during cycle execution||1000||mA|
|m||Mass of the device||65||g|
|RHoperating||Operating humidity (non-condensing)||0||75||%|
Pressing this button for at least 200 milliseconds will toggle the EPM.
An external button can be connected to the pin 6 (GND) and 3 (TXD) of the UART header (J9). Pulling pin 3 (TXD) low momentarily toggles the EPM. External button is supported in firmware builds starting from March 2017.
This LED indicator shows the status of the device derived from the continuous self-diagnostics, according to the UAVCAN node status code:
|Health||Blinking ON/OFF duration, milliseconds|
|ERROR or CRITICAL|
This LED indicates the activity on the CAN bus. Each blink indicates that there was a CAN frame that was successfully transmitted or successfully received during the last few milliseconds. Under a high bus load, this LED indicator is expected to glow constantly.
Note that CAN frames filtered out by the hardware acceptance filters will not cause the LED indicator to blink.
Connect an RC receiver or some other hardware capable of producing RCPWM signal (e.g. Pixhawk) to the RCPWM connector.
The device divides the PWM pulse duration into three ranges:
- Neutral – while the signal is in this range, the device ignores it.
- OFF – while the signal is in this range, the device will be continuously performing the turn-off sequence.
- ON – while the signal is in this range, the device will be continuously performing the turn-on sequence.
|TRCPWM(ON)||RCPWM pulse duration to turn ON||1.75||2.5||ms|
|TRCPWM(OFF)||RCPWM pulse duration to turn OFF||0.5||1.25||ms|
|fRCPWM||RCPWM input frequency||1||50||55||Hz|
|VRCPWM(low)||Low-level RCPWM input voltage||0.3×Vsupply||V|
|VRCPWM(high)||High-level RCPWM input voltage||0.7×Vsupply||V|
This section describes the properties specific for this product only. For general info about the UAVCAN interface, please refer to the UAVCAN interface documentation page.
Use the UAVCAN GUI Tool to interact with the magnet from a computer (Windows/Linux/Mac).
Mode and status codes
OpenGrab EPM v3 employs the following UAVCAN-defined operating modes:
|UAVCAN operating mode||Conditions|
The UAVCAN interface is initializing. The initialization of the interface does not interfere with other functions of the device.
|OPERATIONAL||The UAVCAN interface and the device itself are fully operational.|
The following table describes the meaning of the standard UAVCAN health codes.
|UAVCAN health code||Conditions|
|OK||Everything is OK; the device is functioning properly.|
Possible reasons for the health code being
- Invalid input voltage.
- The high-voltage flyback charger circuit is damaged.
Also, the device reports extended status information via the field
uavcan.protocol.NodeStatus.vendor_specific_status_code. The higher byte is used to store the current voltage on the buffer capacitor, the units are 2 V per LSB. The lower byte is used to store implementation-specific status flags.
This device does not invoke any services.
The following service servers are implemented:
|Controls the magnet, see below.|
|Used to allocate node ID if dynamic node ID allocation is enabled.|
|Status of the magnet, see below.|
|Used to allocate node ID if dynamic node ID allocation is enabled.|
This message allows to control the magnet via UAVCAN. The fields are interpreted as follows:
If the field does not equal the hardpoint ID of the current device, the message will be ignored.
If this field is zero and the magnet is turned on → the magnet will turn off.
If this field is non-zero and the magnet is turned off → the magnet will execute the number of turn on cycles specified in the field, but not less than 3 and not more than 10.
If this field is non-zero, the magnet is turned on, and the field has changed its value → see #2.
In all other cases the command will be ignored.
This message carries the status of the magnet.
Hardpoint ID of the current magnet.
Always set to NaN.
Always set to positive infinity.
Indicates whether the magnet is turned on or off:
- 1 - the magnet is turned on.
- 0 - the magnet is turned off.
CAN bus characteristics
The device will detect the CAN bus bit rate automatically after powering on. The automatic detection is done by means of listening to the bus in the silent mode, alternating between the pre-defined values of supported CAN bit rates (listed in the table) until the first valid CAN frame is received. Unconfigured CAN bus does not interfere with other functions of the device.
|fCAN||CAN bit rate (autodetect)||100|
|VCAN(out)dif-dom||CAN dominant differential output voltage||1.5||0||3||V|
|VCAN(out)dif-rec||CAN recessive differential output voltage||-50||0||50||mV|
|ICAN(out)dom||CAN dominant output current||40||70||120||mA|
|ICAN(out)dom||CAN recessive output current||-5||5||mA|
|RCAN(in)diff||CAN differential input resistance||19||30||52||Ω|
|tCAN(out)to-dom||CAN dominant time-out time||0.3||1||12||ms|
The device is equipped with a 4-position DIP switch that allows the user to configure the Hardpoint ID and enable or disable UAVCAN dynamic node ID allocation:
|DIP switch #||Purpose|
|Hardpoint ID bit 0|
|Hardpoint ID bit 1|
Hardpoint ID bit 2
0 - use dynamic node ID allocation;
1 - use fixed node ID
Hardpoint ID is defined in binary by the lowest 3 switches. The table below clarifies the binary encoding:
|Hardpoint ID||DIP #3||DIP #2||DIP #1|
Node ID selection
If the DIP switch #4 is set to OFF, the device will perform dynamic node ID allocation once the CAN bus bit rate detection is done. This implies that the device will not be available via the UAVCAN interface unless the UAVCAN network contains a functioning dynamic node ID allocation server. Please refer to the UAVCAN specification for more info.
If the DIP switch #4 is set to ON, the device’s node ID will be fixed at (Hardpoint ID + 100). For example, if the Hardpoint ID is set to 5, the fixed node ID will be configured as 105. In this case the device does not require an external dynamic node ID allocation server, and therefore it will be accessible via UAVCAN immediately once the CAN bus bit rate detection is done.
The EPM reports error and status messages over this interface. This interface can also be used to update the firmware – please refer to the source repository for instructions (link below).
Parameters of the serial interface are as follows:
|New line sequence|
|VUART(in-low)||Low-level UART input voltage||0.3 Vsupply||V|
|VUART(in-high)||High-level UART input voltage||0.7 Vsupply||V|
|VUART(out-low)||Low-level UART output voltage||0.4 Vsupply||V|
|VUART(out-high)||High-level UART output voltage||Vsupply - 0.4||V|